Listening for Nassau Grouper at Cayos Cochinos
In March, our Senior Scientist Dr. Krista Sherman flew to Honduras, returning to Cayos Cochinos to work with the Honduras Coral Reef Foundation, four years after her first visit.
Her destination was a small archipelago off the north coast of Honduras called Cayos Cochinos, home to the Garifuna communities who have fished these waters for generations, and the heart of a marine protected area called the Cayos Cochinos Monumento Natural Marino. The reefs here include a spawning site called Roatan Banks. For decades, Roatan Banks was one of the places Nassau grouper went to spawn.
Whether it still is, no one can say for certain. That is the question.
What is a Nassau grouper spawning aggregation?
On most days, a Nassau grouper is a solitary, territorial fish. It hunts a patch of reef, guards it, and rarely strays. But once a year, as the water cools and the moon pulls the tide a certain way, something changes.
Fish travel. Thousands of them. They leave their home reefs and converge on the same spawning sites their grandparents gathered at, often arriving within the same few nights each winter. Our team, led by PIMS Executive Director Dr. Craig Dahlgren, tracked this behavior across multiple years in The Bahamas and found that individual grouper migrate during the December full moon, often making round trips of 70 to over 200 kilometers, and spending only 1 to 2 days at the spawning site before heading home (Dahlgren et al. 2016).
A spawning aggregation is a small miracle. It is also a catastrophic vulnerability. If those sites are known to fishers, or published openly, or unprotected, they can become overfished and eventually collapse or no longer form. In the 20th century, spawning aggregations up and down the Caribbean were found, fished, and collapsed. The Nassau grouper is now listed as Critically Endangered on the IUCN Red List, and in The Bahamas its closed season runs December through February, coinciding with observations from telemetry data and spawning aggregation surveys of peak spawning.
What the tagging data tells us
Why do some spawning sites go quiet?
This is not hypothetical. In 2017, a study co-authored by Krista and Dr. Dahlgren used acoustic telemetry along Andros Island, The Bahamas, to document what the paper’s title calls a “collapsed historic fish spawning aggregation” (Stump, Dahlgren, Sherman & Knapp 2017, Bulletin of Marine Science). The site was one of the best-known grouper aggregations in the country. When the team tracked tagged fish across a full spawning season, the fish did not show up at the expected site. They migrated somewhere else entirely, suggesting an undocumented aggregation, and confirming that the old site had effectively collapsed.
That study is the intellectual foundation for what may be happening at Roatan Banks right now. A spawning site that groupers used may no longer form. There is only one way to know: research.
How does acoustic telemetry work?
You cannot watch a spawning aggregation from a boat. It happens underwater, at night, in winter, often in weather no one wants to be out in. So, sometimes you do not watch it. You listen.
Acoustic telemetry is the quiet engine behind most modern spawning-aggregation science. A small transmitter, roughly half the size of an AA battery, is surgically implanted in a fish. It pings on a set frequency. Receivers, placed at the spawning site, record every ping that passes within about half a kilometer. A single receiver can hold years of data. A network of them can map a fish’s movements across an entire coastline.
Deploy enough receivers along an area you suspect is a spawning site, then wait a year, and the fish tell you everything. When they arrived. When they left. Whether they came back the next year.
That is what Krista went to Honduras to do: train HCRF staff on acoustic telemetry and help them set up receivers at a historically active Nassau grouper spawning site.
What happened during the workshop?
Krista’s job on this trip was training. She is a scientist who also trains scientists, which is the form much of PIMS’s international work takes. The goal for the trip was to provide theoretical and hands-on training to HCRF staff and invited workshop participants on spawning aggregation survey methods, fish capture, transmitter surgeries and release procedures, receiver initialization, deployment, maintenance and retrieval, and acoustic telemetry data management, so the monitoring program keeps running long after the workshop ends. Receivers were programmed and put in the water. Data protocols got hammered into shape around a kitchen table at the research station.
