The Ecology of Body Size and Depth Use by Bloater (Coregonus hoyi Gill) in the Laurentian Great Lakes: Patterns and Hypotheses

Deepwater ciscoes (genus Coregonus, subgenus Leucichthys) radiated into six phenotypes that occupy different depths in the Great Lakes, based upon body size and lipid content. Large, lipid-dense ciscoes occupy greater depths than small, lean ciscoes. This relationship is observed between adults and juveniles of the most prevalent deepwater cisco, the bloater (C. hoyi Gill). Lipid-dense adult bloaters are restricted to the hypolimnion, whereas lean juveniles are found primarily in the epilimnion. This article critically reviews, synthesizes, and provides hypotheses from the literature on the ecology of body size and depth use of bloater. Factors influencing depth use in bloater are categorized by Fry's (1971) environmental factors. The case is made for two parsimonious hypotheses to explain the depth distribution by body size of bloater: (1) the optimal foraging-antipredation (OFA) hypothesis, and (2) the mass-specific metabolism hypothesis. The OFA hypothesis relates abundance of piscivores with abundance and extent of diel vertical migration of bloater. The mass-specific metabolism hypothesis relates body size to density-dependent growth, metabolism, swimming activity and hence depth distribution of bloater. Follow-up hypotheses and predictions are presented and may broaden our understanding of trophic ecology and adaptive radiation of deepwater ciscoes in the Great Lakes.