Effects of Variation in Fishing Rate and Nutrient Loading on Coral Reef Health with Implications for Marine Protected Area Design

Coral reefs rank among the highest-biodiversity habitats in the world, hosting many fish and invertebrate species found nowhere else. Additionally, reef ecosystems generate billions of dollars in revenue annually for coastal communities via fishing and tourism. However, from the Caribbean to Australia, coral reefs are in decline worldwide. Causes of this include anthropogenic stressors such as overfishing and excess input of nitrogen and other nutrients. To evaluate the threats posed to reefs by these processes, I simulate reef dynamics using a mechanistic, spatially explicit model fit using field data. I find three major regimes: one where coral dominates with periodic algal blooms, one where coral and algae coexist, and one where coral is driven to extinction by algae, in order from lowest to highest fishing rates. For moderate fishing rates, both a healthy coral population and a profitable local fishing industry can exist. Also, establishing a marine protected area (MPA) with no fishing in 20 percent of the simulated area is enough to maintain the coral-dominant equilibrium in the rest of the system, even when fishing rates outside the MPA are very high. Decreasing nutrient input into the system can also shift it towards the coral-dominant equilibrium. The rates of nutrient loading at which regime shifts are predicted to occur vary nonlinearly with fishing rate.