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The Gambia Estuary, a "normal" estuary with a decreasing salinity gradient from the mouth towards the head, is moderately exploited by small-scale fisheries and does not receive any severe pollution from either agriculture or industrial activities. Neither the Gambia River nor its estuary are artificially impounded. As the last large West African estuary free of major human disturbance, it is of considerable interest for comparative studies on the effect of major environmental perturbations in West African estuarine ecosystems. The aquatic environment and fish communities of the Gambia Estuary (about 250 km long) were studied by purse seine sampling at different periods in the river cycle, covering all hydro-climatic seasons that are characteristic of West African estuaries. Emphasis was placed on the diversity, composition, structure and distribution of fish assemblages in relation to fluctuations in physico-chemical factors such as water temperature, salinity and turbidity. Results on the aquatic environment, mainly the salinity range (from freshwater to 39) and dissolved oxygen (never a limiting factor for fish in the estuary) and on the main characteristics of the fish fauna (high diversity of life cycles, all the ecological categories represented) indicated that the Gambia Estuary was free of major climatic perturbation and reinforced the choice of this system as a reference for the study of the effects of major perturbations on estuarine tropical fish communities.

International audienceThe influence of salinity on life-history traits was tested using two adjoining West African estuaries: the Gambia with a 'normal' salinity gradient (salinity always <40 and decreasing from the mouth upstream) and the Sine Saloum (Senegal) with an inverse gradient (from 35 at the estuary mouth up to >130 in the upper reaches). The breeding seasons and subsequent fork length (LF) at first maturity (LF50) were estimated for different fish species reproducing both in the Sine Saloum and in the Gambia River estuaries using a database built from experimental fish samplings between 1990 and 2003 with a purse seine (30 locations in the Sine Saloum and 44 in the Gambia). The database contained 30 553 individuals belonging to 60 different species among which only 20 species reproduced in both estuaries. The breeding seasons peaked just before, or at the beginning of the rainy season (June to July), and there were almost no sexually mature fishes at the beginning of the dry season (November to December). Patterns of differences between LF50 of the two estuaries did not follow a general trend (positive or negative), but varied in the same way for females and males of a given species. The LF50 was only systematically reduced with increasing salinity in species living in high-salinity waters (>70). For species living below 70, differences in sizes at first maturity between the two estuaries did not show any clear relationship with salinity. The smallest mature individual found in an environment was a good indicator of the size at first maturity reached in a particular ecosystem because the trend of the species differences between the two ecosystems generally followed that of the differences in size at maturity

A comparative study of the Ébrié lagoon (Ivory Coast) and Lake Nokoué (Benin) was made based on ecotrophic model outputs that describe each system's structure and functioning. Two models were constructed using the Ecopath software to differentiate main biomass flows in the systems.Results indicate that biomasses and productions in both ecosystems are concentrated in trophic levels (TL) 2 and 3. Higher TL biomasses and productions in Lake Nokoué compared to Ébrié lagoon may be explained by the presence of acadjas. High production per biomass (P/B) and food consumption per biomass (Q/B) values indicate the high productivity of these systems and the abundance of juveniles in most groups which utilize these systems as refuge zones and nurseries. The difference, however, lies between the principal source of energy and how it is incorporated in the food web of each ecosystem. Lake Nokoué is a detritus-driven ecosystem while Ébrié lagoon is dominated by the phytoplankton pathway. System indicators suggest different levels of ecosystem stability and maturity. Relevance of other observations on ecosystem functioning and indicators in relation to perturbation is discussed.

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The common West African bonga shad represents a large part of the fish biomass in 2 neighbouring estuaries that function in different ways. The Gambia estuary has a normal salinity gradient, while the Saloum has an inverse gradient. Bonga shad Ethmalosa fimbriata were collected in both ecosystems during a 16 mo period (June 2001 to September 2002) at 5 locations, to investigate the role of salinity on life history traits. The main traits were studied at a spatio-temporal scale: reproduction from macroscopic examination of the gonads, oocyte counting and measuring, and growth from interpretation and measurements of a sub-sample of otoliths. Analysis of genetic differentiation at 3 intronic and 1 anonymous nuclear gene loci was also carried out to investigate differences between estuaries and among locations. The results did not show any allelic frequency heterogeneity between populations, indicating that populations of both estuaries represent 1 single panmictic unit, and that selection is not significantly acting on these loci. Hence, the response of the different traits to environmental variation may primarily represent phenotypic plasticity. The seasonal cycle of reproduction was clearer in the Saloum, occurring during a long period (January to August). The calculated size at maturity was reduced for both sexes in the upper Saloum, where the salinity was highest. The relative fecundity and the oocyte size were larger in the Saloum. On the otoliths, translucent zones, formed each year at the end of the rains (September to October), were used to estimate the age in months. Growth rates were reduced in the hypersaline environment of the Saloum, whereas growth differences were smaller between the Gambia and the pooled Saloum data, with a salinity < 60 psu. Growth was faster in the lower parts of the Saloum, related to better conditions for fish. The results illustrate that an environment with high salinity (> 60 psu) affects the growth, reduces the size-at-maturity and increases the fecundity of E. fimbriata.

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