Impacts of Variation in Thermal Habitat on Lake Trout Population Traits

Climate variation has the potential to significantly reduce the amount of cold, well-oxygenated water available to fish, especially during periods of stratification in temperate lakes.  The volume of habitat available to cold water fish species, like lake trout, reaches a minimum at the peak of stratification when fish become constrained to areas of the lake bounded between upper and lower physiological tolerance limits set by water temperature (15 °C) and dissolved oxygen concentrations (>6 mg/L), respectively.  As such, lake trout are extremely sensitive to anthropogenic disturbance, including climate-mediated losses in cold-water habitat from thermocline deepening – a consequence of increasing air temperatures.  This is especially critical for lakes without cold-water pelagic prey species, whereby lake trout are forced to forage in thermally sub-optimal regions of the lake to access their prey.  In this study, we examine the behavioural and population-level responses of lake trout to climate-mediated changes in habitat volume from well-studied lakes at the Experimental Lakes Area (ELA) in northwestern Ontario.  ELA lakes are characterized as small, oligotrophic boreal systems with littoral cyprinids as the dominant forage.  For the past 10 years we have been monitoring the spatial and pelagic distribution of lake trout in several lakes using acoustic telemetry.  In addition, we have been monitoring the size-structure, age, condition, growth, recruitment and abundance (Jolly-Seber mark-recapture models) of these populations from annual trap-netting data for >30 years.  We predict that climate variability, defined as the intensity and duration of stratification, will limit available cold-water habitat for lake trout, which in turn will result in changes in their ability to access profitable prey in warm-water habitats.  We further predict that limited access to forage fish will be manifested through changes in population traits, such as decreased fish growth and condition.  Our preliminary analyses show that lake trout respond to climate variability, reducing warm water use during periods of more intense and longer stratification.  We are currently examining whether there are population-level changes in traits associated with climate-mediated loss of lake trout habitat.