Shifts in movement behavior of spawning fish under risk of predation by land-based consumers

Animals confront behavioral trade-offs whenever the movements required to attain suitable sites for reproduction or resource acquisition also increase exposure to predators. Hence, the consequences of such trade-offs should be considered in analyses of movement behavior. The objective of this study was to quantitatively link shifts in movement behavior of stream fish during the reproductive season to risk of mortality from land-based tetrapod predators. We tagged 30 brook trout with internal radio transmitters and extensively recorded both movement patterns and mortality from predation over the entire spatial range of the population. Our main result is that most of the individuals (19/30) had marked shifts in movement behavior, characterized by alternating periods of low and high movement rates, which were associated with marked shifts in mortality rate. Mortality rate increased from 0.016/day during periods of low movement rate (8.0 m/day; sheltering state) to 0.071/day during periods of high movement rate (91.0 m/day; searching state). Furthermore, individuals presenting no movement shift had low movement rate (9.6 m/day) and high mortality rate (0.096/day), in association with a preference for spawning ground. Mortality rates at the time of spawning were substantially greater than those previously reported in the literature and depended strongly on the movement state of the individuals and their use of spawning ground. Our quantitative analyses highlight the influence of within-individual variation in prey movement on risk of predation by land-based consumers.