Predictable spatiotemporal dynamics of a dense cuttlefish spawning aggregation increases its vulnerability to exploitation

The giant Australian cuttlefish, Sepia apama, forms a dense spawning aggregation at a single known location across its wide southern Australian distribution. After a rapid increase in fishing pressure on the aggregation in the late 1990s, a series of fishing closures were introduced before any biological information could be collected. We surveyed the habitats, timing, and spatial distribution of the spawning aggregation over 4 years, using underwater visual transects and passive tagging, to assess the suitability of the closures. We found that the annual aggregation was both temporally (April-August) and spatially (over 8 km of coastline) localized and predictable, with a consistent peak in abundances in late May-early June. Cuttlefish densities were generally highest over the shallow, broken bedrock habitat, which was more extensive in several sites left open to fishing. Although the original closure covered about 43% of the hard substrate, it accounted for only 23-37% of the total cuttlefish abundance. The extremely high densities recorded during this study verified that this is a massive spawning aggregation for cuttlefish species worldwide, and that it could be highly vulnerable to overexploitation in the absence of adequate protection, because it is so spatiotemporally predictable and localized.