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December 19, 2011

Nassau Groupers in Trouble: Shedd Field Research

IMG_0999The Nassau grouper was once one of the most important fishery species in the Caribbean region. Not so today: Populations are near extinction in many parts of the Atlantic and Caribbean, and the Nassau grouper is listed as endangered by the International Union for Conservation of Nature (IUCN).

To learn more about the status of this fish in the light of current management strategies, Shedd biologists, along with representatives from the Perry Institute for Marine Science, American Museum of Natural History, New England Aquarium, the Bahamas National Trust and the Bahamas Department of Marine Resources spent a week this month conducting research on selected spawning aggregations of the Nassau grouper in the Bahamas.
Epinephelus striatus, as it’s known to scientists, is extremely important to the health of coral reef ecosystems as well as to the well-being of the Bahamian people. Grouper has been a staple in the diet of Bahamians since their arrival on the islands. The fishery supports thousands of livelihoods and saturates the social fabric of the country. Therefore documented declines of the Nassau grouper, beginning in the 1980s, prompted public and governmental concern about overexploitation of the species.

The Nassau grouper is especially vulnerable because of its unique reproductive behavior. Under the full moons of December and January, adult groupers converge in conspicuous spawning aggregations over limited coral reef sites. These spawning aggregations, which only last for a few days, draw individuals from hundreds of miles. As you might guess, this predictable seasonal massing also attracts fishermen and leaves the groupers open to large-scale overfishing.

The threat of overexploitation prompted the Bahamian government to implement a precautionary, nationwide seasonal closure to the fishery for the last six years. These seasonal closures, however, are not automatic; they must be reassessed on an annual basis.

In 2010, Shedd and its partners began a monitoring program designed to assess the status of Nassau grouper stocks in selected areas of the Bahamas and to evaluate the efficiency of the current management strategy. Preliminary results indicated that despite the seasonal closures, poaching occurs at spawning sites. In addition, while some spawning aggregations are active, several historic and significant spawning aggregations have disappeared.

This year, between Dec. 6 and 12, our team returned to the Bahamas to expand its monitoring efforts. Using Shedd’s research vessel, the R/V Coral Reef II, as a research and dive platform, the team visited three locations around Long Island known for their historic aggregations. The work was challenging at times because seas in the Bahamas can be rough this time of year.

The expertise of the dive team and boat captains was crucial in the success of this research as we conducted daily “live-boat” dives on the schooling groupers. This type of diving required the group to synchronize entry into the water after the boat slowed and stopped engines over the aggregation. Once the dive team was in the water, we made our descent onto the school with various divers equipped with underwater clipboards, 100-meter measuring tape for transects, video and camera equipment, and a dive flag attached to a float at the surface. The diver carrying the float let out line as we descended and then dragged the dive flag (still at the surface) with the group as we swam underwater. At the conclusion of the dive, the safety officer and boat captains followed the flag and picked up the group, a tricky maneuver in often rolling seas.

IMG_0990While underwater the group swam transects to quantify grouper numbers along a known distance. We also estimated grouper numbers through direct counts and by analyzing digital images. Sizes of fish were estimated as well as the different color phases that the fish go through before spawning. We also documented the number of illegal fish traps on the sites, and then team members from the Bahamas National Trust and Bahamas Department of Marine Resources released more than 60 illegally trapped fish.

Of the three sites we visited, two had fish and one appeared to have been depleted completely of spawning groupers. On the sites with fish, one had approximately 250 groupers while the other had more than 500. It is a stirring experience to swim amid such a large school of these massive fish, which are almost 3 feet long. Unfortunately we did not have much luck in observing actual spawning. During multiple late-afternoon dives two days before the full moon, we saw females full of eggs and males changing colors and courting females as they swam off the bottom. We excitedly anticipated observing the spawn on the day of the full moon. In the meantime, the boat ventured eight hours south to a historic aggregation site at the other end of the island. That aggregation appeared to be fished out, so we went back north to dive the other sites.

In our absence, the larger aggregation had spawned—few fish remained and they exhibited little pre-reproductive behavior. We were disappointed, but we continued to monitor both sites and noted a re-emergence of approximately 225 fish at the spawned site. We did observe a tell-tale cloud in the water resulting from released eggs and sperm after a female made her rush to the surface while being trailed by multiple males. One team member managed to observe an isolated spawning event at the smaller aggregation site, but that is where our luck ended.

Though we did observe groupers at two historic spawning sites, their numbers were much lower than in the past. Historic accounts tell of aggregations upward of 10,000 fish. The southern aggregation appears to have been fished out recently because fish were reported there a decade ago.

The closed grouper season is good in principle, but our observations of illegal fishing underscore the necessity of continued monitoring and a revision of current management strategies to ensure the long-term survival of this impressive fish. Such recommendations are forthcoming while Shedd continues to be committed to marine conservation in the Bahamas.

This work was supported in part by the Mohamed bin Zayed Conservation Fund.

—Posted by Chuck Knapp, conservation biologist

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