Depth segregation of deepwater ciscoes (Coregonus spp.) in Lake Michigan during 1930–1932 and range expansion of Coregonus hoyi into deeper waters after the 1990s
Bunnell, David B.; Eshenroder, Randy L.; Krause, Ann E.; Adams, Jean V.
The deepwater cisco (Coregonus spp. ) assemblage in Lake Michigan was among the most diverse in the Laurentian Great Lakes. During the 1930s, seven species were identified, but by the early 1970s, six species were extirpated, primarily due to overfishing, leaving only one species (C. hoyi). To quantify the degree of depth segregation that occurred within this assemblage, we applied generalized additive models (GAM) to gillnet data collected in 1930–1932 from the R/V Fulmar. Our results generally supported findings published earlier. C. hoyi and C. reighardi typically exhibited peak catch-per-unit-effort (CPUE) in the shallowest depths (50–68 m) and C. kiyi and C. nigripinnis always exhibited peak CPUE in the deepest depths (127–150 m). The peak CPUEs for C. alpenae and C. zenithicus, species described previously as occupying waters of “intermediate” depth, occurred at intermediate depths (88–94 m) in the northern region and at shallow depths (50 m) in the southern region. The nearly complete overlap in their depth distributions was consistent with a prior hypothesis that C. alpenae was synonymous with C. zenithicus. The depth of peak CPUE of C. johannae, previously described as inhabiting intermediate depths, was highly variable, ranging from very shallow (50 m) to very deep (150 m), depending on region and season. We found no clear evidence that deepwater ciscoes changed depths for spawning. We found little evidence that deepwater cisco species collected in gillnets during 1930–1932 were distributed by depth as a function of fish size. C. hoyi collected by bottom trawl from 1974 to 2007 exhibited a positive relationship between depth and fish size. When we applied a GAM to the 1974–2007 trawl data, we found that C. hoyi shifted to markedly deeper waters from 1988–1991 to 2004–2007. Based on the ability of C. hoyi to shift to deeper depths, this species should occupy deeper waters in the lower Great Lakes (Erie, Ontario) should managers seek reintroduction of native deepwater ciscoes.