Department of Marine and Fisheries Sciences
Permanent URI for this collectionhttp://197.255.125.131:4000/handle/123456789/4932
Browse
2 results
Search Results
Item Rock Assessment of the Five Major Commercial Species in Yeji Area (Stratum Vii) of the Volta(University of Ghana, 1999-10) Ofori-Danso, P.K.; Vanderpuye, C.J.; University of Ghana, College of Basic and Applied Sciences , School of Biological Sciences , Department of Marine and Fisheries SciencesAn assessment of the fishery of five major commercial species which together contribute more than 70% of the catches in stratum VII (near Yeji) of the Lake Volta has been made based on studies undertaken between March, 1995 and December, 1996. The species studied are: Hemisynodontis membranaceus (Geoffroy St. Hilaire, 1809), Chrysichthys nigrodigitatus (Lacepede, 1809), Chrysichthys auratus (Geoffroy St. Hilaire, 1809), Oreochromis niloticus (Linne, 1757), and Schilbe intermedius (Ruppell, 1832). The study aimed at contributing information to provide a basis for comprehensive developmental strategies, management and sustainable exploitation or conservation of the fisheries particularly in stratum VII of the lake. Sixty-six fish species representing 39 genera and 19 families were encountered throughout the studies. Gill-net is the most important gear and the catches were dominated by the bagrids Chrysichthys auratus, C. nigrodigitatus and the cyprinid Labeo spp. Data on gonadosomatic index and and gonad maturity stages of the major species indicate that Chrysichthys nigrodigitatus and C. auratus-, and the cichlid Oreochromis niloticus spawn throughout the year while the mochokid Hemisynodontis membranaceus shows two clear spawning seasons in February and July. The mean annual catch-per-unit of effort (CPUE) of canoes utilizing gill-nets during the study period were estimated respectively as 10-18 kg canoe'1 d'1 and 7-15 kg canoe 1 d 1 for 1995 and 1996 which indicated declining catches over those years. The total annual canoe catch estimate in stratum VII alone during 1996 was about 25,000 metric tonnes. Among the physico-chemical variables measured during the study, the mean monthly CPUE was found to be positively correlated with the ammonia concentration in the water; total dissolved solids (TDS), and conductivity. The relation with ammonia concentration (AMN) was best described by the equation: Log m CPUE = 0.6076 Log I0AMN + 1.3522 (r 2 = 0.6992) Growth parameters were estimated using length-based stock-assessment methods and the von Bertalanffy growth model for the major species are described as follows: H. membranaceus: Lt = 44. C. nigrodigitatus: Lt = 44. C. auratus: Lt = 31. O. niloticus : Lt = 33. S. intermedius: L, = 30. 0 [ 1 - exp (- 0.55 (t + 0.26))] 5 [ 1 - exp (- 0.65 (t + 0.22))] Of 1 - e x p ( - 0.60 (t + 0.12))] 5 [ 1 - e x p ( - 0 . 5 5 (t + 0.45))] Of 1 - e x p ( - 0.80 (t + 0.38))] The estimated longevity (t max - 3/K) for the major stocks were between 4 and 6 years indicating that they are short-lived. This was expected because tropical fish are known to be fast growing and short-lived. This increase in growth rate accompanied by a decrease in size appeared to be adaptation for survival by the species in the midst of increasing effort. The estimated ages at first capture of the major species based on the estimated length at first capture (Lo50) show that they are caught before reaching one year old which implies that there is growth over-fishing. The length frequency analysis also showed that the fishery operated on three pseudo-cohorts of the major fish stocks. According to the the relative yield-per-recruit analysis, the present level of fishing (Epresent) was greater than the maximum expected exploitation (Emax). Based on the Eopt = 0.5 optimization criterion, this implied that there was over-exploitation of the major stocks. This grave situation is aggravated by the progressive reduction in water level, brought about by poor rains. In order to forestall this situation, some options for management of the fisheries have been recommended. These include: a) limitation of entry into the fishery; b) enforcement of mesh size regulations; c) control over changing fishery technology; d) establishment of “lake reserves” through closure of fishing in selected areas; and e) adoption of a rights-based fisheries management system.Item Comparative Studies of the Dynamics and Management