Effects of overfishing on coral reefs over local and regional scales

Coral reefs are highly connected habitats, with dynamics that take place over very large spatial scales. However, performing field work over these large scales is challenging, and most mathematical modelling of coral reefs has focused on local dynamics. Here, we use a mechanistic, spatially explicit coral reef model to simulate the regional and local effects of three coral reef stressors (overfishing, nutrient loading and crown-of-thorns starfish invasions). We find three different local regimes (coral-dominant, macroalgae-dominant, macroalgae-only with no coral or fish), with sharp boundaries that depend on the interaction between fishing rate and nutrient loading rate. We also find that overfishing within a single patch can decrease coral cover by significant amounts in non-overfished patches. Additionally, increasing the proportion of patches that are overfished causes nonlinear declines in coral cover in non-overfished patches; this decline is strongly dependent on the configuration of which patches are overfished. The combination of crown-of-thorns starfish presence and high nutrient loading increases the variability of coral populations, and limits the space covered by both coral and macroalgae. These effects are present systemwide even when nutrient loading is restricted to one patch. Our findings have implications for both future field work and implementing conservation objectives.