Revisiting the restricted movement paradigm: the dispersal of Atlantic salmon fry from artificial redds

Abstract

The restricted movement paradigm (RMP) asserts that stream fishes are sedentary, living much of their lives within a single reach. To test the RMP, we implanted eyed Atlantic salmon eggs (Salmo salar) into a total of 19 artificial redds, in seven salmon-free streams, in 6 years, and estimated summer fry dispersal through electrofishing surveys. As expected, most fry dispersed downstream, but an average of 35% moved upstream. Surprisingly, fry moved just as far upstream as downstream (medians = 403 and 404 m, respectively). Fry were larger at lower densities and farther from redd sites, consistent with density-dependent growth, and larger upstream than downstream, possibly indicating that larger fry were better able to move upstream against the current. Dispersal distances were normally distributed around all but two of 19 redds, and half of the downstream dispersal curves were best described by unimodal distributions, consistent with a homogeneous movement strategy. Our data suggest that salmon fry were more mobile and move upstream more than previously thought, which should facilitate their stocking or reintroduction to new habitats.