Carbon isotopic ratios reveal complex strategies in the use of the hydrological network by fish in a large river ecosystem

English Abstract:

By reducing the connectivity within large river systems, dams perturb the completion of the life cycle of many mobile organisms that depend on upstream tributaries not only to reproduce but also to feed on alternate food sources. The isotopic signatures of sessile organisms, such as invertebrates, can be used to map the isotopic baselines or "isoscapes" of river systems, against which fish isotopic ratios can be compared for evidence of sendentarity or movement. Here we exploit the clear distinctiviness in carbon isotopic ratios of inorganic and organic fractions from a large tributary of the St-Lawrence River Canada compared to the river main stem to track throughout the year the movements and feeding in nine species of potentially highly mobile fish. Carbon isotope data of fish muscle tissues allowed us to identify three general migration patterns and sources of productivity in this large river system. Our study offers a better understanding of fish movements in large rivers, providing a tool to predict with more confidence the possible impacts of dam building projects on fish assemblages.