Effects of Largemouth Bass Biomass Reduction on Prey Availability and Size Distribution

Predator-prey relationships can greatly impact size structure and abundance of fish communities.  In Cerrillos Reservoir, Puerto Rico, about 20% of the Largemouth Bass Micropterus salmoides biomass was removed annually in 2012 and 2013 in an attempt to restructure the Largemouth Bass fishery. Following this experimental reduction in predator abundance we examined prey populations to determine if abundance or size structure were affected. Mean (±SE) annual catch rates (CPUE, fish/hour) of sunfish Lepomis spp., and tilapia Tilapia and Oreochromis spp. increased from 38±6 and 57±9 to 72±10 (T-test, DF=67, P=0.007) and 162±29 (T-test, DF=46, P=0.001), respectively, following the removal.  Concurrently, mean total length decreased among lepomid (Kolmogorov-Smirnov, D=0.1505, P=0.0021) and tilapia (Kolmogorov-Smirnov, D=0.4147, P<0.0001) species suggesting a shift in the size structure to smaller size classes. Mean (±SE) annual Threadfin Shad Dorosoma petenense catch rates (NPUE, fish/1000 m³) did not significantly increase (T-test, DF=51, P=0.376) following the removal. The reduction in predator abundance resulted in increased prey availability and reduced size structure of lepomid and tilapia populations. These changes could have a major impact on this important sport fishery.