Comparison of Seasonal Bluegill Catch Rates and Size Distributions Obtained with Trap Nets and Electrofishing in a Large, Heated Impoundment

Abstract

To determine potential bias in bluegill Lepomis macrochirus population indices, we compared the catch rates and size structure of bluegills captured by use of trap-netting and electrofishing from 1983 to 2001 in Coffey County Lake, a 2,100-ha cooling impoundment in Kansas. Trap-net catch per unit effort (CPUE) and size structure (proportional stock density (PSD)) were greater in 13-mm-mesh than in 25-mm-mesh trap nets, but length frequencies were similar. Bluegill electrofishing CPUE and PSD were greater during fall than spring, but longer bluegills were obtained by spring electrofishing. Trap nets produced a higher bluegill CPUE than did spring electrofishing, and the trap-net length frequency was skewed toward longer fish. Our data suggest that (1) the 13-mm trap-net mesh size yields more accurate information about harvestable-size bluegill abundance and size structure than 25-mm trap nets and (2) trap-netting (both mesh sizes combined) produces more accurate information about harvestable-size bluegill abundance and size structure than does spring electrofishing. Year effects played a large role in the differences detected among catch rates and population indices. Although fall electrofishing provided a larger bluegill sample than did spring electrofishing, size structure differences were not great enough to warrant a change in the standard electrofishing sampling period (i.e., spring) for bluegills in large Kansas impoundments because black bass Micropterus spp. can be sampled concurrently. This study provides gear and seasonal bias information on bluegill catch rates and size structure. Such information is lacking for large impoundments, which points to the need for further studies like ours.