Factors Affecting Trap-Net Catchability of Black Crappies in Natural Minnesota Lakes

Abstract

We quantified the effects of length-group (150–199, 200–249, and 250–299 mm), sampling period (September, October, early spring, and late spring), lake, and intraspecific density on catchability q of black crappies Pomoxis nigromaculatus in trap nets (modified fyke nets) set in eight natural Minnesota lakes during 1996–2001. The catchability of the two larger length-groups exceeded that of the smallest length-group in all lakes. Catchability in early and late spring exceeded that in September or October in most lakes; otherwise, q did not differ among sampling periods. Catchability differed among lakes during September, October, and early-spring sampling periods but did not differ among lakes in late spring. For 150–199-mm fish, q increased with increasing lake surface area during all sampling periods, decreased with increasing shoreline development ratio during all periods, decreased with increasing maximum depth during fall sampling periods, and was unrelated to the percentage of surface area of lakes less than 4.6 m deep. Catchability of the larger length-groups was unrelated to any of these lake morphometric variables. Catchability of 150–199-mm black crappies decreased with increasing density during all sampling periods. However, the q of 200–249-mm black crappies was density independent during early spring and September, decreased with increasing density in October, and may have been density dependent in late spring. For 250–299-mm black crappies, the q in early spring decreased with increasing density, and the q in late spring was density independent. Trap-net catch per unit effort (CPUE) provides more meaningful data on the relative abundance of 200-mm and longer black crappies than it does for smaller black crappies. Comparisons of trap-net CPUE and length-frequency distributions are most meaningful if trap-net samples are collected during the same sampling period and within the same lake.