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137 Recent Changes in Successional State of the Deep-Water Fish Communities of Lakes Michigan, Huron, and Ontario and Management Implications Randy L. Eshenroder and Brian F. Lantry Of the five Great Lakes, Lakes Michigan, Huron, and Ontario are most alike in their morphometry (fig. 1) and fish communities. All three lakes are characterized by extensive, deep offshore waters and massive hypolimnia that, before European colonization, were dominated by various forms of ciscoes (Coregonus spp.)andLakeTrout(Salvelinus namaycush) (Eshenroder and Burnham-Curtis 1999). Among these fishes, the most-specialized forms were the results of partial speciation events that began at the end of the Ice Age (Pleistocene) and were driven by feeding opportunities in the deeper waters, what here we call the deep pelagia. The deep pelagia comprise a zone of nearly year-round constant temperature (about 4°C) and can be visualized as the core of the lake. It begins at approximately the fifty meter contour, depending on the lake, and does not include surface waters, which, during late spring through autumn, are a separate, warmer water mass. The waters immediate to the lakebed are technically not part of the pelagia, as the term implies open waters, but many of the fishes and invertebrates that utilize the pelagia inhabit the lake bed periodically. The deep pelagia comprise about 60 percent of the average volume of Lakes Michigan, Huron, and Ontario and, therefore, are important not only because of their size, but because they once supported the dominant fishes of these lakes. Lakes Michigan, Huron, and Ontario are among the seventeen largest by volume and twelve deepest (mean depth) lakes in the world (Beeton 1984). The types of freshwater pelagic fishes that could fully utilize their deepest waters (> 100m) are, therefore, rare. In North America, such fishes most likely would have perished during the last Ice Age (Eshenroder et al. 1999; Eshenroder 2008), because the refugia for north temperate fishes during the maximum extent of ice were mostly rivers and small temporary lakes (Power 2002) that would have been inhospitable to such fishes. As the glaciers receded and fishes returned from theirrefugiatotheprecursorsoftheGreatLakes(theproglaciallakes),thosefisheswithafacultyfordeeper, pelagic waters, the ciscoes and charrs, encountered an environment uninhabited by fishes and, to them, novel. The deep pelagia were especially attractive to pelagic-type fishes, because the largest freshwater zooplankter in north temperate waters had also re-colonized the Great Lakes (Väinölä et al. 1994). Once in the Great Lakes, this species, Mysis diluviana, was hypolimnetic during the day and migrated into the Eshenroder and Lantry 138 upper hypolimnion-thermocline at night to feed on zooplankton (Johannsson et al. 2003). M. diluviana is apparently constrained to the deep pelagia by light and temperature (Boscarino et al. 2009). M. diluviana was an especially suitable prey for planktivorous fishes owing to its high energy content. One adult mysid is equivalent in energy to several thousand of the small cladocerans that are common diet items for planktivorous fishes in the Great Lakes (Eshenroder and Burnham-Curtis 1999). M. diluviana makes the most extensive and rapid vertical migration of all freshwater crustaceans (Penak 1953). Beeton (1960) observed nightly migrations of ninety-six meters in Lakes Michigan and Huron; and the rate of ascent was 0.5–0.8 m/min. Vertical migrations of this scope pose buoyancy-regulation problems for fishes, like ciscoes, that have swim bladders. Such fishes cannot maintain neutral buoyancy at both ends of the migration, because, if such a fish is neutral at the start of a migration, as it ascends, its swim bladder will over inflate and it must release gas to continue. Released gas is not available on the descent and cannot be replaced fast enough to compensate for compression of the swim bladder and loss of buoyancy owing to increased pressure with depth. The energetic penalty (increased upward swimming) for loss of buoyancy can amount to up to 60 percent of the energy requirements for a fish swimming at one body length per second (Alexander 1993). Cisco (Coregonus artedi) was likely an evolutionarily advanced planktivore when it reentered the proglacial lakes (Eshenroder et al. 1999), and it is assumed an ancestor of the complex of deepwater ciscoes FIG. 1. Map of the Great Lakes showing the 50-m contour. Map image courtesy of David Bennion, U.S. Geological Survey. Area of Lakes at 50m Depth Lake Erie 425 km2 Lake Ontario 12,625 km2 Lake Huron 30,025 km2 Lake Michigan 37,340 km2 Lake Superior 68,850 km2 Explanation...

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