Life at the top: Lake ecotype influences the foraging pattern, metabolic costs and life history of an apex fish predator

Abstract

We used acoustic telemetry and acceleration sensors to compare population‐specific measures of the metabolic costs of an apex fish predator living in four separate lakes. We chose our study species and populations to provide a strong test of recent theoretical predictions that optimal foraging by an apex fish predator in a typical aquatic environment would be consistent with feeding to satiation rather than continuous feeding. We chose four populations where the primary prey type differed along a body size gradient (from small invertebrates to large planktivorous fish) and along a thermal accessibility gradient (from easily accessible cold‐water pelagic prey to less accessible warm‐water epilimnetic and littoral prey). We expected that these gradients in prey type would evoke distinctly different activity gradients depending on whether predators fed to satiation (e.g., less frequent “rest” detections where primary prey are smaller/less accessible) or fed continuously (e.g., fixed level of “rest” detections under all prey conditions). Our study organism was a fall spawning, cold‐water visual apex predator (lake trout). Therefore, we focused our study on diel (early night, dawn, day, dusk, late night) changes in metabolic costs associated with summer feeding behaviour. The duration (~20 days) and fine temporal scale (~30 min) of our behavioural data provided a uniquely detailed picture of intra‐ and inter‐population differences in activity patterns over a critical period in the annual growing season. In all populations, diel shifts in activity were qualitatively consistent with that expected of a visual predator (e.g., resting state detections were most frequent at night). Between‐lake differences in daytime thermal experience were qualitatively consistent with between‐lake differences in the location of primary prey (e.g., excursions to warm habitats were common in lakes with epilimnetic/littoral fish as primary prey and relatively rare in lakes with pelagic cold‐water invertebrates/fish as primary prey). Daytime activity patterns were more consistent with the feeding pattern expected from feeding to satiation rather than continuous feeding: (a) individuals in all four populations exhibited clearly delineated bouts of resting behaviour and active behaviour; (b) the frequency of resting bouts and the resultant overall cost of daily activity were strongly associated with the size and accessibility of prey—in lakes with smaller and/or less accessible prey, predators rested less frequently, exhibited marginally higher costs when active and had higher overall daytime activity costs. Within each lake, similar changes in activity occurred concurrently with diel changes in prey accessibility/relative density.

Winter in water: differential responses and the maintenance of biodiversity

“…4a and c). Access to a high‐quality prey during open water months, which increases lake trout growth and condition (Cruz‐Font et al ), could carry over and help promote more sustained growth in winter, although more work is needed to explore this idea. Compared to winter‐active species, winter‐inactive species suppress metabolic costs through reduced activity and feeding and therefore have little capacity for overwinter growth (Micucci et al ).…”

Winter in water: Differential responses and the maintenance of biodiversity

“…4A, C). Access to a high-quality prey during open water months, which increases lake trout growth and condition (Cruz-Font et al 2019), could carry over and help promote more sustained growth in winter, although more work is needed to explore this idea. Compared to winter-active species, winter-inactive species suppress metabolic costs through reduced activity and feeding and therefore have little capacity for overwinter growth (Micucci et al 2003).…”

“…However, empirical studies can apply a range of tools for mapping how species interactions change seasonally and from year to year (e.g., across years with short and long winters). Acoustic telemetry to measure the 3D movements and activity of fish in the wild particularly promising (Cruz-Font et al 2019), especially when coupled with repeated sampling to obtain stomach contents or tissue for dietary analysis (e.g., via stable isotope or fatty acids; Guzzo et al 2017).…”

Thermal preference influences depth use but not biomass of predatory fishes in response to lake morphometry

“…Many species are responding to global environmental change by altering their degree of habitat coupling (Bartley et al in press), i.e., the linking of spatially-disparate habitats through movement and foraging (Schindler and Scheuerell 2002). Top predators in particular, which tend to be highly mobile and generalists, seem to respond to environmental conditions in ways that shape their local food webs (Cruz-Font et al 2019, Hammerschlag et al 2019). Habitat coupling appears to be a fundamental component of food architecture that, importantly, can stabilize food webs (Rooney et al 2008, McCann and Rooney 2009).…”

“…During summer, warm surface waters often exceed the temperature preferences of cold-water fish, increasing the metabolic costs associated with occupying nearshore habitats. In response, the cold-adapted top predator lake trout responds by retreating to the offshore to forage and at most accessing nearshore resources in quick forays (Guzzo et al 2017b, Cruz-Font et al 2019). Other cold-water fishes exhibit similar responses across natural gradients in lake temperature (Bartley et al in press, 2018, Tunney et al 2014), suggesting that thermal accessibility may be limiting for many species.…”