Modelling The Relationship Between The West African Mangrove Oyster (Crassostrea Tulipa, L.1819) And The Aquatic And Climatic Environment For Use As A Bio-Indicator In The Densu Estuary

Abstract

Crassostrea tulipa (Lamarck, 1819) in the Densu estuary was investigated from March 2019 to August 2020 for aspects of its ecology; it’s potential as a bio-indicator of environmental variability; and long-term effects of climate variability on shellfish production in Ghana’s artisanal fisheries and its implication on sustainable management of oyster fisheries. Oyster samples were collected monthly and physicochemical parameters namely Temperature (OC), Dissolved Oxygen (mg/L), pH, Total Dissolved Substances (mg/L), Conductivity (µS/cm) and Salinity (0 /00) measured in situ in triplicates. Silicates, Total Alkalinity, Chlorophyll a, microbes (Total Viable Counts, faecal coliform and Escherichia coli) and heavy metals (Lead, Cadmium and Mercury) were measured ex situ following standards of APHA (2015). Relative abundance was measured as Catch Per Unit Effort (CPUE) and growth pattern determined using the TropFishR package in R programming software. The numerical and frequency of occurrence methods were used to determine its food habits. Species-environmental driver relationship was analyzed following Canonical Correspondence Analysis (CCA) approach using the Vegan package (version 2.5-4.) in R studio software (version1.3.1056). CPUE from experimental fishing was significantly (p < 0.05) higher (6-200; 233.33 ± 6.00 kg/hr/fisher/day) than commercial fishing (3-100;78.12 ± 7.11 kg/hr/fisher/day). CPUE was significantly higher (p = 0.0161) at low tide (115-500;50.10 ± 5.3 kg/hr/fisher/day) than high tide (6-200; 62.58 ± 3.12 kg/hr/fisher/day). CPUE was higher (p = 0.023) in the dry season (150.87 ± 1.12 kg/hr/fisher/day) than the rainy season (57.45 ± 0.55 kg/hr/fisher/day). Crassostrea tulipa has a fast growth rate (K= 0.81; L∞ = 13.24 cm). Higher condition index (60 %) was recorded in the rainy season than the dry season (39 %). University of Ghana http://ugspace.ug.edu.gh iv The diet of the oyster was predominated by golden algae (IRI=595), red algae (IRI=209), green algae (IRI=131.37) and diatoms (IRI =172). Densu estuary is a dynamic shallow system with high concentration of total alkalinity and aragonite. Water depth, silicates, e coli and revelle factor were significantly higher (p < 0.05) at high tide than low tide. Also, mean water depth, cadmium, total alkalinity, pH, carbon dioxide, lead, total carbon dioxide, carbon dioxide fugacity and chlorophyll a were significantly higher (p< 0.05) bio accumulated Pb and Cd than big- sized (4.5-5.4g) tissues. Lead, mercury, TVC, faecal coliform and Escherichia coli also bio accumulated in C. tulipa tissues more (BAF > 1) than in the water medium which suggests that it has the ability to provide a measurable response to changes in the estuarine environment. Therefore, C. tulipa in the Densu estuary has the ability to accumulate pollutants from the environment and its morphometric features could give clues on the state of environmental variables. C. tulipa is a good bioindicator for assessing; lead, mercury, Total viable counts, E coli and faecal coliform in the Densu estuary. Densu estuary is high in aragonite and total alkalinity. The predictor variables for; Condition factor is aragonite, shellfish catch is temperature and Shell height & Width are faecal coliform, total carbon dioxide and carbon dioxide fugacity. There is the occurrence of contamination and therefore the need for regular monitoring, enactment of control measures and depuration prior to consumption. Also, the use of refuse dam and sewage outlet should be prohibited. It is recommended that laboratory and field-controlled experiments be conducted on oyster responses to extremes of temperature, aragonite and total alkalinity. There is an urgent need for the collation of data on estuarine/lagoonal shellfisheries in Ghana by Fisheries Commission and other stakeholders on catch trends, gears, effort and income of artisanal oyster collectors.