Of Fish Populations in an Open and Closed Lagoon In Ghana(University of Ghana, 1998-05) Entsua-Mensah, R. E. M.; Vanderpuye, C.J.; University of Ghana, College of Basic and Applied Sciences , School of Biological Sciences , Department of Marine and Fisheries SciencesComparative studies have been done on the dynamics and management of the fish populations in Abrubi lagoon (closed lagoon) and Densu delta (open lagoon) in Ghana from May 1994 to April 1996.Some environmental parameters of the lagoons, which affected fish population were also monitored. There was a noticeable increase in temperature during the dry season (November to March) in both study areas. Temperatures ranged from 26°C to 33°C. On the whole, changes in pH were gradual and steady with no definite pattern in both study areas. The major rains were from May to July with minor rains from September to November; December to March were generally dry months. The depth of the water increased with the onset of the rains. Fluctuations in salinity were closely linked to amount of rainfall. The three stations in each study area showed a pattern of salinity gradation with lowest salinity occurring at Station 1 (riverine portion) and highest salinity recorded at Station 3 the area nearest to the sea. Dissolved oxygen levels were high in the wet season but low in the dry season. Differences in oxygen levels were not significant between months and between stations in both study areas. Analysis of variance tests showed that levels of phosphate and nitrate were not significantly different between months and between stations in both study areas. Levels of total dissolved solids and conductivity followed each other closely. Maximum Catch per Unit Effort (CPUE) values were recorded when the depth of the lagoon was low. Eighty-three thousand, eight hundred and forty-three kilogrammes of fish were obtained from the Densu delta by experimental cast net fishing during the study period and sixty-six thousand nine hundred and ninety-nine kilogrammes of fish from Abrubi lagoon. The five dominant species in Abrubi lagoon were Sa.rotherod.on melanotheron, Hemichromis fasciatus, Tilapia zillii, Liza falcipinnis, and Ethmalosa fimbriata. In the Densu delta they were Sarotherodon melanotheron, Gobiodes ansorgii, Tilapia zillii, Mugil cephalus and Ethmalosa fimbriata. Sarotherodon melanotheron was the most dominant species, accounting for 68% of the catch in the Densu delta and 75% of the catch in Abrubi lagoon during the study period. The overall species diversity was not markedly different between Abrubi lagoon and Densu delta. At 5% significance level (Mann Whitney U test), the species diversity in the dry season was different from that of the wet season in Stations 1 and 3 but not in Station 2 in both study areas. The Eveness component varied in a manner similar to diversity. Breaching of the lagoon affected species diversity in Abrubi lagoon. The peak of diversity occurred just before the peak of rainfall in Abrubi lagoon. Marine incursions and the opening and closing of the sluice gates of the Weija dam affected diversity in the Densu delta. The Forage / Carnivore ratio of 2.86 for Abrubi lagoon and 5.15 for Densu delta indicates that both study areas had balanced fish populations. The Multi-Dimensional-Scaling (MDS) obtained for the six sites in the two study areas indicate that temperature and salinity were strong environmental factors determining the presence of fish species There was monthly variation in male: female ratio with females predominating most of the time. From the Gonadosomatic Index (GSI) obtained S. melanotheron spawns from February to March. The overall length -frequency distribution showed a modal length of 8.5-9.0 cm SL in Abrubi lagoon and 5.0-5.5 cm SL in Densu delta. The length at first maturity was 7.2 cm SL in Abrubi lagoon and 4.8 cm SL in Densu delta. The von Bertalanffy growth parameter (Loo = 12.0 cm SL , K = 0.67 for Densu delta and Loo =15 cm SL, K= 0.79 for Abrubi lagoon implies fast growth which might compensate for the rather high fishing and natural mortality rates. Direct and indirect management practices can be used to manage the fisheries. Direct management involves gear control, catch control, fishermen control and season control. Indirect management involves mangrove afforestation, environmental protection and introduction of alternative trades to the fishermen.