Archive of Spotlight Feature Articles

Studying Nassau grouper spawning aggregations

The key to rebuilding and conserving this important fishery resource

by Craig Dahlgren

Around the world, tropical marine fishery species such as groupers (Family: Serranidae) are being overfished, with devastating ecological and economic consequences to coral reefs. Preventing these harmful effects requires management measures based on the best available information about these stocks, and the causes and consequences of overfishing them. The Caribbean Marine Research Center, the National Undersea Research Program's research center for the wider Caribbean region, is working in collaboration with scientists from North Carolina State University and the National Marine Fisheries Service's Southeast Science Center to study Nassau grouper population dynamics to enhance our ability to manage this important but threatened resource.

The Nassau grouper, a large predatory fish inhabiting coral reefs throughout the tropical western Atlantic, including the Caribbean Sea, Florida Keys and Gulf of Mexico, is among the most important fish species on coral reefs throughout the region. In addition to being important predators on coral reefs, they were also one of the most important fishery species in the region. Overfishing, however, has driven Nassau grouper stocks below sustainable levels and has even eliminated them from much of their historic range. Nassau grouper are currently protected in state and federal waters where they occur, and are a candidate species for listing under the Endangered Species Act. To improve our ability to re-build Nassau grouper stocks to sustainable levels in US waters, NURP funded researchers from the Caribbean Marine Research Center (Dr. Craig Dahlgren), National Marine Fisheries Service (Drs. Anne-Marie Eklund and Stephania Bolden) and North Carolina State University (Dr. David Eggleston, the project's principle investigator, Dr. Peter Rand and Dr. Joe Hightower) are currently studying critical aspects of Nassau grouper biology in an area where these fish are still fairly common and fisheries remain active, The Bahamas.

Stereo-paired images of grouper aggregation at South Point, Long Island, Bahamas. Images were recorded at approximately 16:25 on 10 December 2000. (Photo by P. Rand)

Most of this research focuses on large spawning aggregations, at which thousands of Nassau grouper gather each year during one or more winter full moons. This stage in the life cycle of the Nassau grouper is particularly important to understand since the entire annual reproduction for a region is concentrated at a few particular spots where aggregations occur, for only a few days each year. Furthermore, because fishers often know where and when these spawning aggregations occur, the fish are usually subject to the greatest fishing pressure at these times. In many instances, entire regional stocks have been wiped out due to intense fishing pressure on spawning aggregations. Stocks may take several decades to recover from such intense exploitation.

Although still in progress, preliminary results from this collaborative research effort have yielded much information. Through tagging fish that are captured in the fishery and ultrasonic telemetry, this research has shown that fish may migrate over 100 miles to spawning aggregations, and may spend only a few days at these aggregations. This highlights the importance of sustaining spawning aggregations for protecting stocks throughout an entire region. Furthermore, a comparison of the characteristics of fish at current spawning aggregations to characteristics of fish at the same spawning aggregations over 10 years ago, indicates that even moderate fishing pressure on spawning aggregations over a decade may have a large effect on fish stocks such as a reduction in the number of fish at spawning aggregations, a reduction in the size of fish at spawning aggregations, and a skewed ratio of males to females at spawning aggregations.

Based on what we learn from Nassau grouper stocks in The Bahamas, this research will contribute to the rebuilding of Nassau grouper stocks in U.S. waters and throughout the wider Caribbean region and assist fishery managers with the management of a sustainable fishery when stocks are healthy enough for the fishery to be reopened.

[12/3/01]

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