Empirical links between thermal habitat, fish growth, and climate change

We used historical data to assess the effect of variation in thermal strati fication due to climatic conditions on the annual growth of smallmouth bass (Micropterus dolomieui) and lake trout (Salvelinus namaycush) in Lake Opeo ngo in Algonquin Park, Ontario, and of rainbow smelt (Osmerus mordax) and y ellow 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 shall ower thermocline. Lake Opeongo smallmouth bass. which are at the northern e dge of the species' distribution, grew better as the availability and durat ion of warmer epilimnetic water temperature increased. Notable climate sign als 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 Nino year i n the database. Lake trout growth was poorer in years of early stratificati on, but was not otherwise associated with stratification variables, suggest ing that springtime feeding on littoral prey is a major determinant of grow th for this hypolimnetic species. The growth of young rainbow smelt, which feed in the epilimnion, declined when epilimnion volumes contracted but old er smelt, which occupy the hypolimnion. grew better. Greater growth of yell ow perch was also associated with a shallower, more isothermal epilimnion. Stratification characteristics that reflected the thermal and feeding habit ats' spatial or temporal boundaries accounted for, on average, 44% of the v ariation in fish growth. Our findings should be useful in forecasting how c limate change signals may filter through thermal habitats and effect fish g rowth.