Intraspecific variation in stable isotopes provides insight into adfluvial migrations and ecology of brook trout in Lake Superior tributaries

Identifying streams hosting salmonids with poorly understood adfluvial life histories, such as coaster brook trout, is challenging due to the lack of inexpensive, non-lethal techniques for confirming lake to stream movements for stream-captured fish. In this study, we used stable isotope data from 589 brook trout collected throughout the Lake Superior basin to characterize stream versus Lake Superior foraging. We observed strong isotopic separation in δ 13 C between brook trout inhabiting Lake Superior and lake-inaccessible stream reaches (i.e., those lacking Lake Superior access). Using these data, we developed a linear discriminant function (LDF) which assigned brook trout to Lake Superior or stream habitats with over 97 % accuracy. LDF and Bayesian stable isotope mixing models were then used to estimate stream and lake energy use by brook trout collected from lake-accessible reaches. Brook trout caught in lake-accessible reaches had isotope signatures and sizes that were intermediate to fish from lake-inaccessible reaches and Lake Superior, potentially indicative of Lake Superior to stream migrations or possibly an energy subsidy from adfluvial migrants in streams. The LDF was used to estimate the probability that recently grown fin tissue from brook trout collected in lake-accessible reaches resulted from foraging in Lake Superior. We identified tributaries hosting “likely” coaster brook trout using a fish’s length and LDF probability value. Our findings show the potential and limitations of this approach for confirming adfluvial migrations of brook trout.