Consequences of Predation Threat for a Macrophyte-Associated Fish

Includes bibliographical references (pages 24-29).Predation strongly influences population dynamics and community structure. Besides\ud the direct consumption of prey, however, predators also have important non-consumptive effects (NCEs). During early life stages, marine fishes experience substantial predatormediated mortality, and thus may also experience NCEs, including a physiological stress response. In this study I investigate the nonlethal effects of a piscine predator (kelp bass, Paralabrax clathratus) on the behavior, physiological stress (measured by cortisol concentration), individual growth, and subsequent survival of juvenile giant kelpfish (Heterostichus rostratus). The spatial distribution of giant kelpfish changed in response to visual exposure to kelp bass vs. a similarly sized non-predatory fish and a control (no fish), indicating that giant kelpfish visually recognize predators. Giant kelpfish also respond to predation threat through elevated cortisol concentrations, but there was no difference in cortisol concentration when they were directly exposed to a predator or a non-predator. When giant kelpfish fed ad libitum were exposed to a predator or to a control during crepuscular periods for 2 wk, fish that experienced a predator exhibited slower growth, suggesting that the physiological response per se was responsible (and not lower feeding activity). When subjected to treatments of structural habitat complexity (densities of artificial eelgrass), the highest cortisol concentrations exhibited by giant kelpfish were at medium densities of eelgrass, likely due to habitat-mediated predator movement and thus encounter rates with prey. Most importantly, prior exposure to predation threat resulted in lower predation mortality, possibly due to a faster escape response from elevated cortisol concentrations. These results demonstrate that predation threat elicits a stress response in giant kelpfish, and that this response may have demographic consequences in the form of reduced individual growth and lower predation risk