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No safety in numbers

The urge to reproduce pulls typically solitary adult Nassau groupers to gather in huge numbers at the same location year after year. These spawning aggregations, occurring at predictable locations and times—after the full moon in December or January in the Bahamas—have long made this species an easy target for fishermen. Heavy fishing has led to the collapse of many spawning aggregations, including a spawning aggregation at one of Shedd’s study sites, 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. They are using 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. (Grouper tagging is done under the auspices of permits from the Bahamas Department of Marine Resources and follows Shedd's animal-handling protocols and internal review.)

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.

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.

Finding the aggregations, identifying critical reefs

Shedd scientist Lynn Waterhouse, Ph.D., and her team are building our understanding of Nassau grouper populations by collecting data on distribution and lengths. Using data collected by baited remote underwater video surveys (BRUVS), they can monitor the relative abundance and distribution of Nassau groupers, along with sharks, snappers and other species of groupers. By utilizing a dual camera system, they have turned the BRUVS into stereo-video systems that enable them to measure the length of the fish, which can be used as a proxy for age. By monitoring the age distribution through time, they can assess the health of the population. This data, along with commercial landings data and reef survey data, will enable scientists to update the stock assessment for this species and work with the government of the Commonwealth of The Bahamas to develop a sustainable management plan.

“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.