DEPARTMENT OF MARINE AND FISHERIES SCIENCES 
 
FOOD AND FEEDING HABITS OF THREE TUNA SPECIES LANDED IN 
GHANA 
 
THIS THESIS IS SUBMITTED TO THE UNIVERSITY OF GHANA, LEGON IN 
PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE AWARD OF 
MASTER OF PHILOSOPHY DEGREE IN FISHERIES SCIENCE 
 
ALIDU TUGA 
SID. 10701011 
 
AUGUST 2020
 
DECLARATION 
This thesis is the result of research work undertaken by Alidu Tuga in the Department of 
Marine and Fisheries Sciences, University of Ghana, under the supervision of Dr. Angela 
Manekuor Lamptey.  
 
     
                                          29/ 10/ 21                   
Sign ………………………..                         Date……………………… 
 
Alidu Tuga 
(Student) 
 
              29/ 10 /21 
Sign ………………………..                         Date……………………… 
 
Dr. Angela M. Lamptey 
(Principal Supervisor) 
 
 
 
 
ii 
 
 
DEDICATION 
 
This work is solely dedicated to Almighty Allah whom I trust and worship, my lovely 
wife, Bintu Seidu Tuga, and my mum, Ajarah Ohui Tuga for their sincere love, 
commitment, and support given to me. It is my prayer that this work will improve the 
quality of our lives in subsequent years to come. 
 
 
 
 
 
 
 
 
 
 
  
iii 
 
 
ACKNOWLEDGEMENTS 
 
I wish to express my sincere thanks and gratitude to God Almighty for the many blessing, 
strength, favour, and love granted me throughout my stay in University of Ghana. 
Without Him, this study would have been impossible.  May His name forever be 
glorified, Amen! This work has been executed successfully not only through the prowess 
and the instrumentality of the Researcher, but also with the active involvement of a good 
number of people, and institutions who in diverse ways contributed immensely to this 
work. 
I am grateful to my Supervisor, Dr. Angela Manekuor Lamptey of the Department of 
Marine and Fisheries Sciences for the expert assistance, advice, and guidance during my 
research and write up of my thesis. 
 
Special thanks go to the Head of Department, Lecturers, and staff of the Department of 
Marine and Fisheries Sciences for their assistance and criticisms. I am indebted to Mr. 
Atsu Foli Bennett for his inputs, encouragement, and guidance for my data analysis. 
 
I wish to extend my profound gratitude to my parents, Mr. Adamu Tuga and Mrs. Ajarah 
Ohui Tuga and my wife, Mrs. Bintu Seidu Tuga, for their financial support, guidance, 
and motivation. To my siblings; Abass, Abiba, Isha, Ramatu, Hawa, and all members of 
the Tuga family I say thank you all for the prayers, love, and the time you shared with 
me. 
iv 
 
 
To my friends, thank you for the friendship, I am grateful for the time we have come to 
know each other and the time we have shared during my study. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
v 
 
 
ABSTRACT 
 
Food and feeding habits of the three tuna species landed in Ghana were investigated for a 
period of six (6) months from October 2019 to March 2020. The species are Thunnus 
albacares, Katsuwonus pelamis, and Thunnus obesus. The study sampled a total of 210 
individuals of the three tuna species, out of which 90 individuals were Katsuwonus 
pelamis, 60 individuals of Thunnus albacares, and 60 individuals of Thunnus obesus. 
The study sampled 90 specimens for Katsuwonus pelamis because they are of high 
abundance and very common in Ghanaian waters than Thunnus albacares and Thunnus 
obesus. These tuna species were sampled for stomach content analysis, to determine 
which type of prey item is most important and preferred as food for these three tuna 
species. The samples were transported to the Fisheries Scientific Survey Division 
(FSSD) laboratory in Tema, under the Ministry of Fisheries and Aquaculture 
Development for analysis. 
At the laboratory, some morphometric measurements of the samples were taken. The 
fish samples were dissected and their guts were removed for examination. Gut content 
analysis was conducted on each of the fish sampled. The gut fullness was determined on 
a scale of 0/4 to 4/4 by visual inspection. The commonest prey items discovered in the 
stomachs during the studies were Engraulis encrasicolus, Exocoetus spp, Penaeus 
kerathurus, Sepia hierredda, Loligo spp, and Metapenaeus monoceros.   
Engraulis encrasicolus was found to be the most preferred food item accounting for 58% 
occurrence by number, with Diplodus spp being the least preferred food item accounting 
for 2% for Thunnus albacares. For Katsuwonus pelamis, Penaeus kerathurus was found 
vi 
 
 
to be the most preferred food item accounting for 57% occurrence by number, with Sepia 
hierredda being the least preferred food item accounting for 10% of total prey items 
found. Exocoetus spp was also found to be the most preferred food item accounting for 
86% occurrence by number, with Metapenaeus monoceros being the least preferred food 
item accounting for 12% for Thunnus obesus. Similarly, the most important food items 
identified using Index of relative importance were Engraulis encrasicolus, Penaeus 
kerathurus and Exocoetus spp with Diplodus spp, Sepia hierredda and Metapenaeus 
monoceros recording the least important for Thunnus albacares, Katsuwonus pelamis and 
Thunnus obesus respectively. There was a significant difference among these three tuna 
species with respect to Engraulis encrasicolus, Penaeus kerathurus Exocoetus spp, 
Sardinella aurita, Caranx crysos, Sepia hierredda, Loligo spp, and Brachyuran spp.  
Continuous research on food and feeding habits of these Tuna species to cover all the 
yearly seasons was recommended owing to their commercial importance to Ghana and 
the international community.  
  
vii 
 
 
 
 
TABLE OF CONTENTS 
Content                                                                                                                  Page 
DECLARATION ................................................................................................................ ii 
DEDICATION ................................................................................................................... iii 
ACKNOWLEDGEMENT ................................................................................................. iv 
ABSTRACT ....................................................................................................................... vi 
LIST OF FIGURES ............................................................................................................ x 
LIST OF TABLES ............................................................................................................ xii 
LIST OF APPENDICES ................................................................................................... xii 
LIST OF ABBREVIATION/ ACRONYMS ................................................................... xiii 
CHAPTER ONE ................................................................................................................. 1 
INTRODUCTION .............................................................................................................. 1 
1.1 Background of Study .................................................................................................... 1 
1.2 Problem Statement and Justification ............................................................................. 7 
1.3 Objectives ..................................................................................................................... 9 
1.3.1 Main objectives: ......................................................................................................... 9 
1.3.2 Specific objectives: .................................................................................................... 9 
CHAPTER TWO .............................................................................................................. 10 
LITERATURE REVIEW ................................................................................................. 10 
2.1 State of the world fisheries ......................................................................................... 10 
2.2 Global tuna production and management ................................................................... 12 
2.3 Tuna production in Ghana .......................................................................................... 17 
2.4 Food and feeding habits .............................................................................................. 20 
2.5 Method used in stomach content analysis ………………………………………………………...22 
CHAPTER THREE .......................................................................................................... 26 
MATERIALS AND METHODS ...................................................................................... 26 
3.1 Study Area .................................................................................................................. 26 
3.2 Field procedure ........................................................................................................... 28 
3.3 Laborator yprocedure .................................................................................................. 29 
3.4 Data analysis ............................................................................................................... 30 
viii 
 
 
CHAPTER FOUR .............................................................................................................. 33 
RESULTS .......................................................................................................................... 33 
4.1 Stomach content .......................................................................................................... 33 
4.2 Yellowfin tuna (Thunnus albacares) .......................................................................... 34 
4.3 Skipjack tuna (Katsuwonus pelamis) .......................................................................... 40 
4.4 Bigeye tuna (Thunnus obesus) .................................................................................... 46 
4.5 Prey Species Similarities.............................................................................................. 50 
CHAPTER FIVE .............................................................................................................. 57 
DISCUSSION ................................................................................................................... 57 
5.1Species Composition in the stomach of Thunnus albacares, Thunnus obesus and 
Katsuwonus pelamis.......................................................................................................... 57 
5.2 Comparison of Species Composition found in Thunnus albacares, Thunnus obesus 
and Katsuwonus pelamis ................................................................................................... 59 
5.3 Prey Species Similarities.............................................................................................. 62 
CHAPTER SIX ................................................................................................................. 64 
CONCLUSIONS AND RECOMMENDATIONS ........................................................... 64 
6.1 Conclusion .................................................................................................................. 64 
6.2 Recommendation ........................................................................................................ 65 
REFERENCES ................................................................................................................. 66 
APPENDICES .................................................................................................................. 74 
  
 
 
 
 
 
  
ix 
 
 
LIST OF FIGURES 
Figures                                                                                                                Page 
Figure 3.1: Map of Ghana showing the study area………………………….......27 
Figure 4.1: Proportion of stomach fullness for Thunnus albacares………….…..35 
Figure 4.2: Prey species composition for Thunnus albacares……………….…...36 
Figure 4.3: Proportion of stomach fullness for K. pelamis…………….………....42 
Figure 4.4: Major prey groups for Katsuwonus pelamis ……………………........43 
Figure 4.5: Stomach fullness of Thunnus obesus …………………………….….47 
 
 
 
 
 
 
 
 
 
 
x 
 
 
 
 
LIST OF TABLES 
Tables                                                                                                                 Page 
Table 2.1: Tuna landing records in Ghana………………………………………...18 
Table 4.1: Total number of the stomach samples for the three tuna species…..….34 
Table 4.2: Results of stomach content analysis of   Thunnus albacares…………..38 
Table4.3: Results of stomach content analysis of Katsuwonus pelamis……….....45 
Table 4.4: Results of stomach content analysis for Thunnus obesus………….….49 
Table 4.5: Qualitative Jaccard’s species similarity indices for the three  
Tuna species in the study area……………………………………..........................52 
Table 4.6: Analysis of variances (ANOVA) of prey items found in the  
Stomach of the three tuna species………………………………………………...53 
Table 4.7: Prey items found in the stomach of the three tuna species and their habit 
………………………………………………………………………………………54 
 
 
 
 
 
xi 
 
 
 
LIST OF APPENDICES 
Appendix                                                                                                      Page 
Appendix 1: Biological Data Sheet (Field)…………………………………..74 
Appendix 2: Biological Data Sheet (Laboratory)…………………………….75 
Appendix 3: Tukey test for multiple comparisons of prey items found in the stomachs of 
the three tuna species………………………………….……………………...76 
 
 
 
 
 
 
 
  
xii 
 
 
 
LIST OF ABBREVIATION/ ACRONYMS 
FAO - Food and Agriculture Organization 
LIFDC - Low-Income Food Deficit Countries 
FASDP - Fisheries and Aquaculture Sector Development Plan 
GDP - Gross Domestic Production 
FAD - Fish Aggregating Devices 
UNCLOS – United Nation Convention on the Law of the Sea 
ISSF – International Seafood Sustainability Foundation 
NOAA – National Oceanic and Atmospheric Administration 
RFMO – Regional Fisheries Management Organization 
 
 
xiii 
 
 
 
 
 
CHAPTER ONE 
INTRODUCTION 
1.1 Background of Study 
In recent times, demand for fish continues to rise in some parts of the world as a result of 
increasing population growth and promoting the consumption of animal protein (FAO, 
2020; Addi, 2014). In a well-established economy, demand for high-value fish species 
like tunas, shrimp, and salmon has increased over the years, due to the rise in population, 
income, and urbanization and a shift in preference from red meat to fisheries products 
(Merino et al., 2012).  
Fish is a very essential source of protein and micronutrients, which contains omega 3 
fatty acids and used for managing high blood pressure. According to the Food and 
Agriculture Organization, fish contribute roughly 20 percent of animal protein 
consumption in Low-Income Food Deficit Countries (LIFDCs). In some islands and 
coastal communities such as Sri Lanka, Indonesia, Senegal, and Bangladesh, it provides 
more than 50 percent of animal protein to people in these countries (Buckland et al., 
2017). Moreover, it contributes to about 62 percent in Gambia and up to 63 percent in 
Ghana and Sierra Leone (FAO, 2020). It's one of the most important components in the 
food of poor communities because it is often the cheapest means of getting animal 
protein in the world (Devries and Phillips, 2015). 
1 
 
