Thermal habitat fragmentation in stratified lakes induces resource waves that brook charr track across seasons

The spatial configuration of thermal habitats constrains the thermoregulatory performance of ectotherms. Thermal landscapes also vary through time, which is particularly relevant in seasonal environments such as temperate lakes. Indeed, elevated temperatures in the epilimnion of dimictic lakes during summer could substantially reduce the use of this habitat by cold-stenothermic fish during the stratified period. The main objective of this study was to evaluate whether thermal habitat fragmentation in stratified lakes modulates accessibility to resources that brook charr, Salvelinus fontinalis, which is a mobile consumer, can track across seasons. More specifically, we hypothesize that reduced access to the littoral habitat during summer enhances foraging opportunities in this habitat during winter. We used an automatic acoustic telemetry system offering full coverage of the lake to continuously record brook charr locations across seasons, and we estimated zoobenthos abundances in the littoral habitat using image processing and semi-automatic classification. While brook charr concentrate in the metalimnion of the pelagic habitat in summer, most individuals in winter shift to a shallow bay that is unexploited in summer due to thermal constraints. In this habitat, zoobenthos abundance is more than twice as high at the end of the summer compared to littoral habitats close to the thermal refuge in the pelagic habitat. Surprisingly, brook charr showed strong within-lake site fidelity between two consecutive summers, which suggests that spatial memory could be a key driver of seasonal habitat use in this lacustrine population. Overall, our results suggest that thermal barriers create fragmentation between littoral and pelagic habitats that in turn produces resource opportunities that brook charr can track across seasons.

Methods
The study was carried out in Lake Ledoux (46° 38’ N, 73° 15’ W), Mastigouche Wildlife Reserve, Québec, Canada, from 2016 to 2018. Lake Ledoux is a typical small oligotrophic temperate zone lake with respect to surface area (11.9 ha), mean depth (5.5 m), maximum depth (18.0 m), and general physicochemical characteristics (Gignac-Brassard et al. 2023, Magnan 1988). Brook charr is the only fish species in the lake, and sport fishing is rigorously controlled by the Québec Government (Bourke et al. 1996). The lake was closed to fishing during the three years of the study, but illegal fishing was observed in Lake Ledoux (M. Pépino, personal observation; see details in Supplement 1).

We deployed acoustic telemetry positioning arrays to continuously follow brook charr movements and habitat use at fine spatiotemporal scales (<1 min, <5 m resolution) using two different VEMCO Positioning Systems (VPS). From 5 to 7 July 2016, we deployed 10 receivers (VR2Tx-069k-111-BAT; VEMCO Inc., Halifax, NS, Canada) and two reference tags (V9TP-2x-069k-1-0034m; VEMCO Inc.). From 7 to 8 November 2016, four additional receivers were deployed to allow better coverage of the study lake during winter, especially in the shallow bay in the western section of the lake (Fig. 1; Supplement 1). This VR2 positioning system was operational from 7 July 2016 until 22 May 2017, but full coverage of the lake was not accomplished until we added the four additional receivers in November 2016. Furthermore, the first VPS analyses revealed potential “collisions” between tag signals, which occur when individuals are in proximity and create interference during signal transmission. In 2017, we changed to the HR2 VPS technology, which was specifically developed to circumvent this problem. This system consisted of 23 receivers (HR2-180k-100; VEMCO Inc.) and four reference tags (V9TP-2x-180k-34m; VEMCO Inc.). This HR2 VPS was operational from 21 June 2017 until 6 November 2018 and provided full coverage of the lake. Details of the VR2 and HR2 VPS as well as VPS performance are provided in Supplement 1.