Fish species abundance and distribution in The Gambia estuary

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Darboe1.1 Objectives• To improve understanding of the general fish distribution pattern of theGambia estuary.• To establish and describe fish species abundance and diversity in the Gambiaestuary.It is expected that the present study will give answers to the question as to whetherfish population and distribution in the Gambia estuary have definite patterns and if sowhat are the in-situ factors that determine those patterns. Attainment of theseobjectives will enable the Government, through the Fisheries Department, to giveperiodic advice to small-scale fishermen and to adopt regulatory fishing measures formaximum productivity with minimum costs.1.2 Goals• To identify the indicator (most frequent/ecologically important) species in theestuarine system.• To utilise indicator species to identify highly productive areas in-terms ofoverall species composition.• To describe physico-chemical parameters of the estuary.• To relate and assess the impact of the parameters on distribution and otherbehaviours of fish.1.3 Inception of the studyUnder the umbrella of the Franco-Gambian technical co-operation, this study, vestedin the Institute for Research and Development (IRD), was funded through theEmbassy of the Republic of France in the Republic of Senegal. The present studyrepresents one of the components to be jointly implemented by the Institute and theDepartment of Fisheries of the Gambia. The scientific team in this component iscomposed of one Gambian marine ecologist, four French fisheries researchers and oneSenegalese Eco-sounding technician. The Fisheries Department is fully represented inthe implementation of other components, all geared towards improving the muchneededcapacity of the Department to conduct applied research.UNU – Fisheries Training Programme 6
Darboe2 LITERATURE REVIEW2.1 General description of estuariesThe word estuary was derived from the Latin word “aestus” meaning “tide” and theadjective “aestuarum”, which means “tidal”. The more common definition inliterature was given by Cameron and Pritchard (1963) as: “a semi-enclosed body ofwater which has a free connection with the open sea and within which seawater ismeasurably diluted with freshwater derived from drainage”.This definition however is often considered limiting since it will not include thosewater bodies that are intermittently disconnected with the open sea due to sand barsthat emerge during the dry season when rain and river runoff are greatly reduced. Thisgave rise to an expanded definition by Hopkinson and Hoffman (1983)“...has a freeconnection with the open sea at least intermittently...”.A common categorisation of an estuary lies in its being either positive (normal) ornegative (inverse). An estuary is considered negative or inverse when evaporationfrom it exceeds inflow of fresh water from rivers or local rainfall into it causing waterin the estuary to become more saline than the adjacent seawater. The phenomenonwas observed in the South Senegalese river of Casamance (Albaret 1987), which issituated in the same geographic area as the Gambia estuary. The opposite scenario istherefore an estuary in which annual freshwater loss through evaporation does notexceed the annual gain in freshwater from rain and rivers. An upstream rise of thesalinity gradient could have far reaching consequences on the ichthyofauna of theestuary. Albaret, considering species richness as one of the fundamental parameters inthe study of the fish community in the Casamance River, observed that the parameterdecreased with an increase in salinity during all of the three periods of his study.However, he could not rule out the possibility of incidences of high fish mortalityresulting from emigration and osmo-regulatory problems of the fish encountered.The influence of extreme salinity on the fish community of estuaries was alsoreported by Young and Palter (2001). The authors, working on the Wellstead estuaryin West Australia reported that a marked increase in the mean monthly salinities in thelower estuary from 53 to 112 ppt was accompanied by a decline in the number of fishspecies. In their comparison with the upper estuary during the same period, wheresalinities did not undergo such an extreme rise, the authors could not observe anysuch decline in the number of species. Instead they observed a rise in the fishcommunity density. The comparison suggests that, as a result of exceptionally highsalinities, some freshwater fish species could have suffered mortality in the lowerestuary or moved into the upper estuary where salinities are lower.Among the most emphasised aspects in estuarine and other fish community studies isspecies richness (Albaret 1987) and Baran 2000). The former expanded the definitionof species richness in estuaries and lagoons as the number of species encountered atleast once within their open systems. Baran (2000) compared the species richness ofWest African estuaries with that of world estuaries and positioned West Africanestuaries as intermediate between those of South America and Australia or Asia. Fromthe authors’ modified table of species richness, Diouf (1996) gave the species richnessUNU – Fisheries Training Programme 7
Page 3 and 4: DarboeLIST OF FIGURESFigure 1: Map
Page 5: DarboeShrimp harvests are purchased
Page 9 and 10: Darboe3 MATERIALS AND METHODS3.1 Or
Page 11 and 12: Darboe4 RESULTSThe area covered by
Page 13 and 14: Darboeand the lowest, 23.8 o C, at
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Page 17 and 18: Dissoled oxygen (mgl-1)7,56,55,54,5
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Page 25 and 26: DarboeFigure 14: Aggregated number
Page 27 and 28: Darboeexplanation for low oxygen co
Page 29 and 30: DarboeACKNOWLEDGEMENTSI express gra
Page 31 and 32: DarboeAPPENDIX : TABLES OF DATA REC
Page 33 and 34: DarboeTable 3: Records of environme
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Page 39 and 40: DarboeTable 9: List of fish epecies

Fish species abundance and distribution in The Gambia estuary

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