 
Within Sub-Saharan Africa, the fishery industry directly provides work for about three 
(3) million populace and other extra 7.5 million populace found in fish processing and 
trading sectors (Costello et al., 2016, FAO, 2020). Furthermore, it’s expected that the 
recent yearly income from capture fishery would increase to about US$2 billion which 
will generate a multiplier effect of 2.5 times (US$5 billion) throughout Africa (Costello 
et al., 2016). 
Ghana has valuable fisheries that produced about US$ 1 billion revenue in the year 2011 
to 2016 and accounted for about 5% of Agriculture Gross Domestic Production (AGDP), 
according to Fisheries and Aquaculture Sector Development Plan (FASDP, 2015). 
The tuna fishery is a highly commercial fishing sector that uses industrial fishing vessels 
with either purse seine or baited hooks to catch tuna and tuna-like species (Thunnus 
albacares, Thunnus obesus, and Katsuwonus pelamis). These species are very essential 
and most significant food sources for humans (Bayliff, 2016). In 2017, Tuna landings 
were projected at 100,000 metric tonnes yearly by the Fisheries Commission of Ghana. 
However, the current tuna landings have been much lower than the expected threshold 
(Kwadjosse et al., 2018).   
 
The overall demand for tuna products has been increasing gradually since the 1980s 
(Hassan, 1997). Tuna fishery is a very essential seafood in terms of volume and value of 
catch landings with respect to the three most important tuna species (Thunnus albacares, 
Thunnus obesus, and Katsuwonus pelamis) which accounted for 20.93 % of the overall 
marine fisheries production in 2008 (Addi, 2014). A reduction in tuna stock will, 
2 
 
 
therefore, have a great effect on the protein supply of consumers (Evans et al., 2011). In 
general, Ghana's tuna production has been erratic due to a decline in catches year after 
year (FAO, 2014). 
Fish continues to be the anchor of the country's source of protein intake. In Ghana it's 
estimated to be about 60% of all diet consumption and also holds sufficient potential for 
job creation (Kwadjosse, 2009). The study of food composition in the fish diet plays an 
important role in fisheries management. The examination of food and feeding habits of 
fish has usually been a significant area of study in Fish Biology. However, it is very 
difficult to compare the outcome with research from other areas (FAO, 1974). Food and 
feeding habits of fish cannot be measured in segregation nevertheless; it has to be 
deliberated in relation to the entire aquatic ecosystem, in which fish is a single 
component of the ecosystem (Mahesh et al., 2018; Setyadji, et al., 2017; Sivadas and 
Anasukoya, 1999). 
 
These species serve as a key source of protein, especially to most coastal communities 
found in Ghana. However, converting them to fish meal is also a common practice among 
fish feed manufacturers in Ghana (Addi, 2014; Abdul Hakim et al., 2018). In Ghana, 
these species are often caught and classified as high- target export fishery due to its 
market value in the international communities. The research focused on these three (3) 
common tuna species namely, Thunnus obesus, Katsuwonus pelamis, and Thunnus 
albacares. 
 
3 
 
 
Thunnus obesus (Bigeye tuna) is a species of true tuna in the genus Thunnus, belonging 
to the family Scombridae.  Thunnus obesus are found in open waters of temperate and 
tropical Oceans but are not found in the Mediterranean Sea (Addi, 2014). They are large 
pelagic species with deep bodies. They are streamlined fish with a large head and big 
eyes. The pectoral fins are very elongated to the back beyond the start of the second 
dorsal fin in juveniles and space between the first and second dorsal fins in adults 
(Carpenter et al., 2014). They also have between 13 to 14 dorsal spines. Thunnus obesus 
has a distinctive physiological characteristic that permits them to feed in deep colder 
water and tolerate water with poor oxygen content (Dizon and Brill, 1979).   
Thunnus obesus are observed to stand oxygen level at 1.0 ml/L and usually attain depths 
of despair ambient oxygen content below 1.5 ml/L (Holland et al., 1992; Magnuson, 
1973). This is mainly due to the existence of blood with high oxygen content (Magnuson, 
1973; Dizon and Brill, 1979). The vascular counter of the existing heat exchanger 
maintains the body temperature above the optimum water temperature (Aoki et al., 2020; 
Carey and Lawson, 1973; Dizon and Brill, 1979). This heat exchanger is affianced to 
preserve heat in deep colder waters that are disconnected to permit fast warming as the 
species rise from cold water into warmer surface waters (Holland et al., 1992). This 
provides short latency and physiological thermoregulation of the species (Carey and 
Lawson, 1973; Aoki et al., 2020; Dizon and Brill, 1979). The eyes of Thunnus obesus is 
well modified with a huge sphere-shaped lens that permits them to see very clearly in a 
low light environment (Holland et al., 1992; Dizon and Brill, 1979). Thunnus obesus 
carry out different diel movement in upright manners, normally downward movement to 
deep, cool water and recurring to shallow, warmer water at sunset (Aoki et al., 2020; 
4 
 
 
Dizon and Brill, 1979). In the day time, Thunnus obesus embark on an upright shift into 
waters of about 300m – 500 m downward and can be 20 °C cooler than surface waters 
(Holland et al., 1992). The individuals embark on thermoregulatory behaviour even as 
depth, occasionally recurring from deep, cool water to shallow, warmer waters to rewarm 
(Dizon and Brill, 1979; Carey and Lawson, 1973). The typical vertical behaviour of 
Thunnus obesus movement occurs when they are connected with buoys, seamounts, and 
FAD, with those left behind the surface of waters (Holland et al., 1992). 
Thunnus obesus normally grow up to a maximum length of 250 centimeters (100 inches) 
or 8 feet. The greatest weight of individuals Thunnus obesus is about 180 kg (Carpenter 
et al., 2014). 
 
Thunnus albacares (Yellowfin tuna) belongs to a family Scombridae. These species are 
mostly found in the surface of waters of subtropical and tropical Oceans globally (Aoki et 
al., 2020). For Thunnus albacares, the anal fin and the second dorsal fin in addition to the 
finlets, between the fins and the tail are bright yellow, which accounts for the common 
name (Yellowfin tuna). The anal fin and second dorsal are very elongated in a full-grown 
species, which are prolonged to almost the tail of the fish giving it the appearance of 
scimitars or sickles shape (Carpenter et al., 2014; Aoki et al., 2020; Perera et al., 2015). 
They have long and wilder pectoral fins. 
Thunnus albacares is one of the large tuna species found in the world and has a 
maximum weight of over 180 kg. They are extensively smaller than Atlantic and Pacific 
bluefin tuna, which weight more than 450 kg (Aoki et al., 2020; Dizon and Brill, 1979;). 
5 
 
 
Thunnus albacares are also somewhat smaller than Thunnus obesus (Addi, 2014). The 
body of Thunnus albacares is a very dark metallic blue, varying to silver on the belly. 
They have about 20 vertical lines on the skin (Aoki et al., 2020). 
 
Thunnus albacares normally travel in schools within the same size cohort. They 
occasionally move with other tuna species and mixed together with other schools of 
pelagic species such as barracuda, trigger fish and blue runner in a particular area. They 
are frequently linked themself with various species of porpoises or dolphins, and with 
bigger marine mammals like sharks and whales. They are also known for chasing floating 
objects such as FAD, pallets, and logs. Sonic tagging shows that, some Thunnus 
albacares like chasing moving vessels (Perera et al., 2015). 
 
Katsuwonus pelamis (Skipjack tuna) is an average size perciform species belonging to the 
family, Scombridae. They are also called mush mouth, victor fish, oceanic bonito, aku, 
balaya, arctic bonito, and striped tuna. It can grow up to a length of about 3 ft (1 m) long 
(Carpenter et al., 2014). They are multinational tuna species living in warm temperate 
and tropical waters. They are significant pelagic species in the world.  
Katsuwonus pelamis are fast-swimming pelagic species with a streamlined body, 
common in tropical Oceans all over the globe (Collette et al., 2011). They inhabit the 
surface of waters in large shoals. They are very essential food source for large surface-
dwelling pelagic species and sharks and are frequently used as baitfish especially for 
Marlins (Collette et al., 2011). They are non-scales fish species, apart from the lateral line 
6 
 
 
and on the corselet. The corselets are broad scales forming a ring around the body at the 
back of the head.  
The fork length of Katsuwonus pelamis is about 80 cm and can weigh between eight 
(8kg) to ten (10 kg). The maximum total length for Katsuwonus pelamis is about 108 cm 
and the maximum weight is up to 34.5 kg. The lifespan of Katsuwonus pelamis ranges 
between 8 to 12 years (Collette et al., 2011).They are species that spawn in batches. The 
reproduction occurs throughout the year in equatorial waters. Normally, the fork length of 
the first batch reproduced is around 45cm. Katsuwonus pelamis is also known for its 
potent smell (Carpenter et al., 2014; Collette et al., 2011). 
 
1.2 Problem Statement and Justification 
Fish is the most important source of protein for human beings and offer opportunities for 
people to be employed in the fisheries sector.  The fisheries sector also contributes 
significantly to the development of the economy. On the other hand, with a better 
understanding of fisheries resource management, it has been realized that aquatic 
resources are decreasing rapidly. Although they are renewable resources but need to be 
sustainably managed. Due to the role it plays in the nutrition, economic, and social well-
being of the increasing populace, fisheries resources need to be sustained for future 
generations (Kwadjosse, 2009). 
According to the Fisheries Commission of Ghana tuna stocks  have gradually declined 
over the years. However, studying the food and feeding habit of these three most 
important Tuna species in Ghana could help Fisheries Managers to plan and manage the 
7 
 
 
resource better. The excessive utilization of these fisheries resources without scientific 
research on their feeding habit would endanger the long term sustainability of the tuna 
species.  
 
Studying food and feeding habits of fish species and other organisms is based on the 
investigation of the stomach content and this has become the accepted protocol to assess 
fish feeding habits (Hyslop 1980). The analysis of the stomach content of fish gives an 
imperative understanding of fish feeding patterns in an ecosystem. The quantitative 
assessment of food composition as well as feeding habits of fishes is vital in fisheries 
management and ecological studies of fish. Generally, feeding habits of fish present the 
basis to understand the trophic relations in the marine environment. Feeding habits of fish 
signify a combination of many essential ecosystem mechanisms which encompasses 
weather conditions, behaviour, habitat use, intra/inter specific interactions, and energy 
intake (Mahesh, 2019). 
 
Tunas (Katsuwonus pelamis, Thunnus albacares, and Thunnus obesus) are considered as 
important commercial pelagic species found in Ghanaian coastal waters. Despite its high 
abundance and the economic value of these species in Ghanaian coastal waters, little or 
no information is known about the food and feeding habit of these important fish species. 
This research was conducted to increase the understanding of the food and feeding habit 
of these three Tuna species.  Research has indicated that studying the trophic ecology of 
fishes is very essential to appreciate the functional role of fishes within an aquatic 
8 
 
 
ecosystem (Bahou et al., 2007; Blaber, 1997; Cruz-Escalona et al., 2000; Hajisamae et 
al., 2003). 
 
