Carbon isotopes reveal complex variation in fish migratory strategies at the confluence of two large rivers
Fish undergo migrations in order to access multiple resources located in distinct locations. Variation in migration strategies, observed among- and sometimes within species, range from sedentarity, through spawning with little or no food intake, to more complex patterns of movement and trophic exploitation of different environments. Identifying and quantifying these behaviors is an important prerequisite to multi-species stock management and dynamic food web quantification. Large differences in carbon stable isotopes (+10‰) observed between the St-Lawrence River (Canada) and one of its large tributaries, the St-Maurice River (SMR), provided us with an opportunity to quantify migration patterns in nine species over an annual cycle. To achieve this goal, we used mixing models in a Bayesian framework to estimate the proportion of alternative food sources exploited by individual fish.
Analysis of 556 fish muscle tissues identified three general migration patterns: residency (Ambloplites rupestris, Micropterus dolomieu and Moxostoma macrolepidotum), spawning migration (Acipenser fulvescens, Catostomus catostomus and Sander vitreus) and trophic migration (Esox lucius, Hiodon tergisus and Ictalurus punctatus). These last three species were characterized by highly variable carbon ratios suggesting continuous movement and associated feeding thoughout the year. Our approach allowed us to clearly identify hitherto unsuspected differences in migration strategies between two functionally similar sucker species (C. catostomus and M. macrolepidotum). Presence of multiple contingents within species caught in the SMR suggests that partial migration occured in six of the nine species. Isoscapes near confluences are often very discontinuous and be used to identify migration strategies of individuals and populations, and investigate the potential importance of river connectivity for different fish species.