Studying Nassau Grouper Spawning Aggregations

by Craig Dahlgren

From December 13-19, Dr. Craig Dahlgren of the Perry Institute for Marine science is teaming up with representatives from The Bahamas National Trust, The Nature Conservancy, the Bahamas Reef Environment Educational Foundation (BREEF) and the Ocean Crest Alliance, as well as local fishermen to study Nassau grouper spawning aggregations around Long Island, Bahamas. This will be the third expedition in the past four years to examine the status of these spawning aggregations.  Dr. Dahlgren’s past research (supported by the Shedd Aquarium and Disney Worldwide Conservation Fund) has shown that fish from as far away as over 100 miles travel during December’s Full moon to spawn at the same sites off Long Island year after year.

Nassau Grouper Aggregation

Nassau Grouper spawning aggregation off Long Island in 2011. Photo courtesy of Chuck Knapp, the Shedd Aquarium.

Unfortunately, this strategy for reproducing has been the downfall of Nassau grouper populations throughout much of the Caribbean. Once fishermen know where these spawning sites are, they can wipe out regional Nassau grouper populations within a decade or less, leading Nassau grouper to be considered an endangered species internationally. 

What is the status of Nassau grouper populations in The Bahamas? The Bahamas is one of the few places on the planet where Nassau grouper populations are relatively healthy and spawning aggregations still form, yet even here, populations have been in decline for much of the past 20 years.  Dr. Dahlgren’s previous expeditions to Long Island during the December full moon have shown that spawning aggregations that used to support thousands of fish no longer form and the few spawning sites that are remaining see only a few hundred fish spawn each year.  Similar observations have been made elsewhere in The Bahamas.  The Bahamian government has made some positive steps for protecting Nassau grouper by implementing a fishing ban during the spawning season over the past decade.

Nassau grouper crowded in a poacher’s trap at a spawning aggregation off Long Island. Photo courtesy of Chuck Knapp, the Shedd Aquarium.

Nassau grouper crowded in a poacher’s trap at a spawning aggregation off Long Island. Photo courtesy of Chuck Knapp, the Shedd Aquarium.

While populations of Nassau grouper in various parts of the Bahamas have started to increase over this period, poaching has still been observed on Nassau grouper spawning aggregations during the closed season, limiting its effectiveness. The closed season is also not formally included in fisheries legislation in The Bahamas, which means that each year it is the decision of politicians whether or not it will be implemented and when it will be implemented.

The studies of Long Island’s Nassau grouper populations are just a start in The Bahamas.  Other reported spawning aggregations must be surveyed throughout the spawning season (December through February) to determine their status.

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