Distribution and feeding interactions of the threadfin shad and juvenile gamefish in Patagonia Lake

Abstract

Distribution, diet, and food availability for young-of-the-year (YOY) largemouth bass (Micropeterus salmoides), YOY bluegill (lepomis macrochirus) and threadfin shad (Dorosoma petenense) were studied to reveal any potential interactions in Patagonia Lake, a 170 ha (265 acre) recreational impoundment in Santa Cruz County, Arizona. Threadfin shad, a forage species, were stocked to enhance the growth of bass. As shad became abundant, limnetic zooplankton declined, reducing the primary food supply of the YOY game fish. Diet analysis showed very little food overlap between YOY gamefish and threadfin shad. Shad ate phytoplankton when zooplankton became scarce, and continued to grow well and reproduce. As zooplankton decreased, YOY gamefish shifted their diets to aquatic insects. After the shift, the gamefish did not grow well. The continuous overgrazing of zooplankton by large numbers of threadfin shad had an impact on the growth of the juvenile largemouth bass and bluegills. This impact may result in weak year classes in the future.