Habitat Occupancy by Lake Whitefish Includes An Ecotone In the Profundal Zone of Two Canadian Shield Lakes

Temperature and dissolved oxygen preferences have traditionally defined volumetric habitat for lake whitefish (Coregonus clupeaformis) without regard to spatial structure.  The purpose of this study was to determine if structure was important in defining lake whitefish habitat.   We estimated habitat occupancy for lake whitefish using a multi-pass survey approach based on randomly-stratified gillnets in two lakes (Lake Opeongo and Smoke Lake), separating detection from occupancy as a function of habitat covariates.  Temperature, light, depth, and substrate habitat via acoustic classification were modeled at two different spatial scales, the scale of the net employed and a presumed movement scale.  Both scales demonstrated the importance of depth and proximity to a previously underappreciated ecotone in the profundal zone.   Model selection supported both depth and a substrate transition zone known as the E2 ecotone (the gradient between softness-hardness) for describing occupancy of lake whitefish in Lake Opeongo.  Lake whitefish occupied deeper water in Smoke Lake and did not adhere to the substrate ecotone.  Diurnal behaviour influenced the detection process with morning activity leading to greater detection than afternoon activity.  The E2 ecotone aligns with internal wave structure of the thermocline and may have important implications for other benthivorous fish in Canadian Shield lakes.