Bloater (Coregonus hoyi) is the last remaining member of the Great Lakes deepwater cisco complex in Lake Michigan, a group once composed of six named species. The vertical migration and nighttime vertical distribution of adult bloater were investigated during late summer of 1995 and 1996 in Lake Michigan. This research was conducted over bottom depths of 60-115 m using simultaneous acoustics and midwater or bottom trawling. In order to determine if bloater migrate to feed, bloater diet composition and times and depths of feeding were also determined.

Bloater were distributed at night throughout 30-65 m of water, depending on bottom depth. Maximum distances of migration were not related to incident light levels or cloud cover. Nighttime midwater densities ranged from 0.00 to 6.61/1000 m3 and decreased with increasing bottom depth. Length and weight distributions did not differ between migrating and non-migrating bloaters. In addition, target strength distributions by 5-m strata indicated that migrating bloater did not stratify by size in the water column at night. However, at most sites, daytime bottom trawls collected a greater proportion of larger individuals compared to nighttime midwater or bottom catches. This trend was not likely due to a problem of availability of larger bloater to nighttime trawls, because no size stratification was detected in midwater at night.

Observations of bloater vertical migration showed a limit to the vertical depth changes that bloater experienced. The effects of hydrostatic pressure on swim bladder volume at shallowest depths of migration were compared to the effects at corresponding bottom depths using Boyle's Law. Results indicated that swim bladder volume on lake bottom was reduced to approximately 50-60% of the volume in midwater. This trend suggested that daily vertical movement in bloater may be limited by a maximum difference in swim bladder volume for which bloater can compensate.

Overall, patterns in frequency of empty stomachs and mean digestive state indicated that a portion of the bloater population was migrating to feed, but patterns in dry weight of stomach contents did not suggest nighttime midwater feeding. Trends in dry weight may be masked by cold water temperatures of the hypolimnion and resulting decreases in digestion and evacuation rates. Bloater diet composition indicated some midwater feeding, yet occurrence of benthic prey in the diet suggested that bloater were also feeding on the bottom. In sum, although a portion of the bloater population fed in the water column at night, bloater were not limited to feeding at this time. Current bloater feeding patterns may not reflect historic feeding patterns because of changes in the Lake Michigan fish community in the past 50 years, including the invasion of alewife (Alosa pseudoharengus) and extirpation of the other deepwater ciscoes (Coregonus spp.) in Lake Michigan.