1.3 Objectives 
1.3.1 Main Objectives: 
The main objective of the study is to investigate food and feeding habits of the three most 
common tuna species (Katsuwonus pelamis, Thunnus obesus, and Thunnus albacares) in 
Ghana. 
 
1.3.2 Specific Objectives: 
 
The specific objective of the study is to determine; 
• The frequency of occurrence of prey items in the stomach of the three most 
common tuna species (Katsuwonus pelamis, Thunnus obesus, and Thunnus 
albacares). 
• The percent composition by Number and Weight of prey items in the stomach of 
the three most common tuna species (Katsuwonus pelamis,Thunnus obesus, and 
Thunnus albacares) 
•  Which food type is most important and preferred by the three most common tuna 
species (Katsuwonus pelamis, Thunnus obesus, and Thunnus albacares). 
 
9 
 
 
 
CHAPTER TWO 
LITERATURE REVIEW 
 
2.1 State of the World Fisheries 
Over the years Fisheries Managers have been facing a series of challenges in the fisheries 
sector until The Code of Conduct for Responsible Fisheries (CCRF) was adopted to 
ensure sustainable utilization of fisheries resources. The Code was introduced to assist in 
the improvement of intercontinental mechanisms, guidelines, and programs toward 
conscientious administration of fisheries nationally, regionally and globally. The pains in 
adopting the Code of Conduct have been more solid and important since its introduction, 
to mainly deal with challenges in a rational and harmonized manner (FAO, 2020). This is 
to safeguard the use of marine resources, Seas, and Oceans for sustainable growth (FAO, 
2018).  
 
A 10-year study putting together over 50 organizations and 400 researchers concluded that 
the total marine fisheries landing are 50 percent higher than the reported landing and 
fisheries resources are declining due to improper administration of the resources (FAO, 
2014). The overall catches of the marine fishery are higher than in the past reports, 
contributing more to the local and worldwide food security. 
 
The overall fish landing is expected to have reached about 179 million tonnes in 2018 of 
the overall total catch. 156 million tonnes of fish were consumed by humans as food which 
10 
 
 
is equal to the expected yearly contribution of 20.5 kg per capita (FAO, 2020). In 2018, the 
overall worldwide capture fishery production reaches the maximum point ever documented 
at 96.4 million tonnes increment, of about 5.4% from the average in the past three (3) years 
(FAO, 2020; ICCAT, 2003).  The first 7 countries with high landings of capture fisheries 
include India, Russian Federation, Vietnam, United States of America, China, Indonesia, 
and Peru contributing about 50% of global landings (ICCAT, 2003). 
Landings of Engraulis ringens by Chile and Peru contributes the majority of the increment 
of landings in 2018, following comparatively small landings for this fish species in current 
years, making it the highest fish species of over 7.0 million tonnes.  
Theragrach alcogramma was also the second, of about 3.4 million tonnes, whereas 
Katsuwonus pelamis was graded the third, for nine successive years at 3.2 million tonnes 
(FAO, 2020).  
Finfishes represent 85% of the entire ocean capture fisheries production. The smaller 
surface-dwelling species were the major groups, followed by tuna and tuna-like species as 
well as Gadiformes. The catch of tuna and tuna-like species persist to increase year after 
year, reaching the peak levels in 2018 with more than 7.9 million tonnes (FAO, 2020). On 
the quantity of catch, 78.7% of the recent catch was obtained from biologically sustainable 
resources. In 2017, the under-fished resources contributed about 6.2% and the highest 
sustainably fished resources also contributed about 59.6% of the entire number of assessed 
resources (FAO, 2020).  
In 2018, about 88% of the entire 179 million tonnes of overall fish landing was directly 
consumed by humans, whereas the leftover of 12% was utilized for non-consumption 
purposes (ICCAT, 2003). 
11 
 
 
 
In 2017, fisheries contributed about 17 percent of the overall population ingestion of 
animal protein. People are eating more fish than ever before due to increasing production, 
rising incomes, and increased awareness of the health benefits associated with fish 
consumption (Bene et al., 2010). However, FAO has predicted that due to Africa’s 
projected population growth, per capita fish utilization is estimated to increase, raising 
concerns of food security (Bene et al., 2010; FAO, 2020). 
 
FAO has estimated that 2.9 billion people depend on fish for a substantial component of 
their animal protein. In some countries in Africa and Asia, fish provide more than half of 
their animal protein supply (Kosamu, 2015; FAO, 2014). FAO again has estimated that the 
contribution of marine fisheries to food security may even be higher than before in a lot of 
developing countries (Evans et al., 2011; Kosamu, 2015). 
 
In Ghana, the fisheries industry supports livelihoods of about 10% of the population and 
significantly contributes about 60% of the country's protein intake.  Also, about $87 
million of fisheries resources were exported in 2009 (Samey, 2015). Fish and other seafood 
contribute about 16% of the overall household expenditure on food for an average 
Ghanaian (Ghana Statistical Service, 2014). 
 
2.2 Global tuna production and management 
Tunas have been very significant to human beings over the years. There is an 
archaeological proof that early people harvested this species over six thousand (6,000) 
12 
 
 
years ago, tuna products were the first manufactured fisheries commodities traded among 
the early people (Nootmorn et al., 2008). Catches between 1975 and 1992 increased 
significantly for tuna species throughout the world especially for Thunnus obesus, 
Katsuwonus pelamis, and Thunnus albacares. On the other hand, there were times of 
rapid decrease in tuna production.  From 1991 to 1996 tuna landings continued to be 
reasonably stable, between 3.1 and 3.2 million metric tonnes. Within 1997 tuna 
production (landing) hit 3.4 million tonnes and has sustained the rise year after year, to 
reach 3.9 million tonnes in 1999 (Hasarangi et al., 2012). 
 
The increased tuna production that occurred from 1970 to 1978 was as a result of the 
extension of fishing effort in eastern Atlantic Ocean and the improvement in the latest 
offshore fishing grounds in eastern Pacific (FAO, 2015). The few years of modest rise in 
global tuna productions were attributed to the discovery of novel fishing areas. From the 
year, 1996 to 1999 the tuna landings were augmented by 19%, owing to enhancement and 
improvement in the use of FAD (FAO, 2015). 
 
Tunas are highly migratory species and form part of significant fish species listed in 
United Nations' Convention on the Law of the Sea (UNCLOS) Annex I. Article 64 of the 
Convention states that, the administration of extremely migratory fish species needs 
collaborative approach among coastal countries in the world (FAO, 2015).  Due to the 
high commercial value of tuna species throughout the world, several global 
organizations, as well as scientific research organizations, NGOs and resource protection 
groups such as US National Oceanic and Atmospheric Administration (NOAA), 
13 
 
 
International Seafood Sustainability Foundation (ISSF), and Pew Environmental Group, 
are helping to manage these resources sustainably. These organizations play essential 
roles in the administration of tuna fishery in the world (Pons et al., 2017). 
 
The Regional Fisheries Management Organization (RFMO) has the management powers 
to put in place technical measures, catch and fishing effort limits, and control 
responsibilities over fish productions. Tunas are essential economic commodities and a 
primarily important source of food to human beings. The yearly landings of these tuna 
species have increased from 1.6 million tonnes in 1950 to over 6 million tonnes in 2003 
(ICCAT, 2003; FAO, 2014). 
 
Furthermore, because the fisheries sector gives a direct job to over 200 million populace 
in the world (Dunn et al., 2016), and contributes about 19 percent of the overall animal 
protein consumption by human beings, it needs to be properly managed (Chalamaiah et 
al., 2012). The collapse and decline of these fish species would possibly have an 
extremely social and economic impact on some fishery reliant regions of the world 
(Howarth et al., 2014). 
According to the International Seafood Sustainability Foundation, the fishing pressure on 
tuna species has increased greatly on the global scale indicating an overexploitation of 
these resources. Hence, it is necessary that mechanisms be put in place to preserve these 
fish species (ISSF, 2012). The degree of exploitation should be measured to ensure that 
tuna populations are maintained at preferred levels of abundance (ISSF, 2012). 
14 
 
 
According to Regional Fisheries Management Organization, tuna stocks are approaching 
or exceeding their full exploitation level (RFMO, 2014). Yield per recruit (Y/R) is 
reducing due to increasing juvenile catches. Competition among gear types has been 
accentuated and the global fishing capacity has risen (Miyake et al., 2010). 
 
Water temperature also has a high physiological consequence on tuna species, as it 
affects their cardiac function, swimming abilities, spawning activity, egg hatching, and 
larval growth (Dizon et al., 1977). On the other hand, tolerances in water temperature 
vary widely among tuna species. With respect to Ocean warming, the marine populations 
are inclined to travel in the direction of the poles in the deeper Oceans (Anker-Nilssen et 
al., 2008). Hence, the reflection is as a result of increases in warmer water fish species in 
several higher latitude regions. The change in the biogeography and ecology of marine 
fishery may be valuable signs of climate change (Sharp, 1978). 
 
Tropical species like Thunnus albacares and Katsuwonus pelamis are found in water with 
temperature ranges between 30-32°C and an ambient temperature greater than16°C 
(Boyce et al., 2008). On the other hand, Thunnus thynnus (Atlantic bluefin tuna) feed 
extensively in water with a temperature that is less than 10°C. They may be 
physiologically stressed in temperature greater than 28-29°C (Block et al., 2005). Sea 
surface temperature (SST) is a target area of several studies in order to explain the 
oceanic environment and tuna species distribution (Block et al., 2005).  
15 
 
 
The thermal limits for Thunnus albacares range from 18°C to 31°C with an optimum 
temperature between 21°C and 24°C. For Katsuwonus pelamis the thermal limits ranges 
from 17°C to 28°C with an optimum temperature between 20°C and 22°C and also for 
Thunnus obesus the thermal limits interval is from   11°C to 28°C with optimum 
temperature between 18°C and 22°C (Musyl et al., 2003). Lehodey et al. (1998) pointed 
out that, the greatest profusion for Thunnus albacares is between 20°C and 30°C, for 
Katsuwonus pelamis is between 20°C and 29°C and for Thunnus obesus, the greatest 
profusion depends on fishing gear used.  
 
The estimation of climate change impacts on highly migratory fish species is difficult to 
access due to differences in life stages of the fishery. The ability of tuna to tolerate warm 
water frequently reduces with respect to the size and age of the fish, whereas the ability 
of tuna to tolerate cold water may rise, depending on their physiological distinctiveness 
(Barkley et al., 1978). Due to the extensive distribution of tuna fishery and their reliance 
on the most favorable water temperature, their populations may be an excellent indicator 
of the causes of climate change on worldwide fisheries. 
External reasons such as salinity, oxygen content, ocean fronts, prey accessibility, 
islands, zooplankton, seamount as well as the existence of other organisms including sea 
birds and porpoises are very important indicators. These external factors are assumed to 
correlate with the abundance and distribution of these species (Worm et al., 2006). Sea 
surface temperature has continually been used as the best way to predict species density 
and richness (Worm et al., 2006). This indicates that ambient water temperature may be 
16 
 
 
key to oceanographic variable motivating distribution and abundance of tunas and 
billfishes on a global scale. Although information about the comprehensive distribution 
pattern for individual species of tuna, has highly been developed in recent years such as 
tuna tagging studies (Block et al., 2001). The community of fish migration patterns on 
abundance and richness remains poorly known (Worm et al., 2006). The information on 
patterns and oceanographic factors (temperature) that drives species is important to 
effectively conserve and manage fish populations comprehensively (Worm et al., 2006). 
 
