Fish population surveys in estuaries: a comparison between acoustic at moored stations and purse seine surveys

International audience ; In most estuarine ecosystems, the fish biomass is difficult to estimate, since most of the standard sampling methods are difficult to implement and may be limited to certain biotopes. Moreover, Protected Areas are no longer limited to marine ecosystems and several protected areas have been created in estuaries, lagoons or mangrove environments. In these areas, turbid waters do not allow direct diving observations and fish sampling, often performed using destructive – because efficient and non selective - gears such as trawling or seine nets, is always an ethic problem. As a consequence there is an increased need for liable, non-destructive methods to study fish assemblages in these environments. In order to describe the fish biomass distribution and evolution in the Gambia estuary, five research surveys have been conducted at different hydrological seasons. The fish assemblages were sampled by vertical echo-sounding at moored stations and at the same time and same location with a purse seine. The purpose of this work, still in process, is to compare the two methods, to try to inter-calibrate them and to seek for their complementarity. Though significant, the global direct correlation between estimations of biomass performed with the two sampling methods were too low to be considered satisfactory. Moreover the level of correlation between the two methods varies extensively following the survey, the season or the zone of the estuary taken into account. The role of the main environmental variables: depth, salinity, transparency and current strength on the quality of the correlation of the estimation made by the two methods was emphasized.