Effects of temperature and density on mercury accumulation in Atlantic salmon ( Salmo salar )

Environmental temperature is a master variable governing growth and metabolism of fishes in aquatic food webs, which in turn influences the flow and accumulation of trace elements, including toxic forms of mercury (Hg). Growth can be influenced by population density, with high densities consistently associated with lower growth rates. Given the potential for climate change to simultaneously increase stream temperature and affect population density, understanding how these factors interact to influence Hg accumulation and concentration allows for better understanding of the risks of Hg load to secondary consumers. Under laboratory conditions and ad libitum feeding, we assessed Hg concentrations in age-0 Atlantic salmon (Salmo salar) at cool (near the thermal optimum for growth) and warm (above the thermal optimum for growth) temperatures (16 vs. 23°C) and at low and high fish density (0.53 vs. 2.65 fish/litre). In all treatments, fish grew rapidly, resulting in an approximate 3- to 4-fold increase from initial size. Specific growth rate was not significantly different between the warm (2.98 ± 0.86%/day) and cool (3.15 ± 0.73%/day) temperature treatments but was significantly faster at low (3.41 ± 0.86%/day) vs. high (2.99 ± 0.77%/day) density treatments. Mercury accumulation was related to both temperature and density, with the highest Hg concentrations under warm temperature and high population density conditions. Within treatments, individuals with higher growth had lower Hg at the warm temperature but equivalent or slightly lower Hg at the cool temperature. This interaction among temperature, growth rank and Hg likely contributed to the lack of an overall experiment-wide relationship between individual body size and Hg concentration. Our results indicate that even when fish manage to maintain sufficient growth rates under increasing temperatures, energetic considerations (changes in foraging and metabolic rate) may increase Hg contaminant concentration. In addition, the influence of density on energetics and growth, even when food resources are not limited, makes it essential to consider trajectories of environmental change that also influence abundance and population density when assessing contaminant pathways. These findings help clarify how growth and body size relate to contaminant accumulation, with important implications for understanding Hg contamination under varying environmental change scenarios.