Resource use by salmonids in riverine,lacustrine and marine environments: Evidence from stable isotope analysis

Synopsis We measured stable isotope ratios (δ¹³C and δ¹⁵N) of invertebrates, Atlantic salmon, Salmo salar, and brook trout, Salvelinus fontinalis, in three distinct freshwater environments (headwater tributary, ultra-oligotrophic lake, and main-stem river) in the Western Brook system, Newfoundland, Canada. Large differences in the stable carbon signatures of invertebrates allowed the identification of organic matter assimilation from each environment by resident parr and migrating smolts. Brook trout captured in the headwater tributary in June had a carbon signature characteristic of the tributary, while those collected in August had enriched ¹³C (maximum = −15.6‰) and ¹⁵N (maximum = 12.8‰) values. These enriched carbon and nitrogen signatures were indicative of foraging at sea. There was a low correlation between δ¹³C and δ¹⁵N (r² = 0.198) for individual fish that was likely due to the confounding influence of trout feeding in the lake and the lower main-stem of the river, where δ¹³C of food sources was high but δ¹⁵N was low. Smolts emigrating from Western Brook Pond where they had been foraging (based on lacustrine carbon signatures) were significantly larger than those emigrating from a nursery brook and the main river in the same basin, despite having the same median age. These results suggest better growth opportunities in the lake environment. Trout fork length was positively correlated with δ¹³C and δ¹⁵N, demonstrating that larger individuals had been feeding outside the brook. These results support previous studies that found increased growth potential for salmonids in lacustrine and marine environments, and further, indicate possible adaptive advantages for salmonid movement away from natal brooks.