The nature and implications of variation in a seaweed-epifauna-fish food chain

Abstract

Small crustaceans living on seaweeds (epifauna) have strong links to both the seaweeds they rely on for food and/or habitat, and the predatory fish that prey on them. The studies presented here aimed to better understand the nature and implications of variation in this seaweed-epifauna-fish food chain. A range of factors with the potential to change the structure of this system were investigated, including allochthonous prey availability, cascading effects of fish predation, and fish density and prey selection. A reef-wide budget showed that food requirements of juvenile fish were five times as high as could be met by local epifaunal productivity, and that zooplankton flux onto the reef subsidised these fish. The maintenance of high fish densities resulting from the zooplankton subsidy resulted in an index of predation pressure on local prey nine times higher than at sites with (unsubsidised) low fish densities. The zooplankton therefore functions as an apparent competitor to the local epifaunal prey, which are only 7% as abundant as epifauna from sites with low fish densities. Epifaunal populations released from fish predation increased damage, but reduced fouling, on seaweeds in mesocosm experiments. A cascading effect of these health measures was apparent when epifaunal populations were exposed to fish predators. Algal health did not change with variations in field fish populations as predicted on the basis of this experiment. Co-variation in epifaunal taxa and seaweed palatability with fish density are likely to confound this relationship in the field. Large-scale variation in fish density was used to measure the impact of fish density on epifaunal communities. Fish densities at sites in northeastern New Zealand, southern Chile, and King George Island (Antarctica) varied more than 300-fold. Epifaunal populations were abundant, or individual epifauna were large, only where fish population density was low. The potential to structure prey populations was shown by strongly size-selective foraging by fish in laboratory-based experiments, however fish in the field exhibited an opportunistic foraging strategy.