Climate-mediated changes in habitat use by lake trout ( Salvelinus namaycush)

I determined habitat preferences of lake trout (Salvelinus namaycush ) and investigated the link between climatic conditions, lake thermal stratification, and lake trout distribution. Acoustic telemetry was used to continuously (24 h/d) monitor the depth distribution of lake trout over two climatically different years (2003 and 2004). All data were collected in a small boreal shield lake (Lake 373, Area = 27.3 ha, Max depth = 20.8 m) at the Experimental Lakes Area in northwest Ontario, Canada. Lake trout were distributed over a broad range of temperatures (2--18°C) during thermal stratification, and based on daily median depths, the "classic" thermal habitat boundary (8--12°C) did not adequately define the habitat of tagged fish. Seasonal and annual variations in fish depth were primarily associated with temperature-mediated changes in the timing and depth of thermal stratification. Estimation of theoretical habitat volumes based on combinations of temperature (< 12 or 15°C) and dissolved oxygen concentration (> 4 or 6 mg/L) produced the most accurate estimates of habitat occupied by the tagged fish. Annual differences in habitat use indicated less use of productive shallow water habitats in climatically warmer conditions. To explain the behaviour of tagged fish and explore its potential consequences, I developed an individual-based population model that uses lake trout bioenergetics and life history characteristics to predict depth (and temperature), growth, and survival of lake trout over changing thermal conditions. Depths and temperatures occupied by tagged fish closely resembled simulated fish depths and temperatures, suggesting the model incorporates underlying factors affecting lake trout behaviour. Model output showed a three-fold reduction in growth and fitness during climatically warmer conditions. The model was then run with thermal profiles projected over a series of climate change scenarios by changing observed mean annual air temperatures by -2, 0, +2, +4, and +9°C. This analysis suggested that persistent increases in temperature and longer stratification patterns could challenge the fitness of many lake trout populations.