Bluegill Seasonal Habitat Selection, Movement, and Relationship to Angler Locations in a South Dakota Glacial Lake

While the literature is full of research examining habitat selection of bluegills Lepomis macrochirus, most studies have utilized small (\u3c150 mm) fish in laboratory or small pond experiments. Those few studies that have examined larger bluegills in natural systems have generally occurred over a relatively short time period. The objectives of my research were to 1) quantify seasonal habitat selection by large (\u3e200 mm) bluegills across a full year of environmental conditions, 2) examine changes in movement rates and core home range size across a full year, and 3) compare spatial distributions and habitat fished by bluegill anglers to habitat use by bluegills across seasons. Radio telemetry was used to examine seasonal habitat use and movement patterns of bluegills in Enemy Swim Lake, South Dakota. Transmitters (4 g) were attached externally to large bluegills during September and May, (n=40 and 45, respectively). Tracking occurred from 7 October 2002 through 18 October 2003, only interrupted by ice formation and melt. Transmitter-bearing fish were located 3-4 times each week by boat, all-terrain vehicle, or snowmobile, and positions were recorded by handheld GPS. Bluegill angler locations were recorded as encountered. Habitat variables (i.e., depth, vegetation density and height, visual obscurity, and substrate type) were measured during May, August, and October 2003, and the collected measurements used to create habitat coverages in ArcMap. Bluegill location data were assigned to one of five ‘seasons’ (fall, winter, spring, spawn, or summer) and overlaid on habitat coverages to determine habitat use and selection for the appropriate season. Overall fish loss (i.e., mortality, transmitter expiration, harvest, emigration) was 68% and 79% for September and May tagged bluegills, respectively. Mean seasonal minimum daily movement rates (± 95% CI) ranged from 32.4 m/day ± 13.4 during winter to 195.7 m/day ± 37.3 during the spawning period, while mean 50% minimum convex polygon home ranges varied from 0.8 ha ± 0.8 during winter to 5.0 ha ± 3.5during fall. Mean distance from shore ranged from 105.1 m ± 22.2 during summer to 254.8 m ± 55.9 in spring. Transmitter-bearing bluegills displayed seasonal changes in selectivity for habitat variables, with submerged vegetation extensively utilized during all seasons. The selection for vegetation in a highly heterogeneous lake indicates that it likely provides important resources for feeding and predator avoidance for large bluegills. Angler distance to bluegill home range was not significantly different from random during all months, with anglers concentrating in areas near, but not within, the home ranges of transmitter-bearing bluegills. Anglers also ‘fished’ different habitat than transmitter-bearing bluegills; specifically, areas that were slightly deeper and had less abundant vegetation. Angler locations reflect habitat more conducive to angling compared to areas selected by bluegills during the same period. However, recent increases in bluegill catch and harvest during both summer and winter creel surveys suggest that angler locations are sufficiently similar to bluegill locations for angling success. The results of my study have multiple implications. Sampling for large bluegills should be most efficient if trap net use is limited to periods when movement is high and distance from shore is low, such as during the spawn. However, sampling bias toward male bluegills may occur during this period when males exhibit nest-guarding behavior. Electrofishing is more suitable during other seasons. In addition, trap net sampling has been shown to bias size structure towards larger bluegills, while electrofishing will bias towards smaller individuals. The use of vegetation across all seasons indicates that it plays an important role in the ecology of large bluegills, suggesting that maintaining or, when absent, establishing suitable vegetation densities in lakes to provide habitat heterogeneity. Moderately dense vegetation may be necessary to improve or maintain a large bluegill population size structure. However, in lakes that have a limited distribution of submergent vegetation, large bluegills may be more susceptible to harvest than in lakes with widespread, patchy vegetation. While anglers in my study did not target large bluegills accurately, catch and harvest estimates indicated that anglers locate bluegills with sufficient precision to maintain relatively high harvest rates. If the angling effort on Enemy Swim Lake remains high, managers should monitor this population closely to safeguard against decreasing size structure. Additional information, specifically exploitation rates of bluegills \u3e200 mm should be collected to aid in managing this bluegill population