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No safety in numbers

The urge to reproduce pulls typically solitary Nassau groupers from across the Caribbean region to gather in huge numbers at the same coral reefs year after year. These spawning aggregations, occurring at predictable locations and times—after the full moon in December or January—have long made this species an easy target for fishermen. Heavy fishing has led to the collapse of many spawning aggregations, including one of Shedd’s study groups, because so many fish were removed. By all indications, despite existing protections in the Bahamas, Nassau grouper populations are declining.

Mapping each grouper's movements

Since 2014, Shedd researchers have been studying Nassau grouper spawning aggregations in the Bahamas, along Andros Island, which is bordered by one of the world's longest barrier reefs. They are employing acoustic telemetry, a technology that uses sound pulses—pings—to track individual fish's movements. Working from Shedd's research vessel, the R/V Coral Reef II, our diving biologists moored transmitters to the ocean floor and implanted fish with acoustic tags the size of an AA battery. Now they can follow where individual groupers move throughout the year, including during the long spawning migrations.

Two Shedd researchers in scuba gear crouch on the sandy ocean floor in the Bahamas, securing a floating acoustic telemetry transmitter buoy to a pair of cinderblocks.

Diving biologists moor acoustic telemetry transmitters to the ocean floor.

A tracking device, red, cylindrical, and about the size of a AA battery, sits in the hands of a Shedd Researcher wearing rubber gloves.

Acoustic tags allow scientists to follow where individual groupers move throughout the year.

Finding the aggregations, identifying critical reefs

The acoustic telemetry findings are helping identify coral reef habitats of concern, including critical migration corridors along reef edges that need special protection to ensure successful spawning. Researchers are also describing and assessing spawning aggregations known previously only through anecdotal reports and local knowledge, and gaining an understanding of how grouper populations connect and interact within Bahamian waters. The addition this year of remote underwater videos will expand our ability to assess grouper populations.

“We have made a commitment to preserving the ecological integrity of Bahamian aquatic ecosystems, sustaining populations for some of the world’s most important key indicator species and enhancing human livelihoods that coexist with and rely on them.”

Lynn Waterhouse, Ph.D., research biologist
The beautiful blue waters of the Bahamas, when viewed above, reveal a multitude of sandy shoals dropping into deep darker areas.
An acoustic telemetry transmitter sits anchored to the ocean floor by a cinderblock, in the dark blue waters of the Bahamas among sea grass and small fishes.

Protecting groupers in the Bahamas and beyond

Shedd’s research will inform management decisions for Nassau groupers in the Bahamas and help ensure strategies are based on sound science. This work also has broader international importance for the species because the Bahamas is one of the few countries in the Nassau grouper’s range where populations still support spawning aggregations.