Empirical Links between Thermal Habitat, Fish Growth, and Climate Change

Abstract
We used historical data to assess the effect of variation in thermal stratification due to climatic conditions on the annual growth of smallmouth bass (Micropterus dolomieui) and lake trout (Salvelinus namaycush) in Lake Opeongo in Algonquin Park, Ontario, and of rainbow smelt (Osmerus mordax) and yellow perch (Perca flavescens) in South Bay of Lake Huron, Canada. In these two lakes, years with warmer air temperatures produced an earlier onset of stratification, a warmer epilimnion, a larger thermal gradient, and a shallower thermocline. Lake Opeongo smallmouth bass, which are at the northern edge of the species' distribution, grew better as the availability and duration of warmer epilimnetic water temperature increased. Notable climate signals in the growth of smallmouth bass were detected in 1992, a cool year due to the eruption of Mt. Pinatubo, and in 1983, the strongest El Niño year in the database. Lake trout growth was poorer in years of early stratification, but was not otherwise associated with stratification variables, suggesting that springtime feeding on littoral prey is a major determinant of growth for this hypolimnetic species. The growth of young rainbow smelt, which feed in the epilimnion, declined when epilimnion volumes contracted but older smelt, which occupy the hypolimnion, grew better. Greater growth of yellow perch was also associated with a shallower, more isothermal epilimnion. Stratification characteristics that reflected the thermal and feeding habitats' spatial or temporal boundaries accounted for, on average, 44% of the variation in fish growth. Our findings should be useful in forecasting how climate change signals may filter through thermal habitats and effect fish growth.

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