Fishing, trophic cascades, and the structure of algal assemblages: evaluation of an old but untested paradigm

Removal of important predators by fishing can result in trophic cascades and indirect effects on marine benthic communities. Indirect effects are especially evident when prey populations released from predation by fishing have the ability to modify entire benthic communities, as do sea urchins. Sea urchins have been shown to dramatically alter the underwater landscape by grazing, by converting stands of large erect algae into coralline barrens. In the western Mediterranean, a recent extension of coralline barrens into areas formerly dominated by erect algal assemblages has been attributed to release of predation on sea urchins by overfishing. Most suggestions concerning the transition from erect algal assemblages to coralline barrens, however, have been speculative, and little descriptive and experimental work has been carried out to verify the hypothesis that fish predation on sea urchins land its subsequent release by overfishing) drives this transition. Here we critically review the literature concerning the effect of fishing on sea urchin populations and its subsequent maintenance of different algal assemblages in the Mediterranean. The extant data cannot refute the "fishes as important predators" model, but we argue that other processes (recruitment, pollution, disease, large-scale oceanographic events, sea urchin harvesting, food subsidies, and availability of shelters) may also be important in regulating the structure of Mediterranean algal assemblages.