Indirect predator effects on age-0 northern rock soleLepidopsetta polyxystra: growth suppression and temporal reallocation of feeding

Field observations reveal that age-0 northern rock sole avoid feeding during daylight hours, instead, concentrating their feeding at dusk. Laboratory studies demonstrate these fish to be extremely risk averse in their behavior relative to predators. We hypothesized that dusk feeding may be an adaptation to mitigate the conflict between feeding and predator avoidance, which if unresolved, could negatively affect not only short term survival, but growth as well. We designed experiments to examine firstly, whether growth of juvenile northern rock sole is suppressed by the perception of chronic predation risk, and secondly, whether growth suppression is alleviated by allowing fish to feed at dusk. Replicate groups of 15 fish were grown in the presence or absence of predators (2 walleye pollock) and given access to food under either daylight or dusk conditions over a 6 wk period. Flatfish growth was independently and negatively influenced by both predator presence and daylight conditions; fish fed in daylight in the presence of predators lost weight, while those fed under dusk conditions in the absence of predators grew faster. These results are consistent with our hypothesis that dusk feeding is a behavior that mitigates effects of predation, not only upon survival, but also upon growth. Further, if predator abundance influences growth, as suggested by our data, indirect predator effects such as growth suppression may significantly influence the quality of nursery habitats that differ in predator abundance.