Behavioral Drivers of Hyperstability in an Aggregating Reef Fish

Collapses of marine fish populations are a global concern. Some collapses are attributed to hyperstability, where catch per unit effort remains stable while the population declines. Hyperstability is difficult to detect until after a population is depleted, thus nearly all such studies on the hyperstability phenomenon are retrospective. Using whole-island acoustic arrays to track an endangered, aggregating reef fish on two Caribbean islands, we demonstrate for the first time behavioral mechanisms that might lead to hyperstability. We show that: (1) every reproductive-aged fish aggregates each year, (2) older, more fecund fish aggregate longer, (3) individuals will visit multiple aggregation sites during the spawning period, yet every fish always aggregates and spawns at a single location, and (4) overfishing extends the time spent aggregating, increasing vulnerability to harvest as the population declines. This latter finding is supported by historic accounts from other aggregation sites throughout the Caribbean. Taken together, our results demonstrate that aggregation sites are a surprisingly complete and persistent geographic bottleneck for aggregating species, and this bottleneck is a feature that intensifies with depletion. Our results highlight both the extreme vulnerability of local populations to harvest but the potential for even heavily harvested aggregations to recover.