Under the Full Moon: Tracking Nassau Grouper with Acoustic Telemetry

This blog post was written by Krista Sherman, Research Associate at Shedd Aquarium and Ph.D. Candidate at the University of Exeter. Krista’s research is focused on understanding endangered Nassau grouper populations in The Bahamas.

A Nassau Grouper, Epinephelus striatus, found during a Shedd Aquarium research trip. ©Shane Gross

In the winter, under the light of full moon, Nassau grouper, Epinephelus striatus, migrate great distances, often over 300 km, to specific locations where they converge by the hundreds to thousands to reproduce. These meeting locations are referred to as fish spawning aggregations (FSAs). While the predictability of the Nassau grouper’s behavior allows the animals to migrate easily, it has also made it dangerously simple for the species to become overfished – leading to collapses of traditional FSA’s in The Bahamas and throughout the wider Caribbean. Beyond being an iconic native species, the fish is hugely important to the culture, ecology and economy of the region. To help find and protect these critical spawning grounds in The Bahamas, Shedd Aquarium has partnered with The Bahamas National Trust, Perry Institute for Marine Science and University of Exeter to better understand the status of Nassau grouper spawning aggregations in the country.

Diver deploying a monitor
Diving to deploy a VR2W monitor. ©Tanya Kamerman

Recently, I spent two weeks island-hopping and diving to collect, download, and re-deploy VR2Ws – monitors on the ocean floor strategically placed around The Bahamas with the assistance of Tanya Kamerman (MSc student at Nova Southeastern University) and Tavares Thompson (citizen scientist from Andros). The monitors, along with acoustic transmitters, are used to silently track the movement of Nassau grouper to and from spawning aggregation sites. They do so by picking up signals from more than 60 native fish that have been carefully fitted with surgically implanted transmitters as a part of Shedd Aquarium’s acoustic telemetry project, which began in 2014. Each time a fish with a transmitter passes by one of our arrays, the device records and saves the data. During the course of the trip, we recovered two receivers from Andros, and retrieved and re-deployed seven VR2Ws in Exuma and Long Island.

The Andros receivers were part of a two-year study examining migration patterns between the barrier reef and reported spawning aggregation sites. Results from our study, published earlier this year, suggest the collapse of a historically large Nassau grouper spawning aggregation site and the potential existence of an un-reported site in the northern part of Andros. The two receivers recovered from Andros mark the end of that portion of our telemetry research.

Other aspects of the telemetry work involve exploring patterns of connectivity between the Exumas and Long Island, and movement within a spawning aggregation. Knowing this, we collected and re-deployed receivers around these islands in advance of the upcoming spawning season.

Thus far, data from the Exuma receivers show the migration of several Nassau grouper from a spawning site off Long Island into the Exumas. Additionally, >100,000 detections were recorded from 22 tagged Nassau grouper within a spawning aggregation site. We will be taking a closer look at the within-aggregation data to try and understand what fish are doing during this critical period.

Consistent with previous studies, Nassau grouper are migrating to and from spawning sites within a few days around the full moon, spending very little time at the aggregation before returning to their home reefs. Results from our telemetry studies will be combined with population genetics research, data from spawning aggregation surveys, and socioeconomic surveys to develop a science-based management plan for the iconic species.

Diver in the Bahamas deploying equipment to track Nassau grouper migrations
Deploying equipment in The Bahamas to help Shedd Aquarium track Nassau grouper migrations. ©Shedd Aquarium/Sam Cejtin

Next steps for this work include completing assessments of the FSA’s with the assistance of partners from the Perry Institute for Marine Science and Bahamas National Trust and other volunteers aboard Shedd Aquarium’s research vessel, Coral Reef II, this November and December.

For more information about this research, visit Shedd Aquarium’s website.

At Shedd Aquarium, animals connect you to the living world, inspiring you to make a difference.

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