Sounds that occur naturally in marine environments are known to influence key ecological processes for a variety of organisms. Anthropogenic noise in the ocean, however, has increased substantially in recent decades, inducing physiological stress in animals and disrupting ecological processes, including predation, which can strongly influence population dynamics. Scant knowledge exists regarding the physiological responses of fishes to ecologically relevant anthropogenic noise, and there have been few studies on the impacts of noise on predator-prey interactions. In this study, I investigated the physiological stress effects of boat engine noise on a coastal marine fish (the giant kelpfish, Heterostichus rostratus) by measuring the cortisol responses to boat noise of various temporal dynamics and noise levels. Secondly, I investigated the effects of boat engine noise on predation risk of juvenile giant kelpfish when exposed to adult kelp bass (Paralabrax clathratus) predators to determine whether the presence of noise influences predator-induced mortality. Giant kelpfish exhibited acute stress responses when exposed to intermittent boat engine noise, but not to continuous boat noise or natural ambient sound. In predation trials, boat noise decreased the activity of kelp bass and reduced mortality of giant kelpfish. My results suggest that variability in the acoustic environment may be more important than the period of noise exposure for inducing stress, and also provide potentially useful information regarding noise levels at which physiological responses occur in a marine fish. In addition, this study demonstrates that the presence of boat engine noise can influence the activity of a piscivorous fish and reduce predation risk of its prey, suggesting that anthropogenic noise may have demographic consequences.