2.3 Tuna production in Ghana 
The tuna fisheries are very essential marine fisheries with respect to the volume and value 
of the catch. In Ghana, commercial tuna fishery started in 1962 (Kwadjosse, 2009). 
International Commission for the Conservation of Atlantic Tuna (ICCAT) identified three 
commercial species of tuna fisheries in Ghana namely: T. albacares, T. obesus, and K. 
pelamis. Even though, the Ghanaian tuna fisheries using different types of gears, 
including drift gill net, pole and line, longline, poli/watsa, trolling, purse seines and bait 
boats but the most significant fleet used in the tuna fishery sector are purse seines and 
bait boats (Addi, 2014). Tuna fleets working in Ghana water are limited to catch fish in a 
depth of about 50-75m deep water. However, they go astray often into shallow waters to 
fish. The tuna industry has 24 active vessels landing in Tema and Takoradi (Fisheries 
Commission unpublished data, 2021). The Atlantic Ocean within the 200 nautical miles 
of the Exclusive Economic Zones (EEZs) in Ghana has a very important renewable 
resource and presents the best opportunity for economic development (Secretariat of the 
17 
 
 
Pacific Community, 2010). On a historical basis, the highest tuna landing in Ghana was 
in 2001 recording 88,806.49 tonnes of tuna production (Table 2.1).  
 
Table 2.1: Tuna landing (in metric tonnes) records in Ghana  
Year Yellowfin tuna Bigeye tuna Skipjack tuna Total catch 
1989 28883.3 16.4 23154.2 25053.9 
1990 7710.1 98 29498.6 37306.7 
1991 6628.4 138.3 28249.6 35016.3 
1992 6253.8 95.8 21336.3 27685.9 
1993 10646.8 - 22639.3 33286.1 
1994 7394.1 291.4 23863.3 31548.4 
1995 7119 4 22923 30046 
1996 12242.4 615.1 24284.7 37142.2 
1997 2324.6 27.5 24177.1 47454.2 
1998 19290.48 3920.54 41997.42 65208.44 
1999 28282.18 3680.11 51283.84 83246.13 
2000 15910 1651 34986 52547 
18 
 
 
Year Yellowfin tuna Bigeye tuna Skipjack tuna Total catch 
2001 29303.3 2357.23 56417.15 88077.68 
2002 20310.6 2033.9 38934.4 61278.9 
2003 19030.39 4815.81 32766.14 56612.34 
2004 15137.72 6943.56 33600.2 55681.48 
2005 19833.1 2333.2 54322.05 76488.35 
2006 14548.06 1590.29 42788.85 58927.2 
2007 15107.14 5748.15 46415.35 67270.64 
2008 14250 9269.2 37387.2 60906.4 
2009 18355 10554.4 36063.5 64972.9 
2010 12511 6768 53812 73091 
2011 14389.8 2110.2 34412 50912 
2012     
2013 13920.70 2963 48130.50 65014.2 
2014 18939 4175 50146 73260 
2015 19659 5917 62114 87690 
2016 20217.60 5193.60 54883.30 80294.5 
19 
 
 
Year Yellowfin tuna Bigeye tuna Skipjack tuna Total catch 
2017 20398.40 3837.70 57907.10 82143.2 
Source: Fisheries Commission unpublished data, 2021 
 
2.4 Food and feeding habits 
In fisheries resource management and fish ecology, information about food and feeding 
habit is very important in the decision-making process (Kido, 1996; Mahesh, 2019). Gut 
contents (Stomach contents) can be collected from both freshly died or live fish (Hyslop, 
1980). A research conducted by Lagler (1949) shows that gut content only indicates what 
fishes prefer to feed on in their habitat. 
 
Theoretically, studying the trophic relation of fisheries resource commences with food and 
feeding habits (Mahesh, 2019). Stomach content investigation can be used to assess prey 
selection, habitat preferences, and to develop conservation strategies for sustainable 
management of fisheries resource (Chipps and Garvey, 2007). According to Mahesh 
(2019) feeding habit is used as a scientific approach to determine the frequently consumed 
prey species by an organism. Feeding habit is also used to determine the relative 
importance of different food types to fish nutrition, and to quantify ingestion rate of 
individual food types (Mahesh, 2019). The method of evacuation or dissection and 
assessment of stomach contents is still the most preferred and easiest technique for 
biological/ecological studies in fisheries (Manko, 2016). 
 
20 
 
 
The taxonomic investigation of fish gut contents (Stomach contents) provides very 
important information about both pelagic and benthic community composition as well as 
trophic relationships between fish, benthos, and plankton (Mahesh, 2019). 
Fish gut content analysis is a method used for analyzing fish feeding habits through 
assessment of materials found in dissected fish gut. Fish gut contents are normally assessed 
in terms of mass and volume of prey items identified by taxonomists for their state of 
digestion (Chipps and Garvey, 2007). 
Fish feeding habits and ecology are highly variable among different fish species and can 
vary among developmental phases for a particular fish as well. Fish gut contents can 
consequently be used to identify differences in fish feeding approach, food accessibility, 
fish health, habitat relationship as well as to gather information concerning the trophic 
relationships in an aquatic environment.  
The investigation of fish gut contents is particularly significant to understand fish ecology 
and health, due to the difficulties associated with observing fish feeding habits in their 
natural habitats (Chipps and Garvey, 2007). Fish gut size can vary with the body size of 
the fish which required experience scientist to understand the dynamics fisheries resources 
(Hyslop, 1980; Lagler, 1949; Mahesh, 2019). 
 
Stomach content analyses are mostly using to study different aspects of feeding ecology of 
fishes. (Liao et al., 2001; Chipps and Garvey, 2007; Ahlbeck et al., 2012 ;Manko, 2016; 
Pillay, 1952; Hyslop, 1980; Cortes, 1997; Baker et al., 2014; Hansson, 1998;  Hynes, 
1950). The grading and comparing of prey items in feeding habit, scientist assume that 
some prey is more important than others to the survival, growth, recruitment, size 
21 
 
 
structure, condition, reproductive success, or other aspects of the ecology of the fishes, 
hence it is important to describe the true importance of food item in the stomach (Bowen, 
1996; Mahesh, 2019). 
Stomach content analysis can provide data which aids to doggedness more complex 
questions of fish ecology (Mahesh, 2019; Baker et al., 2014). Information obtain from 
stomach content analysis provides in-depth insight to fish feeding ecology, potential 
competition, resources availability and other aspects of fish biology and ecology, as well as 
predation (Liao et al., 2001; Mahesh, 2019). The individual food item from gut content can 
also provide exclusive information about relative importance of particular food item 
(Hynes, 1950; Pillay, 1952). 
 
2.5 Method used in stomach content analysis  
Fish feeding habitat are measured using different methods. Stomach contents analysis are 
generally categorized into quantitative and qualitative techniques (Mahesh, 2019). The 
qualitative analysis consists of a complete identification of the organisms in the stomach 
contents (Hyslop, 1980; Ahlbeck et al., 2012; Mahesh, 2019). However, the quantitative 
techniques of analysis are also categorized in to three types which includes, volumetric, 
gravimetric and numerical (Manko, 2016; Mahesh, 2019; Pillay, 1952). The numerical 
techniques are centred on the counts of essential food items in the stomach contents 
(Hansson, 1998; Liao et al., 2001). These techniques have been modified in different 
means to evaluate the relative importance of food items and these are classified under 
four different heads such as Dominance, Frequency of occurrence, Number, and Point 
methods (Manko, 2016; Hansson, 1998; Liao et al., 2001; Mahesh, 2019; Pillay, 1952). 
22 
 
 
Volumetric techniques are considered as a more satisfactory method by many researchers 
for quantitative analysis of stomach contents analysis (Mahesh, 2019; Chipps and 
Garvey, 2007). The volume of each food item or of the total food of each fish is given in 
this method (Mahesh, 2019; Chipps and Garvey, 2007). 
Gravimetric technique comprises of the estimation of the mass of each food item, or of 
the total food content of each fish, which is frequently expressed as a percentage of the 
total weight of the fish as in other quantitative techniques (Hansson, 1998; Hyslop, 1980). 
Stomach content can be expressed as wet, dry or ash-free dry weight (Hyslop, 1980). 
Food and feeding habits of fishes begin with the stomach content analysis and can be used 
to evaluate the habitat preferences, prey selection, effects of ontogeny and developing 
conservation policies for the fisheries sector (Mahesh, 2019; Chipps and Garvey, 2007). 
Stomach content analysis is used in many aspects of fish ecology on individual fish 
population and the ecosystem levels (Hynes, 1950; Mahesh, 2019; Pillay, 1952). It also 
helps us to explain specific problems of interactions, nature protection, speciation, 
evolution, invasions and fishery management (Hyslop, 1980; Ahlbeck et al., 2012; 
Mahesh, 2019). Gut content analysis can be integrated into a variety of different research 
objectives (Mahesh, 2019; Chipps and Garvey, 2007). Therefore, the stomach content is 
not only use to identify the diet but also provide source of information on many aspects of 
fish ecology and biology (Hynes, 1950; Hyslop, 1980; Pillay, 1952).   
Food and feeding habits of fish is based on direct investigation of gut content has become a 
standard practice for several years (Mahesh, 2019; Hyslop, 1980). In recent times, many 
other methodologies for instance stable isotope analysis, fatty acid analysis, radioisotopes 
and direct species observations are currently being used for stomach content analysis 
23 
 
 
(Braga et al., 2012; Mahesh, 2019). These methodologies are more accurate and can tell 
even the food items which cannot be identified by microscopic but can expensive and 
complicated procedures (Braga et al., 2012; Cortes, 1997). On the other hand, the stomach 
content analysis involves dissection or evacuation and investigation of gut contents is still 
the most used and easiest method for most ecological/biological studies (Manko, 2016; 
Braga et al., 2012). Other aspects such as prey availability, time of day, sampling location 
and the type of gear used to collect the fishes also needed to be considered before 
beginning a diet studies (Zacharia, 2017; Mahesh, 2019).  The analysis of stomach content 
gives accurate interpretation of fish feeding habits (Zacharia, 2017). 
In stomach content analysis live or dead fishes are normally used (Cortes, 1997). When 
fish samples are not analysed forthwith, fish should be preserved immediately freeze or 
fixing in formalin to avoid continued digestion of food contents (Chipps and Garvey, 
2007). Sampled fish should be measured to its total length to the nearest 1 mm and 
weight to the nearest 0.1 g, then a make a longitudinal cut on the ventral side of the fish 
from just behind the isthmus of the gills posterior to the anal fin (Nootmorn et al., 2008; 
Rohit et al., 2010; Buckland et al., 2017; Kido, 1996; Mahesh, 2019). Two transverse 
cuts at each end of the first cut to open the coelom to expose the viscera and record the 
maturity stage and sex of the fish sampled (Buckland et al., 2017). The digestive tract are 
separated from other visceral organs, then examine the degree of the stomach fullness and 
classify as ‘gorged, such as ‘full’, ‘3/4 full’, ‘1/2 full’ ¼ full, these are than by eye 
estimation and note down the weight to the nearest 0.1 g. (Rohit et al., 2010; Buckland et 
al., 2017).  
24 
 
 
The fishes which do not have a distinct gut, the first half of the intestine can be dissected 
and the contents are transferred into a petri dish for further analysis (Nootmorn et al., 
2008; Mahesh, 2019). However analysing formalin preserved fish samples, keep stomach 
contents out or in water on petri dishes for five (5) minutes to eliminate excess formalin 
in the samples (Hynes, 1950; Hyslop, 1980; Pillay, 1952). Analyse the stomach content 
to the genus and up to species level where it’s possible by identify the large prey item 
using the eye observation and examination of small prey items under binocular 
microscope (Perera et al., 2015; Setyadji. et al. 2012; Buckland et al., 2017; Kido, 1996). 
  
