Influence of prey type and environment on Lake Trout (Salvelinus namaycush) activity, mercury dynamics, and life histories

Abstract

Understanding food web structure in lentic ecosystems is a requirement for determining pathways and rates of energy flow, yet predator-prey interactions can be highly dynamic across space (i.e., between- and within-lakes) and time (e.g., diel daily cycles, seasonal). Differences in prey availability can influence predator life history traits and behaviours, though life histories can also vary across environmental gradients. Metabolic theory predicts that variation in prey and habitat availability affects metabolic rates by modifications to active behaviours, though the precise conditions controlling variation in activity have rarely been empirically assessed. Further, the relative influence of activity via food web structure (compared with environmental characteristics) on life history traits and contaminant dynamics remain poorly understood. Here, I integrate information regarding both prey availability and environmental factors to gain novel insights into their combined effects on Lake Trout activity, contaminant accumulation and life histories. Using stationary hydroacoustics, I directly observed higher activity in planktivorous Lake Trout populations that exhibited sustained day-night swimming and higher active metabolism, which contrasted with piscivorous populations that had distinct day-night behavioural shifts, including nighttime rest. Planktivorous Lake Trout maintained faster, more complex swimming paths, exhibited higher oxygen consumption, and were more frequently observed in suboptimal oxythermal conditions than piscivorous Lake Trout. Secondly, Lake Trout mercury (Hg) concentrations were affected by prey availability but not according to classical contaminant food chain length theory; rather, Hg accumulation slopes against body size were more shallow when Mysis diluviana was present than when they were absent, independent of overall food chain length. Increasing dissolved organic carbon concentrations (DOC) were also positively associated with Lake Trout Hg, and the combined use of prey availability and DOC proved essential for predicting whether Lake Trout Hg concentrations exceeded Canadian consumption guidelines for a given size. Lastly, life history traits in Lake Trout were best explained by models combining both food web structure and environmental variables; adult length and size-at-maturity generally increased with greater prey availability, while environmental variables such as lake size and total phosphorus were key for describing trait variation within categories of prey availability. These findings provide empirical, field-validated support that prey availability can play as important of a role as environmental characteristics in shaping the life history traits of predatory fishes, like Lake Trout, through effects on activity, metabolic rates, and foraging opportunities.