The Gulf of Nicoya, Costa Rica is a relatively shallow, tidally influenced estuary that supports a substantial artisanal fishery on various stocks, primarily corvinas. However, little is known about the population size or productivity of these stocks. While hydro acoustic techniques have been successfully applied to fish population estimation in many circumstances, their application to the Gulf of Nicoya stocks faces a double consideration: economic considerations force a relatively simple, cost-effective approach, but the environment (including the species and size composition) is complex. This study approached the problem of the complex biological composition with Clay's deconvolution technique, which allowed researchers to obtain substantial information about the acoustic size characteristics and density of the fishes with a relatively simple, single-beam echo sounder. Researchers implemented the deconvolution analysis, along with standard echo integration techniques, using the BioSonics ESP acoustic signal processing system. This PC-based system combines portability with substantial processing power and storage capability. The deconvolution technique provides the necessary scaling factor for echo integration, so that absolute population estimates can be made. (Author abstract).