For the seventh year in a row, the Heppell lab has traveled to the Cayman Islands, British West Indies, to participate in the Grouper Moon Project run by the Reef Environmental Educational Foundation (REEF) in conjunction with the Cayman Islands Department of the Environment (CIDOE). The focus of the Grouper Moon Project is the spawning aggregations of Nassau grouper (Epinephelus striatus), a large bodied, top level predator on Caribbean reefs that plays a role in structuring Caribbean reef communities. These amazing fish are typically solitary, maintaining individual territories on home reefs. But, once or twice a year they congregate in a specific location for the express purpose of spawning. This congregation is called a spawning aggregation or SPAG for short.
Unfortunately for the Nassau grouper, the timing and location of their SPAGs are highly predictable. For example, the Nassau grouper in the Cayman Islands return to their traditional spawning grounds year after year on the first full moon that happens 30 days after the winter solstice. Because of this predictability and the ease at which Nassau can be caught on SPAGS, the Nassau grouper is now listed as endangered by the IUCN. Fisherman can simply not resist the lure of thousands of large fish all gathered in one place at one time, and once an aggregation has been discovered it does not take long for the spawning population to be wiped out. Aggregations were once between 30,000 and 100,00 fish strong (Smith 1972). Today, more than 75% of Nassau grouper SPAGS are thought to be defunct or declining and aggregations considered healthy are rarely larger than a couple thousand fish (De Mitcheson et al. 2008). The story of Nassau grouper spawning aggregations in the Cayman Islands is sadly not at all different than the rest of the Caribbean, with one notable exception. The Cayman Islands once hosted 5 Nassau grouper SPAGs; today the only one considered “healthy,” is the aggregation on the West end of Little Cayman Island.
Fortunately for the Nassau grouper, the Cayman government was proactive in placing an eight year long fishing ban at both current and historic aggregation sites throughout the Cayman Islands in 2003 and consequently may have preserved the largest Nassau grouper SPAG left on the planet. It is this aggregation of approximately 3-4,000 fish that we have had the pleasure of visiting over the past two months.
This year was a bit of a strange situation for the Grouper Moon team. The full moon in January fell 29 days after the winter solstice. The fact that this full moon was so close to the 30-day cut off creates what the team refers to as a split moon, which has only occurred once in the history of the project. The team was not entirely sure which moon would be the “big” month, so planning the field season was difficult. Every year there is a large aggregation of fish and a minor aggregation, occurring on either the preceding (if the first full moon of the year is less than 30 days after the winter solstice) or following (if the first full moon is 30 or more days after the solstice) the big month. Confusing, eh? Because the first full moon was so close to the cut-off the team had to make an educated guess as to which month would be the big month and rely on a
skeleton crew of divers to observe any fish in the other month –We picked February as the big month, so in January a small team consisting of Selina Heppell and me (Stephanie Archer) from OSU, Brenda Hitt as a volunteer for REEF, and Keith Neale from CIDOE headed out to the West end of Little Cayman for the January full moon. After rough weather on January 17th and 18th our January team had good weather and virtually no current for the rest of our stay, which led to a very productive month. I am studying the effect of the influx 3,000+ large predators on the reef which hosts the spawning aggregation. Specifically I am looking at trophic interactions between the Nassau grouper and cleaning organisms and planktivores as well as nutrient exchange from the grouper to the host reef via filter feeding organisms such as sponges.
This was a preliminary field season for me and I was able to collect quite a few samples! Because of the early high winds the fish had already started to show up by the time we were able to get out to the aggregation site. While the number of fish never topped 2,000 we were treated to quite a show. Many of the fish that showed up in January were very friendly and were likely from Bloody Bay Wall, a marine park located on the North side of Little Cayman where fishing is prohibited. In addition to the water samples I was collecting we conducted regular counts of the fish and used a video camera with a laser-bracket set up which allows us to get an accurate estimate of the length distribution of the fish on the aggregation site. Counts and the length distribution of the fish at the SPAG are important pieces of information, as they will allow us to understand how the composition of fish attending the SPAG is changing over time.
When we entered the water on the morning of January 25th, we knew that day was different. While on the spawning aggregation Nassau grouper exhibit three distinct breeding color phases that they do not typically show on their home reefs. These color phases are dark, white bellied, and bicolored. The bicolored color phase starts to show up more and more as the fish get closer and closer to the night they are going to spawn.
