Algal turf consumption by sea urchins and fishes is mediated by fisheries management on coral reefs in Kenya

Herbivory is a key process that controls the abundance and accumulation of algal turf on tropical coral reefs. The capacity of reefs to prevent algal accumulation hinges on the balance between algal production and consumption (i.e., grazing). In this study, we quantify algal turf biomass accumulation and grazing using experimental substrata and herbivore exclusion cages across sites in Kenya that represent different levels of fisheries management: heavily fished reefs, community marine-protected areas less than 10 yr old, and older government-managed marine-protected areas. These reefs had different assemblages of grazing herbivores with fished reefs being dominated by sea urchins, while government closures had a high abundance of grazing fishes, in particular parrotfishes. The community fisheries closures had an intermediate mix of sea urchins and grazing fishes, with the latter dominated by surgeonfishes. These management regimes mediated algal biomass on experimental substrata such that urchins consumed as much as 90% on fished reefs and fishes as much as 96% at the government marine-protected areas by the end of the 390-d trial. The younger community fisheries closures lacked the herbivory to significantly reduce algal biomass, and consumption was less than 50% of production and never greater than 2 g algae m−2 d−1. These findings point to the importance of recovery dynamics of herbivorous fishes from heavy fishing pressure. They also suggest that while sea urchins might be effective grazers to prevent macroalgal dominance, they are not a functional replacement for fishes due to their ability to reduce reef accretion through bioerosion and prevent settlement of crustose coralline algae in this system