Depletion of predatory fish by fishing in a temperate reef ecosystem leads to indirect effects on prey, but not to lower trophic levels

Indirect effects of fishing extending 2 or more trophic levels have been observed in a range of ecosystems, but they are not ubiquitously present, and identifying the circumstances in which they occur (or do not occur) is a key challenge for ecologists. We compared a fully protected sanctuary (no fishing allowed) with 2 other types of management zones (one in which all types of fishing were allowed and another in which only recreational line fishing was allowed) at Rottnest Island, Australia. We predicted that fishing would result in lower abundance of predatory fish in the 2 fished zones. We further predicted that, if indirect effects of fishing are important, there would be lower predation on sea urchins and turbinid gastropods, higher abundance of these herbivorous invertebrates and lower biomass of macroalgae in both types of fished zones. The abundance of predatory fish and predation on the sea urchin Heliocidaris erythrogramma were higher in the sanctuary than in fished zones. Predation on the sea urchin Centrostephanus tenuispinus was higher and densities of H. erythrogramma were lower in the sanctuary, but the magnitude of the difference in these measures (and, therefore, the results of statistical tests) was variable among years. However, predation on turbinid gastropods, densities of C. tenuispinus and turbinid gastropods, and the biomass of macroalgae did not differ significantly among management zones. The pattern of higher abundance of predatory fish and higher rates at which prey were consumed in the sanctuary provide evidence for the indirect effects of fishing on the abundance of H. erythrogramma. However, there was no evidence that these indirect effects extend to a second species of sea urchin, or to turbinid gastropods, and no evidence that the effects lead to broad changes in the state of the ecosystem.