Influence of diel period on electrofishing and beach seining assessments of littoral fish assemblages

At two sites in each of two lakes, we sampled days and nights to evaluate diel and gear (pulsed-DC boat electrofisher versus large beach seine) differences in species richness, total fish abundance, assemblage similarity, and size structure of populations of bluegills Lepomis macrochirus. Both gears produced significantly greater species richness at night than during the day. Total catch per unit effort for electrofishing was significantly greater for nighttime than for daytime samples, Diel differences in total density for seining samples were not statistically significant. Ordination of the electrofishing data tended to separate night samples from day samples at sites because of greater abundance of bluegills, black bullheads Ameiurus melas, walleyes Stizostedion vitreum, and white suckers Catostomus commersoni in night samples. Ordination of the seining data indicated a high degree of similarity among day and night samples within three of the four sites and separated West Okoboji sites from East Okoboji sites. We found few significant diel differences in bluegill size distributions for a given gear, but the two gears generally produced differing size distributions within a given diel period. Diel differences were more prevalent for clectrofishing than for seining, whereas differences among sites were more apparent for seines, Our results should help biologists make more informed choices regarding diel periods to sample and gear to use in their littoral zone sampling programs.