On the morning of January 25th the number of fish donning the bicolored color phase was much higher than previous days. There was also a palpable increase in the energy of the aggregation. We predicted the fish would spawn that night, and we weren’t disappointed. We entered the water close to 5:30 and the show had already started. There was a milky white band of gametes hovering just over the wall of the reef where the fish usually spawn. We had not been in the water long when we saw our first spawning burst. By the time our bottom time was used up there were spawning bursts every few seconds and the visibility was dropping fast. We left the water exhilarated. The next day the fish were all still there and still all dressed up in their spawning colors and we were treated to a second night of spawning, although this night was slightly subdued compared to the night before. The next morning, our last dive for the month of January, many of the fish had gone home and the ones remaining were back to their typical coloration. Another month of documented grouper spawning was complete and most of our small team prepared to return to their everyday lives. Selina, however, was gearing up for a month of semi-sabbatical on the tropical paradise that is Little Cayman Island.
When I returned for the February full moon the conditions could not have been more different. The crew was significantly larger with Drs. Christy Pattengill-Semmens and Brice Semmens from REEF, REEF volunteers Dr. Steve Gittings, Denise Mizell, Sheryl Shea, and Heather George, Our World Underwater scholar Josh Stewart, Guy Harvey and his camera man George C. Schellenger, the OSU contingent of Drs. Scott and Selina Heppell and myself, Wayne Sullivan, the owner of the Glen Ellen and his tech-diver crew of Thor Dunmire and Doug Kesling, and our collaborators from CIDOE Phil Bush, Croy McCoy, James Gibb, Keith Neale, Delwin McLaughlin, and Kevin Jackson. In addition to the dramatically expanded crew we had strong winds and even stronger wind driven currents. While more fish showed up in February, the currents led to some strange behavior by the fish and spawning was only observed by divers operating on rebreather diving equipment based off of the Glen Ellen. The specialized diving equipment allowed them to extend their bottom time and stay in the water after dark, long past when the rest of us on open-circuit dive gear had to get out of the water.
February was the month we released our four oceanographic drifters. The drifters are composed of a waterproof ball that floats on the surface and contains a GPS and a satellite phone, which is attached by a tether to a long “sock” which extends into the water column and allows the device to travel with the currents, not the wind. The drifters, once activated, transmit their location every 10 minutes to a satellite. We can then track the position of the drifters to understand what may be happening to the larval fish as they are transported off the SPAG site and perhaps gain an insight into why the fish are choosing that place and the nights they do to spawn. We released one of these devices each night leading up to spawning and retrieved it 12 or more hours later. There were three nights of spawning in February and a drifter was released at the spawning site each night of spawning (we did not retrieve these drifters). You can follow the tracks of these drifters right now, and read about previous years of drifter work on the Baby Grouper Adrift website.
One reason for the expanded crew in February was the production of several educational videos being made about the spawning aggregation. These videos will range in length from 5 minutes to 1 hour and will be vitally important as CIDOE begins their fight to extend protections for the spawning aggregations. The proactive closures the Cayman Government had the foresight to implement 8 years ago will expire before the next spawning season. If you are interested in learning more about the push to protect Nassau grouper spawning aggregations in the Cayman Islands into the future you can read more here and here and always at the Grouper Moon Project website.
Grouper Moon Links:
Sign a petition to help save the Little Cayman spawning aggregation here : http://www.thepetitionsite.com/106/protect-the-world39s-last-spawning-aggregation-of-nassau-grouper/
The Grouper Moon Project http://www.reef.org/programs/grouper_moon
The Cayman Islands Department of Environment grouper page http://www.doe.ky/groupers/
Baby Grouper Adrift http://www.reef.org/programs/grouper_moon/adrift/
NOAA’s Nassau grouper site http://www.nmfs.noaa.gov/fishwatch/species/nassau_grouper.htm
Grouper in the News:
De Mitcheson, Y. S., A. Cornish, M. Domeier, P. L. Colin, M. Russell, and K. C. Lindeman. 2008. A global baseline for spawning aggregations of reef fishes. Conservation Biology 22:1233-1244.
Smith, C. L. 1972. Spawning aggregation of nassau grouper, epinephelus-striatus (bloch). Transactions of the American Fisheries Society 101:257-&.