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Constraints to growth of Lake Superior lake trout, Salvelinus namaycush
T.B. Johnson1, L. Hartt1, and D.M. Mason2
1 Ontario Ministry of Natural Resources
Lake Erie Fisheries Station
Wheatley, Ontario N0P 2P0
2 National Oceanic and Atmospheric Administration
Great Lakes Environmental Research Laboratory
2205 Commonwealth Blvd.
Ann Arbor, MI 48105-2945
We describe lake trout Salvelinus namaycush growth in Lake Superior management unit WI-2 between 1950 and 1999. Growth was described using both interannual and cohort based analyses employing size-at-age, instantaneous growth and vonBertalanffy growth models. Growth of lake trout has declined over the past 50 years. Reduction in growth was most notable after the early 1980s and was greater for wild than hatchery lake trout. In an attempt to explain the decline in growth, we evaluated several constraints including density-dependence, prey resource limitation, prey quality, thermodynamics and sea lamprey parasitism. We found little support for any of our expected hypotheses on growth constraints. A simplified bioenergetic and foraging model suggests that, while prey abundance and biomass are low in Lake Superior, they are sufficient to meet energetic demands in most years. The large thermal mass of Lake Superior may buffer against any significant inter-annual differences in climate that could affect lake trout thermal habitat volume and associated thermodynamic effects on growth. Future analyses such as these would benefit from more precise aging techniques, including using back-calculated growth rates from otoliths of single fish.