After the winter full moon, at three sites around the Cayman Islands, scientists gather to study and document one of the wonders of the underwater world.
Hundreds, in some cases thousands, of Nassau grouper, typically solitary and territorial animals, gather together to spawn.
The phenomenon typically occurs in January or February and is essential to the survival of the species.
That is why a similar aggregation of scientists, policymakers and volunteers tracks the herd, bringing new methods and technology each year to monitor the health of the Caribbean’s most iconic fish.
This year, the research team began using “facial recognition” technology to identify individual Nassau grouper by their facial markings, which are as distinct as a human fingerprint.
The overall diagnosis at the end of the 2018 effort is mixed, according to Croy McCoy, of the Cayman Islands Department of Environment.
The department has been keeping tabs on spawning sites as part of the Grouper Moon project around the three Cayman Islands along with the Reef Environmental Education Foundation since 2002.
“Little Cayman is the real success story and that is due to the efforts of local fishermen who gave up a deep-rooted tradition of fishing the aggregation to comply with the rules and regulations set out for that area,” Mr. McCoy said.
“It is now the largest aggregation of Nassau grouper that we know of in the Caribbean basin, which is something every Caymanian should be proud of.”
At one stage, early in the 17-year research project, there were as few as 1,500 grouper at the site, off the west end of the island.
But a ban on fishing during spawning season has helped revive the population to the point where there are now between 6,000 and 7,000.
The picture on Cayman Brac is also positive, though there are far fewer grouper – between 800 and 1,000 – seen at the site.
“It is good in the sense that it is not declining,” Mr. McCoy said.
Grand Cayman is a different story.
Bradley Johnson, a Department of Environment research officer, said grouper populations had diminished around the main island.
“We are still trying to assess how many fish we have here,” he said. “The population is so low that we are not sure it is functioning as a spawning site any longer.”
The aggregations are believed to involve every Nassau grouper in the reproductive population, so the low numbers at the Grand Cayman site, off East End, are particularly concerning.
“We believe that the location is correct but because the population is so small, it makes it difficult to assess,” Mr. Johnson added.
He said the methods and data from Little Cayman showed that good management plans were effective but took time to work.
Mr. McCoy believes the current policy of protecting the sites from fishing during spawning season is enough to protect the species – provided the ban is adhered to.
“These are the maternity wards of the ocean,” he explained.
“If you kill the mothers headed to the maternity ward, it is not rocket science that you will not have an increasing population, but a declining one.”
The Department of Environment also encourages restaurants to do their part by taking Nassau grouper off the menu at certain times of year.
“If you start to see a lot of grouper on the menu around spawning season, it is probably because someone has been illegally fishing an aggregation site,” Mr. Johnson said.
The methods used to count the grouper at aggregation sites have become ever more sophisticated.
This year, scientists used a mix of “floy tagging” – counting the prevalence of previously tagged fish within a group to extrapolate a population estimate – and stereo-video – using two cameras in consort to take a “3-D video” that can be used to estimate length and abundance of fish and a video pan of the whole aggregation site.
They have also begun using facial recognition technology.
Mr. McCoy said each fish has a distinct facial pattern. One regular visitor to the Little Cayman site was christened “The Phantom” because he was black on one side of his face, and white on the other.
The technology aims to take that type of visual recognition to the next level, potentially allowing researchers to identify which individual fish are at the aggregation site each year.
“We take photos of fish faces side on and map the face using reference points that recognize the coloration pattern and store the data. When we retake photos, the software searches in our database of photos to match the facial image.
“The good thing about this is the fact we can use images we have taken over the past 17 years. This allows us to recognize each fish individually due to its unique facial pattern.”