Ecological Study of Irogami Lake, Wisconsin, with Emphasis on Mitigating Bluegill Spawning Habitat

Abstract

An agreement in 1993 between the Wisconsin Department of Transportation and Wisconsin Department of Natural Resources called for mitigation of 0.54 acres (0.22 ha) of lost centrarchid spawning habitat in Irogami Lake, Waushara County, Wisconsin, due to future relocation of State Trunk Highway 21. An ecological study was conducted to determine recommendations for restoring the lost habitat, as well as to assess the pre-construction characteristics of the bluegill Lepomis macrochirus and largemouth bass Micropterus salmoides populations and species composition of aquatic invertebrates and plants in the construction area. Data were collected from April through August in 1993 and 1994. Physical habitat characteristics of bluegill spawning colonies in all areas of the lake were documented. Substrate, bottom gradient, water depth, distance from shore, and relation of nests to cover are the characteristics on which the mitigation plan was based. Fish were caught with fyke nets, boat-mounted electrofishing gear, and seines. Scale samples were collected from bluegills and largemouth bass for age determination. Bluegills built colonies in areas with mean (range) bottom gradient of 6% (1.1-13.2%), depth of 89 cm (65-119 cm) and distance from shore of 19.1 m (1.5-86.9 m). Spawning substrate particle size ranged from fine (< 0.5 mm) to cobble (64-256 mm). About half of the colonies (56%) were greater than 5 m from cover. Length increments of bluegills of ages 1-7 were not significantly different from state (P = 0.17) and regional (P = 0.36) increments. Length increments of largemouth bass were significantly smaller than average state (P = 0.05) and regional (P = 0.02) increments. Mortality rates, based on fyke net catches, of bluegills of ages 2-7 were Z = 0.314 and A= 27%, and of ages 7-8 were Z = 2.1 and A= 88%. Mortality rates, based on electrofishing catches, of largemouth bass of ages 4-7 were Z = 0.27 and A = 24%, and of ages 7-8 were z = 1.9 and A= 85%. Recommendations for recreating spawning habitat include: placing the spawning bed along the new highway; rip-rapping of the lake shore in that area; laying a 15.2-cm blanket of substrate consisting of 80% (by weight) of gravel 6.3-19 mm diameter with an allowance of no more than 10% gravel of 19 -200 mm diameter, and 20% consisting of fine gravel and coarse sand containing no more than 5% of particles less than 0.5 mm diameter, on the preexisting bottom slope; maintaining 0.46-1.2 m of water on top of the substrate; and adding a 2:1 slope to the outer edge of the new spawning bed. The new spawning bed would cover 0.6 acres (0.25 ha) of lake bottom.