Abstract
Changes in sea urchin behavior following detection of chemical cues from predatory fishes may influence key ecological dynamics but have rarely been experimentally quantified. Here, we measured feeding rates on a habitat-forming macroalgae by two size classes of the New Zealand sea urchin (Evechinus chloroticus) exposed to either ambient seawater or seawater carrying excretions from a predatory fish (blue cod; Parapercis colias). We created a Bayesian model that combined uncertainty in kelp growth rates and probability of urchin feeding to generate robust estimates of fish-exposure effects on multiple feeding metrics. We then compared our results to re-analyzed data characterizing behavioral responses of E. chloroticus to lobster cues (Jasus edwardsii). Larger urchins (6-8 cm test diameter) consumed ~ 40% less kelp in both predator treatments, exhibiting indistinguishable responses to blue cod and lobster despite being less susceptible to fish predation. Responses of smaller urchins (3-5 cm test diameter) to both predators were equivocal, though were more consistent with reduced feeding in the presence of lobster. Here we provide novel evidence that fish cues can suppress urchin feeding rates, even in the absence of urchin alarm cues, and discuss our findings in the context of the specificity of predator cue detection-reception pathways and possible mechanisms for risk-induced reductions in urchin feeding.