Size-specific predation on an overgrazing sea urchin reveals dominant role of large predatory fish

Predators can play a key role in structuring ecological communities through top-down control, which may be influenced by size-specific relationships, abundance and richness of predatory guilds. In southeastern Australia, the diadematid sea urchin Centrostephanus rodgersii reaches high abundances, overgrazing kelp and maintaining extensive barrens. Despite its ecological importance, predator identity and size-specific nature of predation on this urchin remain incomplete. Here, we conducted urchin tethering assays with remote underwater videos to identify predators across a full size range of urchins (16-110 mm test diameter). We identified 5 predatory fishes from a total of 94 arrays and 61 predation events from 7 sites in the Sydney region, Australia. The eastern blue groper Achoerodus viridis (Labridae) accounted for 75% of events, and the other 4 predatory fishes accounted for the remainder, which involved small urchins only (≤25 mm test diameter). Overall, predation risk decreased as urchin size increased, while time to predation decreased with predator richness. Smaller urchins were more likely attacked from the aboral side, while larger urchins were flipped and attacked from the oral surface. A. viridis was the only predator observed consuming all urchin sizes. These findings show that predator size and richness are key factors determining the predation pressure on C. rodgersii . As many of these factors have been heavily impacted by historical overfishing, regulating urchin populations through effective predator management may be crucial for sustaining ecosystem services in kelp forests.