Spatial distribution of lake trout (Salvelinus namaycush) across seasonal thermal cycles in a large lake
Authors: Ivanova Silviya V. , Johnson Timothy B. , Metcalfe Brent , Fisk Aaron T. 
Published: December 2020

Animal movements are influenced by the environment they inhabit and the need to maximise fitness and minimise cost. As such, seasonal thermal cycles in temperate lakes play an important role in the selection of habitat by species. Lake trout (Salvelinus namaycush) is a native top predator in the Laurentian Great Lakes and currently under rehabilitation in Lake Ontario. This cold, deep‐water species is known to migrate to shallower depths in the autumn to spawn, but their spatial and seasonal distribution have not been examined in detail.

We quantified the residency and home ranges of 24 lake trout in eastern Lake Ontario across a full year (1 May 2017 to 30 April 2018) using acoustic telemetry to assess the influence of seasonal thermal cycles. Specifically, we used three thermal logging stations in the eastern basin, 164 acoustic receivers, and a total of over 1,000,000 detections to describe seasonal distribution. We also documented occurrences of long‐distance movements (via 130 acoustic receivers located in the western basin of Lake Ontario), and thus the potential spatial overlap of populations from the eastern and western basins.

During stratification (1 July–1 November), lake trout (n = 24) showed a horizontally restricted distribution in regions of deeper water in eastern Lake Ontario. A variable and broad distribution was observed around the shallower Kingston Basin shoals during isothermal (1 January–30 April), spring warming (1 May–30 June; spring mix), and autumn cooling (2 November–31 December) periods. Home range sizes ranged from 0.1 to 3,966 km2 among all thermal cycles, with the largest observed during cooling autumn mix conditions driven by four individuals. Large variation in home range area was observed among individuals, and, thus, means were not statistically different between seasons. Three individuals occupied shallower shoals even during stratified conditions, and another individual crossed the entire lake from the east to the west end of Lake Ontario, travelling over 200 linear km in 17 days.

Our results confirm that thermal regimes and spawning needs affect the spatial use of lake trout in Lake Ontario, but demonstrate that there are broad distributions during isothermal conditions, highly individualistic spatial utilisation, and inter‐individual variation in spatial distribution and exploratory behaviour. All of these behaviours are consistent with other top predator species.

Defining the spatial utilisation and distribution of individuals are important steps toward a better understanding of reintroduced species ecology in freshwater ecosystems. The variation of individual lake trout distribution across seasons implies that individuals may have different influences on the overall ecosystem function and potentially different responses to increasing water temperatures. These results suggest that an adaptive management approach is required when rehabilitating populations of this native fish where populating multiple habitats in an ecosystem is an objective in the face of a changing environment.