Grouper sex

I have just returned from an absolutely marvelous visit to the Cayman Islands where I was participating in the Nassau grouper spawning project, called Grouper Moon, which is run jointly by the Cayman Islands Government and REEF ( This species of grouper gather together near the full moon in winter months to spawn at specific sites every year. Unfortunately this fidelity has made them very vulnerable to overfishing as the fishers know exactly when and where the aggregation will form. The historical aggregations consisted of tens of thousands of hungry fish so fishing was lucrative and very heavy. The largest fish were being harvested just as they are making the next generation, which seems nonsensical but such traditional fisheries are very difficult to stop. The Cayman Islands finally closed their grouper aggregations in 2003, and the Grouper Moon project has been studying the spawning ever since. Although greatly depleted from historical times, the 3500-4000 fish at the Little Cayman aggregation is relatively healthy. I saw several spawning ‘rushes’ where a large female shoots up through the water column releasing eggs as she goes, surrounded by several males releasing sperm. The eggs are fertilized in the water, and the larvae spend several weeks in the plankton before settling in their nursery habitats to grow big enough to move out to the reef. Nassau groupers’ natural range extends from northern Florida to South America, but intensive fishing has severely depleted populations and unfortunately the historical spawning aggregations that were fished out decades ago have still not returned. The Little Cayman spawning aggregation is one of the very few remaining in the Caribbean, and fortunately seems to be holding its own, with more juveniles appearing every year. Thanks to the considerable efforts of the Caymanian scientists and the REEF program, this aggregation will hopefully survive into the future.


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