The Secret Life of Nassau Grouper Revealed Using Advanced Technology

Nassau grouper are an endangered species, appearing on the IUCN Red List since 1996

Tracking Nassau Grouper Mating Rituals Helps Inform Conservation Policies

Groupers, and a wide variety of other reef fish such as snappers, triggerfish and grunts, migrate long distances from their home sites to certain locations for spawning events which take place in large groups, or aggregations. Little is known about when, where and how often different species aggregate to spawn. Even less is known about how far fish swim or the area from which they migrate to reproduce a few times a year, usually around the full moon.  Understanding the timing and frequency of spawning, and how far and where these different species migrate is important to improving their management, and necessary for understanding the connectivity of home sites to spawning sites.

Acoustic Transmitter Technology

In order to understand the recovery of an overfished Nassau grouper (Epinephelus striatus) spawning aggregation south of St. Thomas, Dr. Rick Nemeth, research professor at UVI’s Center for Marine and Environmental Studies, uses acoustic transmitter technology to map the movements of Nassau at the Grammanik Bank Marine Preserve which is closed to fishing February 1 to April 30 each year. Dr. Nemeth and his team tagged 25 Nassau grouper with acoustic transmitters between 2007-2008. Data gathered from this effort provided critical information on how often and when Nassau grouper aggregate and how much territory is used for their aggregations. Findings show that Nassau grouper aggregate for about a week after the full moon each month from January to April, sometimes as late as May and June, using an area approximately 4.25 miles around the central aggregation site. Because the Nassau grouper are considered a critically endangered species, appearing on the IUCN Red List since 1996, this information is essential to determining when and where seasonal closures should be put in place. 

Tracking animal movements underwater is extremely difficult and requires specialized equipment, such as acoustic transmitters and receivers. Acoustic receivers are set up in an array around the study location and are suspended from moorings which consist of a concrete block on the ocean floor and a styrofoam float which suspends the receiver 10 meters (32 feet) above the bottom.  The transmitters are either attached externally to a fish or surgically implanted into the body cavity. Each transmitter emits a unique signal and when the tagged animal is within a few hundred meters of a stationary receiver.  As the fish passes, the receiver records and stores the transmitter data until it can be retrieved from the ocean and downloaded to a computer. This information includes date, time and even depth, and the approximate location of the fish within about 150 meters. From this data, Dr. Nemeth knows when the fish was at the spawning site, how often it returned and how long it stayed each month.

This photo taken in February, 2018 at the Grammanik Bank shows a Nassau male in courtship coloration courting a normal colored female. Photo courtesy of Dr. Rick Nemeth.

Spawning is a busy time for the Nassau

During spawing season Nassau grouper begin to arrive at the Grammanik Bank site around 3:00 – 4:00 pm, and concentrate on the shelf edge at sunset. During this time the males are trying to attract the attention of females. To do this males swim around the female, change their colors to a dramatic black and white pattern and make special courtship-associated sounds. A 2015 study, in collaboration with Dr. Richard Appeldoorn and Dr. Michelle Sharer, both with the Department of Marine Science, University of Puerto Rico, recorded the courtship sounds with an underwater hydrophone. Information from both the hydrophones and acoustic receivers showed that Nassau most likely spawn at sunset and a few hours after, at a depth of 160-200 feet. They begin to disperse beginning around 9:00 pm – midnight, departing the site in the early morning hours. Nassau then swim 3 – 6 miles away before starting the cycle over again the next day.  We don’t yet know if they go all the way home or just part of the way, but we do know that they continue to make courtship sounds while they are traveling. We believe that they do this each day to attract and guide more fish, especially inexperienced fish who may not have migrated to the spawning site before, to the spawning aggregation site. 

After a week of spawning, Nassau grouper may travel as far as Vieques Island, just off Puerto Rico’s eastern coast, or move inshore to bays like Brewers Bay on St. Thomas, to their home sites where they will stay until the next spawning season. Some ambitious fish make this trip as often as four months in a row, but most visit spawning sites only one to three months a season.  During the migration they are vulnerable to many hazards including fishing. Because they are protected in federal and local waters we need to do everything we can, such as releasing Nassau when caught, to help them continue to make their annual spawning migrations so that we can rebuild the population of this important species.

Dr. Rick Nemeth is VI-EPSCoR's Senior Scientist and a Research Professor at UVI.

ongoing Research

In an ongoing effort to narrow down the exact location of the Nassau spawning site within the Grammanik Bank seasonal fishing closed area, a follow-up study has taken place in 2018.  Dr. Nemeth has surgically implanted 15 more Nassau with depth/accelerometer transmitters. The accelerometer records when and at what depth the fish engage in spawning behavior, specifically, swimming rapidly forward, such as during a spawning rush when a female and several males swim upward quickly so that eggs may be fertilized as they are released. The acoustic receivers have also been placed very close together at the suspected spawning site which will allow Dr. Nemeth and his team to track Nassau movements more precisely within a few meters of their actual location. This study has just recently ended and the 2 million detections downloaded from the acoustic receivers are currently being organized and analyzed. Nemeth and his team are excited about the prospect of verifying that this important species is in fact spawning in the Virgin Islands.  Stay tuned for the next update.

For more information on Nassau grouper in the USVI watch the video in Part 3, to see how conservation efforts and partnerships with local fishermen are helping bring this species back from the brink of extinction

This work has been supported by several grants including NSF VI-EPSCoR, Puerto Rico Seagrant, NOAA and the Lana Vento Charitable Trust.