Fishing, Trophic Cascades, and the Process of Grazing on Coral Reefs

TLDR

Since the 1983 mass mortality of the urchin *Diadema antillarum*, parrotfishes have become the dominant grazer on Caribbean reefs, yet their grazing capacity may be limited if marine reserves restore large predators that prey on them. The study compares the negative effects of increased predation with the positive effects of reduced fishing mortality on parrotfishes within marine reserves. Because large parrotfishes avoid predation by Nassau grouper, predation reduces grazing by only 4–8%, but the resulting density increase in reserves doubles grazing, leading to a fourfold drop in macroalgae cover and underscoring reserves’ role in coral reef resilience.

Abstract

Since the mass mortality of the urchin Diadema antillarum in 1983, parrotfishes have become the dominant grazer on Caribbean reefs. The grazing capacity of these fishes could be impaired if marine reserves achieve their long-term goal of restoring large consumers, several of which prey on parrotfishes. Here we compare the negative impacts of enhanced predation with the positive impacts of reduced fishing mortality on parrotfishes inside reserves. Because large-bodied parrotfishes escape the risk of predation from a large piscivore (the Nassau grouper), the predation effect reduced grazing by only 4 to 8%. This impact was overwhelmed by the increase in density of large parrotfishes, resulting in a net doubling of grazing. Increased grazing caused a fourfold reduction in the cover of macroalgae, which, because they are the principal competitors of corals, highlights the potential importance of reserves for coral reef resilience.