 
 
  
 
 
 
 
 
 
 
 
 
 
 
 
25 
 
 
 
CHAPTER THREE 
MATERIALS AND METHODS 
3.1 Study Area 
Ghana coast is divided into three zones using the length of the coastline namely: Eastern, 
Central, and Western coasts. The Volta and Greater Accra regions constitute the eastern 
coast being the shortest coastline of both regions; the Central region for the central coast 
and Western region constitute the western coast and also has the longest coastline in 
Ghana (Akyempon et al., 2014). 
The industrial tuna sector has only two landing sites, one in Tema in the Greater Accra 
Region and the other one in Takoradi in the Western Region. The country has 24 active 
vessels fishing within the Atlantic Ocean. Twelve (12) of these vessels are operating 
with pole and line and the other 12 are using purse seine. All the 12 pole and line vessels 
berth at Tema port. Out of the 12 purse seiners, 8 berths at Tema port and 4 berths at 
Takoradi port (Fisheries Commission data, 2021). Also, there are seven (7) non – 
Ghanaian registered vessels that discharge tuna in Tema port. Five (5) of these vessels 
are from Berlin and the other two (2) are from Liberia.   The various sampling stations 
(fishing position) were presented in the map below (Fig 3.1). 
 
 
 
26 
 
 
 
 
 
Figure 3.1: Map of Ghana showing the study area. 
 
 
 
 
 
 
27 
 
 
3.2 Field procedure 
 
From October 2019 to March 2020, data were collected onboard from three tuna fleets 
(Atlantic Queen, Panofi Frontier, and Marine 707).  The data were collected from 
different fishing grounds, off the Ghanaian coast, on stomach content analysis of the 
three most common species of tuna (Katsuwonus pelamis, Thunnus obesus, and Thunnus 
albacares). The samples were taken during the fishing activities of the vessel. After the 
samples were taken from the fisher folks onboard, a total of 10 samples of each species of 
tuna (Thunnus albacares, Katsuwonus pelamis, and Thunnus obesus) were randomly 
selected from the population.  
These samples were taken each month during fishing activities for six months, which 
started from October 2019 to March 2020. A total of 210 individual tuna species were 
sampled, out of which 90 were Katsuwonus pelamis, 60 Thunnus albacares, and 60 
Thunnus obesus respectively. The study sampled 90 specimens for Katsuwonus pelamis 
because they are of high abundance and very common in Ghanaian waters than Thunnus 
albacares and Thunnus obesus. Samples were preserved in iced chests with ice block 
placed on top of the samples in the chest to prevent it from going bad or spoiling. The 
samples were transported to the Fisheries Scientific Survey Division (FSSD) laboratory 
in Tema for analysis.    
 
 
 
 
28 
 
 
3.3 Laboratory procedure 
At the laboratory, total lengths, as well as fork length of the samples, were taken. The 
samples were measured to the nearest 0.1 cm using a 100 cm plastic fish measuring 
board.  The weights of samples were also taken using an electronic digital balance with 
0.01g accuracy (Abdul Hakim et al., 2018). 
The fish samples were dissected and their guts were removed for examination. Gut 
content analysis was conducted on each of the fish sampled after some morphometric 
data were collected on the field. The gut fullness was determined on a scale of 0/4 to 4/4. 
The 4/4 means full stomach, 3/4 implies a three-quarter full stomach, 2/4 indicates half-
full stomach, 1/4 also implies quarter full stomach, and 0/4 means empty stomach. This 
information was determined using eye estimation (Abdul Hakim et al., 2018). 
 
The contents of the stomach were emptied into petri dish and spread it out with a forceps 
for ease of identification. The individual food items were sorted and identified using FAO 
fish identification guides 2012, FishBase (Electric Media, 2020) research.calacademy.org 
(Electric Media, 2020), FAO Fishfinder, 2014 and World Register of Marine Species 
(Electric Media, 2020). 
The contents were observed under a binocular microscope which was magnified to about 
100X for further identification.  The individual food items were counted and the volume 
was determined using the displacement method. The prey occurrence was determined as 
a ratio of the number of the stomach in which each food item occurs over the total 
29 
 
 
number of stomachs containing food items (Abdul Hakim et al., 2018). The research 
identified individual food items to the species level where was possible.  
 
 
 
 
3.4 Data analysis 
The feeding habit of the three most common species of tuna (Katsuwonus pelamis, 
Thunnus obesus, and Thunnus albacares) were assessed using percent composition by 
number (N), percent composition by weight (W) model, and frequency of prey 
occurrence (O). The Index of Relative Importance (IRI) was also applied in the study. 
The following mathematical formulas were used to calculate the data taken from the 
samples. 
  
Percentage by Number,       …………………... (1) ∑
    
Where Ni is the total number of prey items i and Nt is the total number of all prey items 
found in the stomach of the fish (Hureau, 1970). 
  
Percentage by Weight,       ……………….…... (2) ∑   
  
Where Wi is the total weight of prey item i and Wt is the total weight of all prey items 
found in the stomach of the fish (Hyslop, 1980). 
  
Frequency of occurrence,          …………………...... (3) 
 
Where Ji is the number of fishes containing prey items i and P is the number of fishes 
with food in their stomach (Hyslop, 1980). 
30 
 
 
The Index of relative importance was also used to summarize and rate food items found 
in the stomach of these tuna species (Thunnus albacares, Katsuwonus pelamis, and 
Thunnus obesus) in the order of importance to the species. This is mathematically 
expressed as: 
Index of relative importance, IRI= (%N+%W) % FO……………..…. (4)  
Jaccard’s index was used to compare the prey item similarities between Thunnus 
albacares, Katsuwonus pelamis, and Thunnus obesus. The Mathematical formula for 
Jaccard’s Similarity index (Cj) was;  
Jaccard’s Similarity index, Cj=J/ (A+B-J)…....................................... (5) 
Where, Cj= Jaccard’s Similarity index, A= number of prey items found in the stomach of 
species one (1), B= number of prey items found in the stomach of species two (2), J= 
number of prey items found in the stomach of both species. The interpretation of values 
from result obtain using Jaccard’s Similarity index is important in ecological studies 
(Aikins et al., 2017). The following are used to explain Jaccard’s Similarity index results; 
If the value is < 0.25, then the similarity is very low (indicating a very high ß-diversity). 
If the value is between 0.25 - 0.50, then the similarity is moderate (indicating moderate 
diversity). 
If the value is between 0.50 - 0.75, then the similarity is high (indicating low ß-diversity). 
If the value is > 0.75, then the similarity is very high (very low ß-diversity). 
 
31 
 
 
Microsoft Excel was used to compile all the data collected on the species before the 
analysis was done. All the data were analyzed using Analysis of variances (ANOVA) and 
Tukey test for multiple comparisons of the prey items found in the stomachs of all the 
three tuna species (Thunnus albacares, Katsuwonus pelamis, and Thunnus obesus). 
GenStat (12th Edition) and SPSS (23rd Edition) were the statistical packages used in the 
data analysis. The degree of freedom used in the analysis for both ANOVA and the 
Tukey multiple comparisons test was 95%. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
  
32 
 
 
 
 
CHAPTER FOUR 
RESULTS 
 
4.1 Stomach content 
Table 4.1 shows the number of Thunnus albacares, Katsuwonus pelamis, and Thunnus 
obesus stomachs sampled. 
It was observed that the stomach contents consisted of cephalopods, fishes, molluscs, and 
crustaceans. In addition to these, some squid beaks particles were also found in the 
stomachs of these tuna species which were used in determining the prey item in the 
stomach. The percentage composition by the number of stomach contents shows that 
crustaceans formed 36%, fishes 53%, molluscs 11%, and unidentified 4% with respect to 
Thunnus albacares.  
 
 
 
 
 
 
 
33 
 
 
Table 4.1: Total number of stomachs sampled for the three tuna species   
 Empty Non-empty 
stomach stomach 
Species Number of sample Length (FLT) N % n % 
Skipjack tuna 90 46 – 67 9 10 81 90 
Yellowfin tuna 60 52 – 120 3 5 57 95 
Bigeye tuna 60 56 – 112 1 1.7 59 95 
 
 
4.2 Yellowfin tuna (Thunnus albacares) 
The size range of Thunnus albacares sampled for the study ranges from 52cm to 120 cm 
with an average fork length of 82 cm. In all the 60 Thunnus albacares stomachs 
examined, 3 samples were unfilled. The investigation was based on 57sampled stomachs 
that contained prey items. The Visual examination of the dissection of tuna stomach 
showed that the proportion of 4/4, 3/4, 2/4and 1/4 was 24.3%, 9.2%, 32.5%, and 29.1% 
respectively (Fig 4.1). The food items in the stomach formed 0.1 to 1.7% of the wet body 
weight of Thunnus albacares. The food items were grouped into four (4) categories 
namely; fishes, crustaceans, molluscs, and unidentified (Fig 4.2), fishes formed the 
majority (53%) of the diet of Thunnus albacares. 
34 
 
 
 
 
empty stomach, 
5.00% 
full stomach, 
24.30% 
quarter full 
stomach, 
29.00% 
three quarter 
full stomach, 
9.20% 
half full 
stomach, 
32.50% 
 
Figure 4.1: Proportion of stomach fullness for Thunnus albacares 
35 
 
 
60
50
40
30
20
10
0
Fishes Molluscs Crustaceans Unidentified
Series1 53 11 36 4
Group of prey items identified  
 
Figure 4.2: Prey species composition in the stomach of Thunnus albacares 
 
 
The diversity in the prey composition was 15 families of fishes, crustaceans, molluscs, 
and other unidentified species which were observed during the research (Table 4.2).  
Fishes were the most dominant food item in the gut content (53%) followed by 
crustaceans (36%) and molluscs (11%). The partially digested unidentified food content 
36 
 
Number of prey items identified 
 
comprises 4%. Fish dominated the food by occurrence (58 %), and crustaceans by 
number (40%). Table 4.2 shows that Engraulis encrasicolus, Exocoetus spp, and Penaeus 
kerathurus were the important prey species found in the gut of Thunnus albacares. 
Engraulis encrasicolus, the most important and most preferred prey item to the Thunnus 
albacares. The Sepia hierredda is a medium important food item found in the feeding 
habit of Thunnus albacares. Boops boops and Diplodus spp was less important prey item 
consumed by Thunnus albacares. 
 
Index of Relative Importance (IRI), the main prey item for Thunnus albacares was fishes 
(69.47%) where Engraulis encrasicolus was the most preferred and important food item 
representing 57.9 % of the entire occurrence of the prey items with IRI of 37.6% (Table 
4. 2).  The additional important food items by occurrence were juveniles and adults of 
Exocoetus spp, Caranx crysos, Sardinella spp, Pagellus spp, and Belone belone. The 
occurrence of juvenile species of Atherina spp, Scomber spp, Diplodus spp, and Boops 
boops as food content was marginal. Crustaceans were also abundant prey in the gut of 
Thunnus albacares.   
The Index of Relative Importance of the total crustaceans analyzed was 16.2%. Although 
Penaeus kerathurus was more numerically abundant (40.4%) but Metapenaeus 
monoceros was less important in the diet forming 10.5% of entire prey wet weight with 
an IRI of 14.5. The Sepia hierredda and Loligo spp represented the molluscs prey item in 
the stomach with an IRI of 12.2%. These food items are presented in Table 4.2.  
 
37 
 
 
Table 4.2 shows the results obtained from the analysis of Thunnus albacares gut 
contents. A total of 1361 prey items belonging to 15 families were identified and 67 prey 
items were unidentified (partially digested and unknown organisms), which consist of 8 
families of fish, 2 families of molluscs, and 5 families of crustaceans. 
                                  
