Effects of sampling time, intraspecific density, and environmental variables on electrofishing catch per effort of largemouth bass in Minnesota Lakes

摘要

Effects of sampling time (day or night and fall or spring), target fish density, water clarity, water temperature, water conductivity, and lake morphometry on electrofishing catch per effort (CPUE) of largemouth bass Micropterus salmoides 200 mm total length (TL) and longer were determined. Electrofishing catch per hour (CPH) and catch per kilometer (CPK) were also compared to determine if each expression provided similar trends in CPUE. Correlations between day CPH and day CPK (r = 0.99; P < 0.0001) and night CPH and night CPK (r = 0.97; P < 0.0001) suggested that both measures provided similar trends in CPUE. Night CPH significantly exceeded day CPH, and spring CPH significantly exceeded fall CPH. Catchability (q) decreased with increasing density; therefore, CPH increased nonlinearly with density. Day CPH in fall decreased with increasing Secchi depth and water temperature but was unrelated to largemouth bass density. Day CPH in spring decreased with increasing Secchi depth and water temperature and increased with increasing density of largemouth bass and water conductivity. Night CPH in fall increased with increasing density and decreased with decreasing water conductivity, and night CPH in spring increased with increasing density and decreasing percent littoral area (percent of lake with depth less than 4.6 m) among lakes. These variables explained 44 percent of day CPH in fall, 75 percent of day CPH in spring, 28 percent of night CPH in fall, and 59 percent of night CPH in spring. Effects of density on q must be determined and environmental conditions must be similar before CPUE can be a useful index of largemouth bass density.