Vertical migration and nighttime distribution of adult bloaters in Lake Michigan

The vertical migration and nighttime vertical distribution of adult bloater s Coregonus hoyi were investigated during late summer in Lake Michigan usin g acoustics simultaneously with either midwater or bottom trawling. Bloater s remained on or near bottom during the day. At night, bloaters were distri buted throughout 30-65 m of water, depending on bottom depth, Shallowest de pths of migration were not related to water temperature or incident light. Maximum distances of migration increased with increasing bottom depth. Nigh ttime midwater densities ranged from 0.00 to 6.61 fish/1.000 m(3) and decre ased with increasing bottom depth. Comparisons of length distributions show ed that migrating and nonmigrating bloaters did not differ in size. However , at most sites, daytime bottom catches collected a greater proportion of l arger individuals compared with nighttime midwater or bottom catches. Mean target strengths by 5-m strata indicated that migrating bloaters did nor st ratify by size in the water column at night. Overall, patterns in Frequency of empty stomachs and mean digestive state of prey indicated that a portio n of the bloater population fed in the water column at night. Bloater diet composition indicated both midwater feeding and bottom feeding. In sum, alt hough a portion of the bloater population fed in the water column at night, bloaters were not limited to feeding at this time. This research confirmed that bloaters are opportunistic feeders and did not fully support the prev iously proposed hypothesis that bloater vertical migration is driven by the vertically migrating macroinvertebrate the opossom shrimp Mysis relicta.