The local Garifuna fishers know these waters, and their families have worked and lived in Cayos Cochinos for centuries. Their knowledge is not a footnote in this science. It is the reason the science happens at all.
As Krista put it after she got back: “Honestly, the highlights for me were spending time with the Garifuna fishers and seeing how much everything at Cayos Cochinos has grown since my first visit in 2022.”
How has the partnership evolved?
Since Krista’s first visit in 2022, the research facilities at Cayos Cochinos have grown. HCRF’s scientific staff has grown. The workshops are deeper, the questions are sharper, and the work is shared across a growing team. A new generation of Honduran scientists are now undertaking this work alongside PIMS, and that is exactly what a long partnership is supposed to produce.
This is the slow, unglamorous center of gravity of international conservation. It does not make headlines in the week the receivers go in the water. It makes a difference a decade from now, when partners across the Caribbean are trading tools, training each other, and answering the next set of questions together to better manage fish populations.
What will the receivers tell us?
The receivers HCRF set up in March will spend the next year recording detections from tagged Nassau grouper. Next spawning season, when the water cools and the moon is right, either tagged grouper will show up at Roatan Banks, or they will not.
If they do, the site is still a spawning aggregation. Then the work becomes protection: enforcement during the closed season, closer coordination with the fishers who share these waters, and possibly expansion of the receiver network to capture the full extent of the migration. If they do not, the science shifts. Is it gone? Are fish using another spawning site?
Either answer shapes the fisheries policy the Honduran government will build on, with the support of DIGEPESCA, the national fisheries authority. The answers matter well beyond Honduras because this critically endangered species is important throughout the Wider Caribbean. Our team has spent decades studying Nassau grouper, including habitat use, abundance and distribution, migration patterns, spawning aggregations, population genetics, and stakeholder perspectives, all to improve management for this species (Stump et al. 2017; Sherman et al. 2016; Sherman et al. 2018; Sherman et al. 2020; Sherman 2025). One species, one aggregation, one answer at a time, the picture comes together.
Who made this work possible?
This work does not happen alone.
MAR Fund (Mesoamerican Reef Fund)
Funded the workshop. Their support of capacity building across the Mesoamerican Reef region is why HCRF now has the equipment, the training, and the methodology in-house.
Fundación Cayos Cochinos / HCRF
Manages the Cayos Cochinos Monumento Natural Marino and carries the weight of this monitoring program year-round. The workshop happened because of their invitation, their planning, and their team.
DIGEPESCA
Honduras’s national fisheries authority. Provides the policy and management backbone that turns receiver data into real protection.
Garifuna fishers of Cayos Cochinos
Their knowledge and participation are crucial for this project.
All photos courtesy of HCRF. We are grateful for the partnership.
Support the work that connects reefs across the Caribbean
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Support our workFrequently asked questions
What is a Nassau grouper spawning aggregation?
A Nassau grouper spawning aggregation is a transient, predictable gathering of grouper at a specific site, usually during the winter full moons, where thousands of fish come together to reproduce. An individual grouper may travel more than 200 kilometers round-trip to reach the site and spend only 1 to 2 days there before returning home (Dahlgren et al. 2016). Because the aggregation is predictable in time and place, it is also catastrophically vulnerable to overfishing.
Why are Nassau grouper critically endangered?
Nassau grouper (Epinephelus striatus) are listed as Critically Endangered on the IUCN Red List because their spawning aggregations have been historically targeted by commercial fisheries. When fishers arrive during the few days when most fish are concentrated, this fishing pressure can functionally collapse the aggregation. PIMS research has documented the extirpation of a historic aggregation in The Bahamas (Stump et al. 2017), and our team supports the Bahamian government’s closed season (December 1 through February 28) and formal conservation management plan.
How does acoustic telemetry track fish?