 
Table 4.2: Results of stomach content analysis for Thunnus albacares 
Group  Family Name of prey Prey Prey Prey Prey Frequency Index of 
species number number weight weight of Relative 
(%) (g) (%) occurrence Importance 
(%) (IRI) (%) 
Fish  Engraulidae Engraulis 312 22.9 4351.7 28.2 57.9 37.59 
encrasicolus 
Sparidae Boops boops 3 0.2 76.1 0.5 3.5 0.03 
Diplodus spp 2 0.1 64.2 0.5 1.8 0.01 
Pagellus spp 18 1.3 234.6 1.5 14.0 0.49 
Scombridae Scomber spp 5 0.4 19.8 0.1 5.3 0.03 
38 
 
 
Group  Family Name of prey Prey Prey Prey Prey Frequency Index of 
species number number weight weight of Relative 
(%) (g) (%) occurrence Importance 
(%) (IRI) (%) 
Clupeidea Sardinella aurita 28 2.1 412.3 2.7 24.6 1.50 
Atherinidae Atherina spp 3 0.2 69.2 0.4 3.5 2.67 
Belonidae Belone belone 6 0.4 382.6 2.5 7.0 0.26 
Exocoetidae Exocoetus spp 270 19.8 3457 22.4 49.1 26.36 
Carangidae Caranx crysos 82 6.0 36.4 0.2 29.8 2.35 
Molluscs Sepiidae Sepia hierredda 120 8.8 2876.4 18.6 35.1 12.22 
Loliginidae Loligo spp 137 10.1 90.3 0.6 8.8 1.19 
Crustaceans  Penaeidae Penaeus kerathurus 147 10.8 2704.7 17.5 40.4 14.53 
Metapenaeus 28 2.1 212.6 1.4 10.5 0.47 
monoceros 
39 
 
 
Group  Family Name of prey Prey Prey Prey Prey Frequency Index of 
species number number weight weight of Relative 
(%) (g) (%) occurrence Importance 
(%) (IRI) (%) 
Squillidae Squilla mantis 134 9.8 134.2 0.9 8.8 1.19 
Isopoda  38 2.8 67.8 0.4 7.0 0.28 
Decapoda  6 0.4 17.6 0.1 10.5 0.07 
 Brachyuran spp 18 1.3 5.2 0.0 7.0 0.12 
Unidentified    67 4.9 211.8 1.4 15.8 1.26 
 
 
4.3 Skipjack tuna (Katsuwonus pelamis) 
The fork length of Katsuwonus pelamis measured during the studies ranged from 46 to 67 
cm. Out of 90 stomachs sampled, only 9 samples were empty, whereas the remaining 81 
samples contain at least one prey item in the stomach. The investigation was based on 81 
guts containing food items. The stomach content analysis conducted on Katsuwonus 
pelamis identified four (4) groups of prey species, namely molluscs, fishes, crustaceans, 
and unidentified. The partially digested prey items are made up of fish, crustacean, and 
molluscs. A total of 925 food items were encounter, belonging to 7 families and 26 prey 
40 
 
 
items were partially digested (unidentified), which comprise 4 families of fish, 2 families 
of molluscs, and 1 family of crustaceans.  
 
The observations made on the dissected Katsuwonus pelamis stomach indicates that 
proportion of stomach fullness were full (4/4), three-fourth full (3/4), half full (2/4) and 
one-fourth full (1/4) was 33%, 28%, 21%, and 8% respectively (Fig 4.3). The prey 
contents were found to be 0.1 to 1.5% of the wet body weight. The food items were 
grouped into four (4) categories namely; fishes, crustaceans, molluscs, and unidentified 
(Fig 4.4).  
 
Crustaceans were the most dominant food item in the gut content (54.7%) followed by 
fishes (31.5%) and molluscs (11%). The partially digested (unidentified) prey items are 
made up of 2.8% of total prey items found in the stomach (Fig 4.4). 
 
 
41 
 
 
empty 
10% 
one-fourth full  
8% full 
33% 
half full  
21% 
three-fourth full  
28% 
 
Figure 4.3: Proportion of stomach fullness for K. pelamis 
 
42 
 
 
Unidentified, 
2.8 
Fishes, 31.5 
Crustaceans, 
54.7 
Molluscs, 11 
Fishes Molluscs Crustaceans Unidentified
 
Figure 4.4: Major prey groups in the stomach Katsuwonus pelamis 
 
 
Table 4.3 shows that Crustaceans dominate the diet by occurrence (57%), followed by 
fish by representing (42%) in terms of prey items groupings.  From the results presented 
43 
 
 
in (Table 4.3) shows that both juvenile and adult of Penaeus kerathurus, Engraulis 
encrasicolus, Brachyuran spp, and Mackerel spp were the preferred and important prey 
species found in diets of Katsuwonus pelamis. The Sardinella spp, Loligo spp as well as 
Lancet fish were also moderate preferred prey items found in diets of Katsuwonus 
pelamis. Lobster and Sepia hierredda are less preferred food items for this species.  
 
For Index of Relative Importance (IRI), the key prey item identified in the stomach of 
Katsuwonus pelamis was crustaceans (42%), of which the majority of this food item is 
made up of Penaeus kerathurus. Penaeus kerathurus were the most important and 
preferred prey item representing 57 % of a total occurrence of the prey items with IRI of 
39% (Table 4.3).  The other essential food items by occurrence were Brachyuran spp and 
Lobster. The occurrence of these two prey species as food items in the stomach of 
Katsuwonus pelamis was marginal. Fishes were also abundant food items found in the 
stomachs of Katsuwonus pelamis. 
 
The Index of Relative Importance of the total fishes analyzed was 36.4%. Even though 
Engraulis encrasicolus were more numerically abundant (42%), Mackerel spp were 
important in the diets forming 37% of total food wet weight with an IRI of 22.5%. The 
Loligo  spp and Sepia hierredda represented the molluscs prey item in the stomach with 
an IRI of 6.3%. 
 
44 
 
 
 
 
Table 4.3: Results of the stomach content analysis for Katsuwonus pelamis 
Group  Family  Name of Prey Prey Prey Prey Frequency Index of 
prey number number weight weight of Relative 
species (n) (%) (g) (%) occurrence Importance 
(%) (IRI) (%) 
Crustacean Penaeidae Brachyuran 180 12.5 582 17.4 37.0 14.6 
spp 
 Penaeus 210 22.7 956 28.6 56.8 38.5 
kerathurus 
 Lobster 116 6.1 185 5.5 14.8 2.3 
Fish Engraulidae Engraulis 56 19.5 706 21.1 42.0 22.5 
encrasicolus 
Scombridae Mackerel 111 12.0 132 4.0 37.0 7.8 
spp 
Clupeidae Sardinella 46 5.0 79 2.4 34.6 3.4 
aurita 
Alepisauridae Lancetfish 78 8.4 112 3.4 17.3 2.7 
Molluscs Loliginidae Loligo spp 72 7.8 415 12.4 23.5 6.3 
45 
 
 
Group  Family  Name of Prey Prey Prey Prey Frequency Index of 
prey number number weight weight of Relative 
species (n) (%) (g) (%) occurrence Importance 
(%) (IRI) (%) 
Sepiidae Sepia 30 3.2 98 2.9 9.9 0.8 
hierredda 
Unidentified    26 2.8 75 2.2 16.0 1.1 
 
 
 
4.4 Bigeye tuna (Thunnus obesus) 
The size range of Thunnus obesus sampled during the studies ranged from 56 to 112 cm. 
The observed samples of all the 60 Thunnus obesus stomachs analyzed, one was empty 
while 59 stomachs were with food. The examination of the dissected Thunnus obesus 
showed the quantity of full stomach, three - fourth full stomach, half-full stomach, one - 
fourth full stomach and empty were 44%, 10%, 16%, 28%, 2% respectively (Fig 4. 5).  
 
The stomach content analysis conducted on Thunnus obesus identified four categories 
(groups) of food items, namely fishes, molluscs, crustaceans, and unidentified. The fully 
digested prey items were made up of fish, crustacean, and molluscs. Fishes were the most 
46 
 
 
dominant prey items found in the stomach content (75%), followed by Crustaceans (22%) 
and molluscs (3%). The unidentified prey item was 1%.  
 
 
2% 
28% 
44% 
16% 
10% 
full three fourth full half full one fourth full empty
 
 
Figure 4.5: Stomach fullness of Thunnus obesus 
 
 
Table 4.4, shows that the undigested prey items found in the stomach of Thunnus obesus 
were both adults and juveniles of Exocoetus spp, Engraulis encrasicolus, Sardinella 
47 
 
 
aurita, Caranx hippos, Decapterus spp, Brachyuran spp, Penaeus kerathurus, 
Metapenaeus monoceros, Loligo spp and other unidentified species. The most copious 
prey item observed in the gut of Thunnus obesus was Exocoetus spp followed by 
Engraulis encrasicolus. Metapenaeus monoceros was very low in abundance.  Based on 
the grouping of prey items observed in the stomach of Thunnus obesus, fishes were the 
most preferred food in their diet (75%), while Crustacean as the complementary food 
item (22%), Molluscs as the additional diet (3%) and unidentified fishes (1%). 
 
For Index of Relative Importance, the main food item identified in the gut of Thunnus 
obesus were fish species (83%), of which the majority of this food item is made up of 
Exocoetus spp. Exocoetus spp was more important and preferred prey items representing 
86% of the overall occurrence of the food items with IRI of 27% (Table 4.4).  The other 
important prey items by occurrence were Decapterus spp, sardinalla spp, Engraulis 
encrasicolus and Caranx hippos. The occurrence of this species as prey items in stomach 
content was marginal. Crustaceans were also abundant prey items observed in the 
stomach of Thunnus obesus.  
The Index of Relative Importance of the total crustaceans analyzed was 15%. Although 
Penaeus kerathurus was more numerically abundant (54%), Brachyuran spp, was less 
important in the diets making up to 42% of overall prey wet weight with an IRI of 13%. 
The Loligo spp also represents the molluscs prey item in the stomach sampled with an 
IRI of 2%. Table 4.4 shows the various prey items and their frequency of prey 
occurrence, IRI and other parameters.  
48 
 
 
 
 
Table 4.4: Results of stomach content analysis for Thunnus obesus 
Group  Family  Name of Prey Prey Prey Prey Frequenc Index of 
prey numbe numbe weigh weigh y of Relative 
species  r (n) r (%) t  (g)  t (%) occurrenc Importanc
e (%) e (IRI) 
(%) 
 Exocoetida Exocoetus 386 26.6 1236 17.0 86.4 26.9 
e spp 
 
Engraulida Engraulis 342 23.5 1012 13.9 67.8 18.1 
 
e encrasicolu
Fish  s 
Clupeidae Sardinella 52 3.6 1190 16.4 78.0 11.1 
aurita 
Carangidae Caranx 30 2.1 921 12.7 67.8 7.2 
hippos 
 
Decapterus 272 18.7 1146 15.8 81.4 20.0 
spp. 
Crustacea Penaeidae Brachyuran 28 1.9 382 5.3 42.4 2.2 
n  spp 
49 
 
 
Group  Family  Name of Prey Prey Prey Prey Frequenc Index of 
prey numbe numbe weigh weigh y of Relative 
species  r (n) r (%) t  (g)  t (%) occurrenc Importanc
e (%) e (IRI) 
(%) 
 Penaeus 282 19.4 986 13.6 54.2 12.8 
kerathurus 
 Metapenaeu 13 0.9 132 1.8 11.9 0.2 
s 
monoceros 
Molluscs Loliginidae Loligo spp 38 2.6 122 1.7 47.5 1.5 
Unidentifie   10 0.7 148 2.0 8.5 0.2 
d  
 
 
 
 
4.5 Prey Species Similarities 
Out of the twenty-three (23) prey species encountered during the research, nine (9) of 
them were shared prey species between the three tuna species at the study area. The 
shared species between the Thunnus obesus and Katsuwonus pelamis (8 species) was 
higher compared to the shared species between Thunnus albacares and Katsuwonus 
pelamis (5 species) as well as the Thunnus obesus and Thunnus albacares (Table 4.5). 
50 
 
 
However, from the qualitative Jaccard’s Similarity Index (CJ), the species similarity was 
moderate for Thunnus albacares and Katsuwonus pelamis (CJ =0.29). There was also a 
moderate species similarity (CJ=0.33) between Thunnus obesus and Thunnus albacares, 
this means that prey species detected in both tuna species were not different from each 
other. There was high similarity (CJ=0.52) between Thunnus obesus and Katsuwonus 
pelamis species, this indicates that the species detected in both Thunnus obesus and 
Katsuwonus pelamis are the same. Table 4.5 illustrate the calculated Jaccard's Similarity 
Index (CJ) for the three tuna species. 
 
