Use of Consumer-Grade Sonar Units with Generalized N-Mixture Models to Estimate Fish True Abundance

Estimating abundances of large populations of fish presents unique challenges for which methods such as mark recapture may be inadequate.  We describe and apply a novel method to estimate White Perch (Morone americana) and Gizzard Shad (Dorosoma cepedianum) abundances in two eastern Nebraska reservoirs.  Point-count and species composition data were collected using a consumer-grade sonar unit, vertically deployed gillnets, and boat electrofisher. We calculated site-specific abundances and detection probabilities as a function of water depth, bottom slope, and sampling period using a generalized form of the N-mixture model for open populations.  From site-specific abundances, we estimated total population size through area expansion of model predictions. In Branched Oak Lake, we estimated that there were approximately 1.1 million White Perch and 1.2 million Gizzard Shad.  Our models predicted that abundance increased with depth and that detection probability differed between sampling periods.  In Pawnee Reservoir, we estimated that there were 1.5 million White Perch and 0.5 million Gizzard Shad.  Our models predicted that detection probability decreased with depth.   The method we described provides researchers and managers with a new method to estimate fish abundance when other methods may not be applicable.