Abstract

Will the protection of historic spawning sites help recover overfished stocks? Answering this question requires an understanding of how Nassau grouper respond to heavy harvest on spawning aggregations. It is commonly believed that Nassau grouper respond to heavy aggregation harvest by either 1) ceasing to aggregate, or 2) shifting aggregation locations. An understanding of the reproductive biology and spatial ecology of Nassau grouper in regions where aggregations have been heavily harvested can help guide recovery efforts for this species. We acoustically tagged and tracked Nassau grouper on Cayman Brac, an island where heavy harvest is believed to have either exhausted the aggregation, or resulted in a shift in its location. In previous years, we had conducted similar research on Little Cayman, and island with a large (~3000 fish) and active aggregation. We found that Nassau grouper on Cayman Brac continue to use the historic spawning aggregation site on the eastern end of the Island, although the number of aggregating individuals remains highly depressed compared to the Little Cayman aggregation site. Despite the stark difference in the aggregation size between the two islands, the aggregating behaviors appear virtually identical. On both islands, the entire reproductive aged population aggregated each season, and the specific nights of spawning were the same. Taken together, these findings demonstrate that aggregation sites represent a surprisingly complete and persistent geographic bottleneck for local spawning stocks. In a management context, these results highlight the extreme vulnerability of local stocks to harvest on aggregations.

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