Lake Habitat Occupancy By Brook Trout Across a Lake Size Gradient

Preferred temperatures define habitat envelopes for fish in many studies.  We conducted multi-pass surveys of nine lakes across a lake size gradient to determine if habitat occupancy by brook trout matches their habitat envelope defined by preferred temperatures.   North American standard gillnets were employed in short duration sets (1 hour) in stratified random depth surveys of the lake set.  We modeled habitat occupancy as a non-linear function of depth for brook trout living in the lakes and compared this to preferred temperature range as provided by temperature depth profiles.  In general habitat occupancy captured some of the habitat envelope but diverged in warmer waters.  In large lakes (>500 ha) there was a common lake model of occupancy (high rank; AIC) but in small lakes it appeared that lake-specific factors limited a common model as a representation of occupancy (low rank; AIC).  Reconciling occupancy (where fish live) with envelopes (where fish ought to live) will continue to be a challenge for understanding the effects of climate change on fish populations when scoping relies purely on a habitat envelope approach.