Hydroacoustic estimation of fish biomass in the Gulf of Nicoya, Costa Rica

A stratified sampling design was used for a hydroacoustic survey of the inn er parts of the Gulf of Nicoya in 1987 and 1988. The bottom topography of t he inner Gulf was modeled by introducing the concept of a topographical bas in model, as the basis for the projection of the sample survey estimates to the entire inner gulf. The bottom depth contours and volumes for the basin model were constructed from nautical charts. The estimates of sample abund ance were made for the fish in the inner Gulf using the acoustic methods, E MS (Expectation Maximization and Smoothing) and echo integration. The estim ates of population were made by the multiplication of the topographic model 's estimate of water volume and a model of fish density dependent on bottom depth. The results showed a general decrease in fish density biomass with bottom depth, and a simultaneous tendency for maximum concentrations over b ottom depths of about four meters. The Four meter bottom depth includes a b road expanse of the inner Gulf located south of Isla Chira. Overall estimat es of volumetric density (0.269 Fish/m(3)) and of areal densities (1.88 fis h/m(2)) ate comparable to other estuarine shallow water environments.