Variation in the relative importance of sublethal effects of predators and competitors on growth of a temperate reef fish

Sublethal effects of predators on the growth of their prey have been little studied in marine systems and consequently their importance relative to other factors is poorly known. Using a set of field experiments conducted on small artificial patch reefs, we measured the relative importance of the effects of predators and conspecific competitors on the growth of a temperate reef fish, the blackeye goby Coryphopterus nicholsii, and we measured how the importance of the 2 processes varied over time. We found that predators and competitors reduced the growth of the gobies at some times, but not others, and that the relative importance of the 2 processes varied among time periods. Moreover, the importance of the 2 processes varied as a function of body size, with large, fast-growing individuals suffering relatively more from the effects of predators than small, slow-growing individuals, which felt the effects of competition more keenly. Competition increased the variation (CV) in growth rates within populations, but exposure to predators had no effect on growth variability. The strength of both the competitive and predatory effects on growth rates declined from summer to winter, but the effect of competition on growth variability did not change appreciably over time. The decline in the effects of competitors and predators on growth rates corresponded to predictable seasonal declines in water temperature and predator abundance. The relative importance of the sublethal effects of predators was greatest during the summer, when predators were most abundant. This study highlights the potential importance of sublethal effects of predators in marine systems and it suggests that the relative importance of predatory and competitive effects may vary in predictable seasonal and ontogenetic ways.