Factors Related to Crappie Indices of Abundance in a Large, Fluctuating Canyon Reservoir in Idaho

Abstract Crappie (Pomoxis spp.) populations are challenging to manage due to highly variable year-class strength; however, such variability has rarely been investigated in western North America, where crappie often occupy large, steep-sided reservoirs prone to severe drawdown. We investigated the influence that various factors had on crappie abundance, as indexed by long-term trawling for larval fish and long-term electrofishing for older fish. Our primary findings were that: 1) autumn age-0 crappie abundance was higher in years when larval abundance and reservoir flow were higher in the summer; 2) spring age-1 crappie abundance was higher in years when fish were larger and more abundant entering their first winter, when hydraulic residence time was reduced and the reservoir volume was higher during the winter, and when predatory-sized smallmouth bass (Micropterus dolomieu) were more abundant in the spring (though the latter relationship was likely not causative but rather a parallel response to mutually advantageous environmental conditions in the reservoir); and 3) age-0 crappie entering their first winter were larger in years with lower summer larval crappie abundance and warmer summer water temperature. We recommend autumn electrofishing to monitor crappie populations in large canyon reservoirs, where shorelines are often too steep to sample fish with trap nets, because it provides an index of age-0 crappie abundance and size at the onset of their first winter as well as data on older crappie year classes and sympatric species; it also requires less sampling effort than summer trawling.