Search

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. [ABSTRACT FROM AUTHOR]

The relationship between food‐web structure (i.e., trophic connections, including diet, trophic position, and habitat use, and the strength of these connections) and ecosystem functions (i.e., biological, geochemical, and physical processes in an ecosystem, including decomposition, production, nutrient cycling, and nutrient and energy flows among community members) determines how an ecosystem responds to perturbations, and thus is key to understanding the adaptive capacity of a system (i.e., ability to respond to perturbation without loss of essential functions). Given nearly ubiquitous changing environmental conditions and anthropogenic impacts on global lake ecosystems, understanding the adaptive capacity of food webs supporting important resources, such as commercial, recreational, and subsistence fisheries, is vital to ecological and economic stability.Herein, we describe a conceptual framework that can be used to explore food‐web structure and associated ecosystem functions in large lakes. We define three previously recognised broad habitat compartments that constitute large lake food webs (nearshore, pelagic, and profundal). We then consider, at three levels, how energy and nutrients flow: (a) into each basal resource compartment; (b) within each compartment; and (c) among multiple compartments (coupling). Flexible shifts in food‐web structures (e.g., via consumers altering their diet or habitat) that sustain these flows in the face of perturbations provide evidence for adaptive capacity.We demonstrate the conceptual framework through a synthesis of food‐web structure and ecosystem function in the Laurentian Great Lakes, with emphasis on the upper trophic levels (i.e., fishes). Our synthesis showed evidence of notable adaptive capacity. For example, fishes increased benthic coupling in response to invasion by mussels and round gobies. However, we also found evidence of loss of adaptive capacity through species extirpations (e.g., widespread collapse in the abundance and diversity of ciscoes, Coregonus spp., except in Lake Superior).In large freshwater lakes, fishery managers have traditionally taken a top‐down approach, focusing on stocking and harvest policy. By contrast, water quality managers have focused on nutrient effects on chemical composition and lower trophic levels of the ecosystem. The synthesised conceptual model provides resource managers a tool to more systematically interpret how lower food‐web dynamics influence harvestable fish populations, and vice versa, and to act accordingly such that sustainable resource practices can be achieved.We identify key gaps in knowledge that impede a fuller understanding of the adaptive capacities of large lakes. In general, we found a greater uncertainty in our understanding of processes influencing energy and nutrient flow within and among habitats than flows into the system. [ABSTRACT FROM AUTHOR]