Age, body size, growth and dietary habits: What are the key factors driving individual variability in mercury of lacustrine fishes in northern temperate lakes?

Fish total mercury concentration ([THg]) has been linked to various fish attributes, but the relative importance of these attributes in accounting for among-individual variation in [THg] has not been thoroughly assessed. We compared the contributions of ontogeny (age, length), growth (growth rate, body condition), and food web position (δ13C, δ15N) to among-individual variability in [THg] within populations of seven common fishes from 141 north temperate lakes. Ontogenetic factors accounted for most variation in [THg]; age was a stronger and less variable predictor than length for most species. Adding both indices of growth and food web position to these models increased explained variation (R2) in [THg] by 6-25% among species. Fish [THg] at age increased with growth rate, while fish [THg] at length decreased with growth rate, and the effect of body condition was consistently negative. Trophic elevation (inferred from δ15N) was a stronger predictor than primary production source (inferred from δ13C) for piscivores but not benthivores. Fish [THg] increased with δ15N in all species but showed a more variable relationship with δ13C. Among-individual variation in [THg] is primarily related to age or size in most temperate freshwater fishes, and effects of growth rate and food web position need to be considered in the context of these ontogenetic drivers.

The trophic niche of sculpins Cottus spp. in forage fish assemblages of boreal lakes.

We compared the trophic niches of freshwater sculpins Cottus spp. with those of other co-habiting forage fishes in two groups of boreal lakes with distinct habitats and fish communities. Near North Lakes (45° 00' to 47° 30' N) were deeper, stratified and contained lake trout Salvelinus namaycush as the apex piscivore, whereas Far North Lakes (51° 10' to 52° 20' N) were shallower, did not stratify and contained pike Esox lucius and walleye Sander vitreus as the apex piscivores. Trophic niches of sculpins and other forage fishes were compared based on niche metrics calculated from muscle stable carbon (δ13 C) and nitrogen (δ15 N) isotope ratios. In Near North Lakes, sculpins were found almost exclusively in deep, offshore waters and their niche positions reflected a greater reliance on pelagic production (lower δ13 C) and a higher trophic elevation (higher δ15 N) compared with most other forage fishes. Furthermore, sculpins in Near North Lakes tended to have larger trophic niches (occupied greater area in δ13 C- δ15 N space), particularly in the food chain (δ15 N) dimension, than other cohabiting forage fishes. In contrast, sculpins in Far North Lakes were commonly found in the nearshore and had trophic niche positions and sizes that were similar to those of the other cohabiting forage fishes. This study illustrates the flexibility in the realised trophic niches of sculpins in relation to habitat availability and fish community composition in boreal lakes.