Nassau grouper (Epinephelus striatus) migrate to specific sites during the winter full moons in order to reproduce in mass aggregations. The Nassau grouper is listed as ‘threatened’ by the International Union for Conservation of Nature and Natural Resources. Intense harvesting of spawning aggregations is the primary cause of the precipitous decline in populations throughout the Caribbean. In 2003 the Cayman Island Marine Conservation Board instituted an 8-year total fishing ban on all known Nassau grouper aggregation sites in the Cayman Islands. Of the eight known (and now protected) aggregation sites, six appear to be inactive due to over fishing. Only one protected aggregation, at the west end of Little Cayman Island, maintains annual aggregations of more than 1,000 grouper. In order to justify the no-take status of aggregation sites, and in order to assess the likelihood that the closures are effective, the Cayman Islands Department of the Environment needed a clearer understand of how local populations of Nassau grouper use aggregation sites.
Click here to watch a 12 minute documentary about this project.
What Questions Were We Trying To Answer?
1) What proportion of Nassau grouper on the Cayman Islands use the aggregation sites receiving protection?
2) Are there any undiscovered (and thus unprotected) aggregation sites?
3) How often do individual fish participate in aggregations?
4) Where do aggregating individuals come from and where do they go afterward?
5) Does demographic status (sex and size) influence participation in aggregations?
How Did We Answer These Questions?
We acoustically tagged Nassau grouper both on and off the Little Cayman west end aggregation site, and monitored movements of the tagged fish over a two year period using an array of passive autonomous hydrophones surrounding the island. Fish tagged on the aggregation site will allow us to determine where fish go after they leave the aggregation. Fish tagged around Little Cayman outside of the aggregation season will allow us to determine the proportion of fish from the Island that attend aggregations, and the frequency of aggregation attendance by individual fishes as a function of demography. Ultimately, this information will allow us to assess the current and future impacts of protections afforded Cayman’s spawning aggregations. Additionally, the study will define an aggregation’s “sphere of influence” both geographically and demographically and will thus aid in the management of aggregations throughout the Caribbean and elsewhere.
To view images and videos of our research methods, click here.
What Did We Learn?
In a nutshell, we learned that 1) all Nassau grouper attending the spawning aggregation on the West End of Little Cayman are from Little Cayman (none are traveling from other countries, or even the other two Cayman Islands), 2) all reproductively-aged Nassau grouper on Little Cayman attend the aggregation each year (and often on multiple months each year), 3) larger (older) Nassau grouper arrive earlier and stay longer at the aggregation site, and 4) the fish move back and forth off the site during the aggregation period and often will circumnavigate the island during the day.
What Were the Movement Patterns of the Fish?
Videos showing the movement of each fish between the aggregation site and their home reef can be found here.