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.

A fyke net survey of the juvenile and small fish assemblage of the Gambia estuary was conducted on three sampling occasions during the maximum recruitment season, from May to November 2002. This is the first detailed description of that particular component of the fish community in the Gambia estuary. The overall fyke net fish assemblage comprised 51 species, with a majority of young individuals of large and medium size species and the adults of small species. This assemblage is defined as the "small fish component of the mangrove channels". The assemblage is dominated by true estuarine and certain marine species, all able to complete their life cycle within the estuary. The sea spawning species whose juveniles use the estuary mainly as a nursery ground, are not very diverse or abundant during the recruitment season. They are limited to the lowest part of the estuary due to the low salinity prevailing in the middle and upper reaches during the rainy season. This suggests that extreme seasonal variability (river flooding) in certain West African tropical estuaries at the time of maximum recruitment by certain marine species may reduce the nursery ground value for these taxa.