Estimating the scale of fish feeding movements in rivers using δ13C signature gradients

Isotopic signatures of consumers provide a time integration of their feeding history, and as a result of movements, are often out of line with signatures of their local resources. Such disequilibrium can be useful for inferring the spatial scale of consumer movement. delta C-13 signatures of dissolved inorganic carbon as well as periphyton and invertebrates, exhibit pronounced gradients along rivers. We outline an analytical framework to estimate the spatial scale of movement of riverine fish by comparing the slopes of their delta C-13 signature gradients to that of the stream invertebrates they consume. For free-ranging juvenile Atlantic salmon (Salmo salar L.), unconstrained by barriers, delta C-13 signatures departed considerably from invertebrate signatures, and along-stream slopes were as little as half those recorded for stream invertebrates. Movement estimates for these fish, based on their signature slopes, are similar to 20 km. By contrast, stream resident salmonids (whose movements are constrained by physical barriers) and sedentary taxa such as sculpins and sticklebacks, have carbon signatures much closer to invertebrate signatures where they were collected. For these groups, our method yields negligible estimates of movement, similar to those of invertebrates. Although this method cannot provide precise estimates of how much individual organisms move, or reveal details of movement history, it may provide an effective complement to telemetric and other methods of studying movement.