Effects of a predator and prey on a foraging reef fish: implications for understanding density-dependent growth

A temperate reef-fish, magpie morwong Cheilodactylus nigripes , was more abundant in habitats (turfing algae) that supported larger numbers of prey at four different locations across >1000 km of continuous coastline. Cheilodactylus nigripes fed at greater rates in patches of turfing algae known to contain greater abundance of prey (primarily gammarid amphipods). In the presence of the piscivorous New Zealand fur seal Arctocephalus forsteri , however, these fish fed at a substantially reduced rate. These results have severe implications for understanding density-dependent growth. If predators are attracted to larger numbers of prey (this study) and prey suffer slower rates of growth under heavier predation risk (previous studies), then experiments that manipulate densities of prey to test the effects of competition may confound the effects of predators and competitors on growth. Separation of the density-dependent effects of foraging (predation) and rivalry over the acquisition of limited resources (competition) is needed.