Table 4.5 shows that, although these three tuna species belong to the same Family 
Scombridae but do not have the same preference, with respect to the type of food they 
feed on in their respective habitat. This implies that they feed on different prey items and 
have specific prey items that are most important and most preferred to a particular tuna 
species. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
51 
 
 
Table 4.5: Qualitative Jaccard’s species similarity indices for the three tuna species in the 
study area 
 
Tuna species Qualitative Jaccard’s Similarity 
index (shared species) 
Thunnus albacares and  
Katsuwonus pelamis 0.29 (5) 
 
Thunnus obesus and Thunnus  
albacares 0.33 (6) 
 
Thunnus obesus and 0.52 (8) 
Katsuwonus 
pelamis 
CJ=J/ (A+B-J) 
 
 
Table 4.6 shows that there is no significant difference among the three (3) tuna species 
with respect to Metapenaeus monoceros (0.509) species of prey item found in their 
stomachs. Since significance value is greater than (0.05) for the objective. 
On the other hand, there is a significant difference among these three tuna species with 
regard to Engraulis encrasicolus (0.000), Sardinella aurita (0.000), Exocoetus spp. 
(0.017), Caranx crisos (0.016), Sepia hierredda (0.002), Loligo spp. (0.004), Squilla 
mantis (0.009), Isopoda (0.006), and Brachyuran spp. (0.000) (Table 4.6). 
 
 
 
 
 
52 
 
 
 
 
 
Table 4.6: Analysis of variances (ANOVA) on prey items found in the stomach of the 
three tuna species 
 
Sum of df Mean F Sig. 
Squares Square 
Engraulis Between Groups 538.177 2 269.089 11.401 .000 
  
encrasicolus Within Groups 3091.882 131 23.602 
   
Total 3630.060 133 
Sardinella aurita Between Groups 16.109 2 8.055 12.593 .000 
  
Within Groups 49.891 78 .640 
   
Total 66.000 80 
Exocoetus spp. Between Groups 179.751 1 179.751 5.901 .017 
  
Within Groups 2802.207 92 30.459 
   
Total 2981.957 93 
Caranx crysos Between Groups 119.490 1 119.490 6.093 .016 
  
Within Groups 1431.630 73 19.611 
   
Total 1551.120 74 
Sepia hierredda Between Groups 41.287 1 41.287 10.882 .002 
  
Within Groups 223.860 59 3.794 
   
Total 265.148 60 
Loligo spp. Between Groups 49.173 2 24.586 5.792 .004 
  
Within Groups 390.512 92 4.245 
   
Total 439.684 94 
Metapenaeus Between Groups .056 1 .056 .444 .509 
  
monoceros Within Groups 4.250 34 .125 
   
Total 4.306 35 
Squilla mantis Between Groups 77.652 2 38.826 4.920 .009 
  
Within Groups 946.998 120 7.892 
   
Total 1024.650 122 
Isopoda Between Groups 33.469 1 33.469 7.921 .006 
  
Within Groups 321.146 76 4.226 
   
Total 354.615 77 
53 
 
 
 
Sum of df Mean F Sig. 
Squares Square 
Brachyuran spp. Between Groups 368.207 2 184.104 11.464 .000 
  
Within Groups 1927.159 120 16.060 
   
Total 2295.366 122 
 
 The mean difference is significant at the 0.05 level 
 
 
 
 
The result from table 4.7 show that, about 12 prey items were pelagic and 11 prey items 
were demersal species found in the stomach of the three tuna species. These prey items 
consumed by the tuna species are both adult and juvenile species of fish, crustacean and 
molluscs in the study area.       
 
 
Table 4.7: Prey items found in the stomach of the three tuna species and their habitat 
Tuna species Name of prey item Habitat of prey item 
T. albacares, K. pelamis and T. obesus Engraulis encrasicolus Pelagic species 
T. albacares Boops boops Demersal species 
T. albacares Diplodus spp Demersal species 
T. albacares Pagellus spp Demersal species 
T. albacares Scomber spp Pelagic species 
T. albacares, K. pelamis and T. obesus Sardinella aurita Pelagic species 
T. albacares, Atherina spp Pelagic species 
T. albacares Belone belone Pelagic species 
T. albacares and T. obesus Exocoetus spp Pelagic species 
54 
 
 
T. albacares Caranx crysos Pelagic species 
T. albacares, K. pelamis and T. obesus Sepia hierredda Demersal species 
T. albacares, K. pelamis and T. obesus Loligo spp Demersal species 
T. albacares, K. pelamis and T. obesus Penaeus kerathurus Demersal species 
T. obesus Metapenaeus monoceros Demersal species 
T. albacares Squilla mantis Demersal species 
T. albacares Isopoda Demersal species 
T. albacares Decapoda Demersal species 
T. albacares, K. pelamis and T. obesus Brachyuran spp Pelagic species 
 K. pelamis  Lobster Demersal species 
K. pelamis Mackerel spp Pelagic species 
K. pelamis Lancetfish Pelagic species  
T. obesus Caranx hippos Pelagic species 
T. obesus Decapterus spp. Pelagic species 
 
 
 
 
Appendix 3, shows, the significant difference among Thunnus albacares, Thunnus 
obesus, and Katsuwonus pelamis using Tukey test for multiple comparisons of prey items 
found in the stomach of these species. 
 
For Thunnus albacares and Katsuwonus pelamis, the significant level is (0.000). Since 
this significant level is less than the (0.05) value needed for mean significance, these two 
tuna species are significantly different with respect to Engraulis encrasicolus. 
55 
 
 
For Thunnus albacares and Thunnus obesus, the results indicate no significant difference 
(sig= 0. 814 which is greater than (0.05). After each of the three tuna species were 
compared with each other with regard to Engraulis encrasicolus, the final results show 
that the feeding habits of Thunnus albacares and Katsuwonus pelamis are significantly 
different from Thunnus obesus, but Thunnus albacares and Thunnus obesus are not 
significantly different from each other in terms of prey items consumed. 
 
 
 
 
 
 
 
 
 
 
 
 
 
56 
 
 
 
 
 
CHAPTER FIVE 
DISCUSSION 
 
5.1 Species Composition in the stomach of Thunnus albacares, Thunnus obesus and 
Katsuwonus pelamis 
 
The study sampled 210 individual tuna species, out of which 90 individuals were 
Katsuwonus pelamis, 60 individuals for Thunnus albacares as well as 60 individuals of 
Katsuwonus Thunnus obesus were collected for analysis. The study sampled 90 
specimens for Katsuwonus pelamis because they are of high abundance and very 
common in Ghanaian waters than Thunnus albacares and Thunnus obesus. This is in line 
with a research conducted in Eastern India Ocean by Setyadji et al. (2012) and 
Nootmorn, et al. (2007). Therefore, the study sampled high number of Katsuwonus 
pelamis as against the other two tuna species which are of low abundance.  
  
The diversity in the prey composition was about fifteen (15) families of fishes, 
crustaceans, molluscs and other unidentified species which indicate that, they are not 
selective feeders and that the feeding habit depends on prey availability rather than 
selectivity (Menard et al., 2006; Junior et al., 2003; Perera et al., 2015). The analysis of 
food composition and feeding habit of tuna species in the Atlantic Ocean (Ghana) as 
discovered from the stomach contents analysis indicated table 4.2, 4.3, and 4.4, showed 
that fishes, crustaceans and molluscs were the most important component of food items 
57 
 
 
found (Dissanayake et al., 2008; Prathibha Rohit et al., 2010; Perera et al., 2015; 
Nootmorn et al., 2008). 
The study identified diverse prey items in the gut of these three tuna species during the 
analysis. The diversity of prey items identified is in concordance with a research 
conducted by Aikins et al., (2017); Mahesh, (2019) and Mahesh et al. (2019), which 
indicates that the researcher's familiarity with the identification of stomach content can 
affects the prey numbers during analysis. 
 
The observations made from this study indicate that most of the tuna species were an 
opportunistic predator (Setyadji et al., 2012; Menard et al., 2006). The feeding on a 
multiplicity of suitable sized prey species, such as crustaceans and squids and this could 
be attributed to the size of the predator (tuna species) (Setyadji et al., 2012; Nakamura, 
1965). 
 
The food composition of three tuna species was dominated by a group of fishes as their 
main food items, crustaceans as complementary food item as well as molluscs as an 
additional food items (Perera et al., 2015; Setyadji. et al. 2012; Dissanayake et al., 2008). 
Similar result reported by Allain (2005a) in Western and Central Pacific for Thunnus 
obesus, Thunnus albacares and Katsuwonus pelamis where the important prey item found 
in the stomachs were fish, molluscs and crustaceans. According to research carried out by 
Setyadji et al. (2012) in Eastern Indian Ocean on Katsuwonus pelamis, Thunnus 
albacares and Thunnus obesus show that, the most important food items found in their 
58 
 
 
stomachs were crustaceans, fish and molluscs. The finding from this report is in line with 
the outcome of the studies carry out in Ghanaian waters.  
Setyadji et al. (2012) and Manko (2010), research studies indicates that fishes were the 
most important food items by weight for Thunnus albacares in equatorial zones of Indian 
Ocean. This is in support of the findings obtain form the study of food and feeding habits 
of the three tuna species investigated. 
 
5.2 Comparison of Species Composition found in Thunnus albacares, Thunnus 
obesus, and Katsuwonus pelamis 
 
There is no significant difference among the three tuna species with respect to 
Metapenaeus monoceros (0.509) species of prey item (Table 4.6) since the significant 
value is greater than the given significance level (0.05) for the research problem. The 
quantity of Metapenaeus monoceros consume is almost the same for all the three tuna 
species that’s Thunnus albacares, Thunnus obesus, and Katsuwonus pelamis. This can be 
attributed to the work of Nootmorn, et al. (2007) and Perera et al. (2015), which indicates 
that the quantity of this prey item consumed by the three tuna species could be attributed 
to the fact that they were consumed together with other most important and preferred 
food items. It could also be attributed to the fact that they were the only readily available 
food for all the three tuna species (Setyadji. et al. 2012). 
 
Moreover, there is a significant difference among the three tuna species with regards to 
prey items such as Engraulis encrasicolus, Sardinella aurita, Excoetus spp, Caranx 
59 
 
 
crysos, Sepia hierredda, Loligo spp, Squilla mantis, Isopoda and Brachyuran spp (Table 
4.6). The quantities consumed among the three tuna species were significantly different. 
This could be attribute to the fact that each of the three tuna species has different prey 
preferences that are important to them (Nootmorn, et al., 2007; Perera et al., 2015). 
 