Acoustic telemetry tracks fish movement using small ultrasonic transmitters surgically implanted in individual fish, paired with strategically placed underwater receivers. Each transmitter emits a coded ping on a set frequency; receivers within approximately 500 meters log every ping. Over time, the data can be used to map when tagged fish arrive at a site, how long they stay, when they leave, and whether they return the following year. The technique is standard practice for studying spawning aggregations because the events happen at night, underwater, often in conditions that prevent or limit direct observation.
Where is Cayos Cochinos and the Cayos Cochinos Monumento Natural Marino?
Cayos Cochinos is a small archipelago off the northern coast of Honduras, home to several Garifuna communities and the center of a marine protected area called the Cayos Cochinos Monumento Natural Marino. The MPA protects coral reefs, mangroves, and seagrass beds along the Mesoamerican Reef, the second-largest barrier reef system in the world. It is managed by the Honduras Coral Reef Foundation (Fundación Cayos Cochinos / HCRF).
What is the Honduras Coral Reef Foundation (HCRF)?
The Honduras Coral Reef Foundation (Fundación Cayos Cochinos) is a Honduran non-profit that manages the Cayos Cochinos Monumento Natural Marino. Founded in 1994 in partnership with the Smithsonian Institution, HCRF leads conservation, research, and community programs across the archipelago. HCRF’s scientific staff runs the long-term monitoring at Cayos Cochinos.
What is the significance of Roatan Banks?
Roatan Banks is a dive site within the Cayos Cochinos Monumento Natural Marino that was historically a known Nassau grouper spawning aggregation. Whether the aggregation still occurs is an open scientific question. The receivers deployed in March 2026 will spend the next twelve months listening for tagged grouper during the next spawning window, providing the first answer to this question.
Why are the Garifuna fishers central to this research?
The Garifuna fishers of Cayos Cochinos possess generations of on-water knowledge about these fish, including when they arrive at spawning sites. This local knowledge makes the science possible.
What happens next with the receiver data?
The receivers will listen continuously for approximately twelve months. During the next spawning season, either tagged grouper will arrive at Roatan Banks or they will not. If they do, HCRF and PIMS will work with DIGEPESCA, the Honduran fisheries authority, to strengthen protections during the closed season. If they do not, the search widens to find where Nassau grouper are aggregating.
References
All papers below are available on our research hub.
Dahlgren, C.P., Buch, K., Rechisky, E., & Hixon, M.A. (2016). Multiyear Tracking of Nassau Grouper Spawning Migrations. Marine and Coastal Fisheries, 8: 522-535. doi.org/10.1080/19425120.2016.1227404
Sherman, K.D., Dahlgren, C.P., Stevens, J.R., & Tyler, C.R. (2016). Integrating population biology into conservation management for Nassau grouper. Marine Ecology Progress Series, 554: 263-280. doi.org/10.3354/meps11771
Sherman, K.D., Dahlgren, C.P., & Knowles, L.C. (2018). Nassau Grouper (Epinephelus striatus) Conservation Management Plan for The Commonwealth of The Bahamas. Department of Marine Resources, Nassau.
Sherman, K.D., Shultz, A.D., Dahlgren, C.P., et al. (2020). RAD-seq analysis and in situ monitoring of Nassau grouper reveal fine-scale population structure. Frontiers in Marine Science, 7: 157. doi.org/10.3389/fmars.2020.00157
Sherman, K.D. (2025). Conservation as a marathon vs. a sprint: The race to save Nassau grouper in The Bahamas. In Navigating Our Way to Solutions in Marine Conservation. doi.org/10.11647/obp.0395.02
Stump, K., Dahlgren, C.P., Sherman, K.D., & Knapp, C.R. (2017). Nassau grouper migration patterns during full moon suggest collapsed historic fish spawning aggregation and evidence of an undocumented aggregation. Bulletin of Marine Science, 93(2): 375-389. doi.org/10.5343/bms.2016.1042
