Fish Biomass Structure at Pristine Coral Reefs and Degradation by Fishing

Coral reefs around the world have experienced a dramatic decline during the past 25 years. Overfishing is believed to play a major role. There is significant interest in stabilizing and restoring damaged reefs, and first steps include understanding the functioning of reefs in their natural state and examining the effects of fishing. The isolated Kingman and Palmyra reefs are believed to provide a baseline for the natural state of coral reefs. At Kingman, it was recently discovered that apex predators constitute 85% of the total fish biomass. This is in sharp contrast to most reefs, where the prey biomass substantially dominates the total fish biomass. The recent study at the two pristine reefs also indicates that the predator:prey fish biomass ratio is an increasing function of reef size. Based on these field observations, we model the fish biomass structure at a pristine coral reef. Since the prey hide from predators in the coral, a key component of our model is a refuge. Our model yields both the inverted biomass pyramid and the increasing dependence of the predator:prey biomass ratio on reef size. Thus, refuge may provide a general new mechanism in ecology to create an inverted biomass pyramid, that does not require mass action interactions between predators and prey. We add various forms of fishing to our model, and show that sufficiently high fishing pressure with quite general types of fishing transforms the inverted biomass pyramid to be bottom heavy. We also show a counterintuitive result that prey fishing alone has the same effect.