For Thunnus albacares and Katsuwonus pelamis comparison, there is a significant 
difference between these two tuna species with respect to Engraulis encrasicolus. Since 
the significant value (0.000) is less than 0.05 (Appendix 3). This implies that the 
quantities of Engraulis encrasicolus consumed by the two tuna species are statistically 
different from each other (Dissanayake et al., 2008; Prathibha Rohit et al., 2010; Sivadas 
and Anasukoy, 1998).   
 
For Thunnus albacares and Thunnus obesus comparison, there is no significant difference 
between the two (2) species with respect to Engraulis encrasicolus. Since the significant 
level (0.814) is greater than 0.05. This means that the quantity of Engraulis encrasicolus 
consumed by these two tuna species is almost the same. This finding can also be 
attributed to a study by Setyadji. et al. (2012) in Indian Ocean which indicates that, 
Thunnus albacares and Thunnus obesus preferred to feed on Engraulis encrasicolus.   
 
For Katsuwonus pelamis and Thunnus obesus comparison, there is a significant 
difference between the two species with respect to Engraulis encrasicolus. Since the 
significant value (0.000) is also less than 0.05 (Appendix 3). Sivadas and Anasukoy, 
60 
 
 
(1998) and Manko (2010), investigation shows that Katsuwonus pelamis and Thunnus 
obesus feed on different prey items. 
 
For Thunnus albacares and Katsuwonus pelamis comparison, there is no significant 
difference between the two tuna species with respect to Sardinella aurita.  Since the 
significant level (0.894) is greater than (0.05). This finding is in line with research report 
by Perera et al. (2015) and Sivadas and Anasukoy, (1998) which shows that Katsuwonus 
pelamis and Thunnus albacares don’t feed on the same prey items in Sri Lanka and 
Minicoy, Lakshadweep. 
 
For Thunnus albacares and Thunnus obesus comparison, the significance value is 0.000 
for that matter, there is a significant difference between the two tuna species with respect 
to Sardinella aurita. Since the significant value is less than 0.05 (Appendix 3). This 
finding can also be attributed to a study by Setyadji. et al. (2012) in Indian Ocean which 
indicates that, Thunnus albacares and Thunnus obesus preferred to feed on Sardinella 
aurita. 
 
For Thunnus obesus and Katsuwonus pelamis, there is a significant difference between 
the two tuna species with respect to Sardinella aurita. Since the significant value of 0.000 
is less than (0.05). This could attribute to the fact that each of the three tuna species has 
different prey preferences that are important to them (Nootmorn, et al., 2007; Perera et 
al., 2015). 
 
61 
 
 
 
For Thunnus albacares and Thunnus obesus, the significant value is (0.205). Since this 
value is greater than (0.05) level required for statistically significant, these imply that 
there is no significant difference between the two tuna species with respect to Loligo spp. 
This could attribute to the fact that each of the three tuna species has different prey 
preferences that are important to them (Nootmorn, et al., 2007; Perera et al., 2015). 
 
 
For Thunnus albacares and Thunnus obesus comparison, there is a significant difference 
between the two tuna species. Since the significant level (0.003) is less than (0.05) with 
respect to Caranx crysos. This result can be attributed to the fact that Thunnus albacares 
and Thunnus obesus has different prey preferences that are relatively important to them 
(Dissanayake et al., 2008; Prathibha Rohit et al., 2010; Nootmorn, et al., 2007; Perera et 
al., 2015).  
 
 
 
5.3 Prey Species Similarities 
There is a moderate species similarity of prey items identified in stomach content 
analysis among the three tuna species. This could be as a result of the fact that most prey 
tends to be captured by chance and may require a longer time in other to identify more 
prey items. However, Crustaceans are the most exclusive food source of surface 
swimming species including Thunnus obesus, Thunnus albacares, and Katsuwonus 
62 
 
 
pelamis which has contributed to the moderate species similarity during the study 
(Batlagla et al., 2013). These tuna species feed primarily on pelagic and demersal fish 
species in Ghanaian waters which is similar to the finding of this study in Minicoy, 
Lakshadweep (Sivadas and Anasukoy, 1999). Similar result appeared in this study, with 
Caranx crysos, Engraulis encrasicolus, and Penaeus kerathurus also found in the 
stomach of tuna species. 
The occurrence of Caranx crysos in all three tuna species stomachs was interesting 
because it by-catch in almost every tuna fisheries vessel in Ghana and Indonesia 
(Nugraha and Nurdin, 2006; Prisantoso et al., 2010; Nugraha and Wagiyo, 2006; Barata 
and Prisantoso, 2009; Nugraha and Triharyuni, 2009). This happened because Caranx 
crysos plays an important role on pelagic food chain. These predators appeared to be the 
most active chaser of prey in the Ocean (Potier, 2004; Setyadji et al., 2012). 
 
All the tuna species sampled for stomach content analysis were captured on Fish 
Aggregation Device (FAD) due to the behaviour of tunas following floating objects 
(Perrin et al., 1973) The gut content examines food composition associated with 
aggregation conducted by Grubbs et al., (2001) found that Thunnus obesus feed on 
vertically migrating species rather than crustaceans on FAD associated environment in 
waters. The result from this study also indicates that, the three tuna species feed on 
crustaceans that are constantly following FADs in Ghanaian waters. This finding is in 
line with a research conducted by Setyadji et al. (2012) and Grubbs et al. (2001).   
 
63 
 
 
 
CHAPTER SIX 
CONCLUSIONS AND RECOMMENDATIONS 
6.1 Conclusions 
In conclusion, it was observed that these tuna species are not opportunistic feeders. Their 
prey item comprises mainly juvenile teleosts, crustaceans, and molluscs as their major 
food items. The most important and preferred prey items to these three tuna species 
(Thunnus albacares, Katsuwonus pelamis, and Thunnus obesus) were Engraulis 
encrasicolus, Penaeus kerathurus and Exocoetus spp, although Sardinella aurita, 
Brachyuran spp. Decapterus spp. and  Loligo spp. were equally important prey items to 
these tuna species. 
 
Engraulis encrasicolus was observed to be the most preferred food item accounting for 
58% occurrence by number, with Diplodus spp being the least preferred food item 
accounting for 2% for Thunnus albacares. 
The Index of Relative Importance of the total crustaceans analyzed was 16.2%. Although 
Penaeus kerathurus was more numerically abundant (40.4%) but Metapenaeus monoceros 
was less important in the food item forming 10.5% of entire prey wet weight with an IRI of 
14.5%. The Sepia hierredda and Loligo spp represented the molluscs prey item in the 
stomach with an IRI of 12.2%. 
64 
 
 
Penaeus kerathurus was found to be the most preferred food item accounting for 57% 
occurrence by number, with Sepia hierredda being the least preferred food item 
accounting for 10% for Katsuwonus pelamis. 
The Index of Relative Importance of the total fishes analyzed was 36.4%. Even though 
Engraulis encrasicolus were more numerically abundant (42%), Mackerel spp were 
important in the food item forming 37% of total food wet weight with an IRI of 22.5%. 
The Loligo spp and Sepia hierredda  represented the molluscs prey item in the stomach 
with an IRI of 6.3%. 
Exocoetus spp was found to be the most preferred food item accounting for 86% 
occurrence by number, with Metapenaeus monoceros being the least preferred food item 
accounting for 12% for Thunnus obesus. 
The Index of Relative Importance of the total crustaceans analyzed was 15%. Although 
Penaeus kerathurus was more numerically abundant (54%), Brachyuran spp, was less 
important in the diets making up to 42% of overall prey wet weight with an IRI of 13%.   
 
 
6.2 Recommendation 
Based on the study conducted, the following was recommended. 
 In order to determine the seasonal preferences for various prey species, further 
research should be conducted on these tuna species throughout the year. 
 
 
65 
 
 
 
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APPENDICES 
 
University of Ghana 
Department of Marine and Fisheries Sciences 
Appendix 1: Biological Data Sheet (Field) 
 
Date …………………………. 
Number of samples……………………… 
No Species Fork Length(cm) Set number GPS Position Remarks  
      
      
      
      
      
      
 
74 
 
 
 
University of Ghana 
Department of Marine and Fisheries Sciences 
Appendix 2: Biological Data Sheet (Laboratory) 
 
Date ………………………….                              
Number of samples………………………            
Total weight (kg)……………………………… 
No Species ForkLength(cm) Stomach Stomach Stomach Contents Remarks 
weight(g) fullness contents weight(g) 
        
        
        
        
 
 
 
75 
 
 
 
 
University of Ghana 
Department of Marine and Fisheries Sciences 
Appendix 3: Tukey test for multiple comparisons on prey items found in stomachs 
of these three tuna species 
Dependent Variable (I) (J) Mean Std. Sig. 95% Confidence Interval 
SPECIES SPECIES Difference (I- Error Lower Upper 
J) Bound Bound 
*
Engraulis 1.0 2.0 4.3676  1.0956 .000 1.770 6.965 
encrasicolus 3.0 -.5882 .9621 .814 -2.869 1.692 
*
2.0 1.0 -4.3676  1.0956 .000 -6.965 -1.770 
*
3.0 -4.9559  1.0956 .000 -7.553 -2.359 
3.0 1.0 .5882 .9621 .814 -1.692 2.869 
*
2.0 4.9559  1.0956 .000 2.359 7.553 
Sardinella aurita 1.0 2.0 -.0969 .2147 .894 -.610 .416 
*
3.0 -1.0435  .2358 .000 -1.607 -.480 
2.0 1.0 .0969 .2147 .894 -.416 .610 
*
3.0 -.9466  .2147 .000 -1.459 -.434 
*
3.0 1.0 1.0435  .2358 .000 .480 1.607 
*
2.0 .9466  .2147 .000 .434 1.459 
Loligo spp. 1.0 2.0 .8784 .5118 .205 -.341 2.098 
*
3.0 1.7245  .5118 .003 .505 2.944 
2.0 1.0 -.8784 .5118 .205 -2.098 .341 
3.0 .8462 .5714 .305 -.515 2.207 
*
3.0 1.0 -1.7245  .5118 .003 -2.944 -.505 
2.0 -.8462 .5714 .305 -2.207 .515 
Squilla mantis 1.0 2.0 -.2171 .5674 .923 -1.564 1.130 
*
3.0 1.8989  .7257 .027 .177 3.621 
2.0 1.0 .2171 .5674 .923 -1.130 1.564 
*
3.0 2.1159  .6939 .008 .469 3.763 
*
3.0 1.0 -1.8989  .7257 .027 -3.621 -.177 
76 
 
 
Dependent Variable (I) (J) Mean Std. Sig. 95% Confidence Interval 
SPECIES 2S.P0E CIES Differ-e2n.c1e1 
*
5(I9-  Er.r6o9r3 9 .008 -3.763 -.469 
Brachyuran spp. 1.0 2.0 J-)2 .4412 1.1011 .072 -5.054 .172 
*
3.0 -5.0716  1.1375 .000 -7.771 -2.372 
2.0 1.0 2.4412 1.1011 .072 -.172 5.054 
*
3.0 -2.6304  .7854 .003 -4.494 -.767 
*
3.0 1.0 5.0716  1.1375 .000 2.372 7.771 
*
2.0 2.6304  .7854 .003 .767 4.494 
 
 The mean difference is significant at the 0.05 level. 1. Thunnus albacore 2.Katsuwonus pelamis 3. Thunnus obesus 
 
 
 
77