Baby Grouper Adrift!

Why "Adrift"?

Pick a point on the map that would be the worst imaginable place to go swimming or scuba dive - cold and deep, with big waves meshing over the confluence of strong currents. That's most often where grouper go to spawn on winter nights. Why there? What is the importance of such places?

For a long time, scientists thought fish spawned at these sites to spread larvae all over. Recent evidence, however, suggests that fish spawn at these sites, and at certain times of the day and tide cycle, in order to keep larvae close to home. 

Through the Grouper Moon Project research efforts, drifters will help researchers visualize where grouper larvae might end up after spawning. The technology allows us to follow the larvae as they complete a 45-day ocean journey, and with any luck, make it to coastal areas to settle and grow. 

Why Track Larvae?

Through the Grouper Moon Project, REEF scientists and volunteers are evaluating the importance of Nassau Grouper spawning aggregations to local fisheries and coral reef ecosystems in the Cayman Islands. During spawning aggregations, thousands of large grouper meet at known reef sites for short periods of time (days to weeks) and release their gametes in massive spawning bursts at dusk. The millions of fertilized grouper eggs form great plumes in the water column that fade away into the darkness of night. Where do the resultant grouper larvae end up?  Answering this question is critical to managing Nassau Grouper stocks effectively.

If grouper larvae from the Cayman Islands mostly spread to other Caribbean locations and very few return “home”, efforts to recover Nassau Grouper in the Caymans may be dependant on spawning aggregations elsewhere. On the other hand, recent evidence from both modeling and genetics studies suggest that reef fish larvae spawned locally ultimately settle locally. In order to understand where grouper larvae from Cayman spawning aggregations go, the Grouper Moon Project uses satellite drifter buoys, released during the spawning period, to track the water column that the grouper eggs are released into. The subsequent 35-40 day paths of these drifters  yield fundamental insights into the final destination of grouper larvae spawned in the Caymans.

How Are Larvae Tracked?

The Grouper Moon drifter buoy project began in 2008, led by Grouper Moon scientist Dr. Scott Heppell of Oregon State University.  The objective is to determine where the currents surrounding Little Cayman carry larval grouper following mass spawning. Nassau Grouper larvae settle onto the reef approximately 35-40 days after they are spawned.  Thus, the drifter buoys used to emulate passive larval transport (e.g. track the water column where larvae were spawned) were programmed to report positions via satellite at regular intervals for at least 40 days post spawning.  

While Nassau Grouper spend as much as 12 days at the spawning site on Little Cayman, they only spawn on 2 or 3 of those nights. During these spawning nights, the drifter buoys are released. The buoy consists of a large hollow plastic float containing the GPS and satellite up-link electronics, and a large “sock” tethered below the float. This sock forces the drifter to follow the sub-surface water column where the larvae are spawned.

Are We Really Tracking Larvae?

Larvae move both passively (go with the flow of the ocean!) and, especially as they get older and bigger, actively (adjust their position in the water column and/or swim to control their path). Recent research suggests that larvae can respond to cues such as reef sounds in order to find suitable reef areas to settle on. Thus, grouper larvae movement is driven by ocean currents in a regional context, but through behavior, larvae can navigate to good settlement sites once they get close to reef areas.

By tracking larvae with satellite drifters, we are capturing the potential for passive movement of larvae, but not the active movement larvae may make.  As such, our grouper tracking represents a conservative estimate of the entrainment of Nassau Grouper larvae in the Cayman reefs.  If we could track BOTH active and passive movement of larvae, it is likely our findings would show grouper larvae finding reef sites and behaving in ways that kept them close to local reef sites. However, even without the active part of larval transport, the satellite drifters provide key insights into where larvae from the spawning site likely end up in a regional context.

What Did We Learn?

The first insight that the drifter program has provided into the nature of the spawning event has to do with the behavior of the near-shore currents before, during, and after the spawning. On the nights leading up to the spawning event, the currents carry the buoys in seemingly random directions, but on the night of, and for several days after the spawning, there is a fundamental change in the currents’ behavior.  While before the spawning night, the buoys were carried away with the currents, on nights of peak spawning they are held close to shore in eddies. Over a twelve-hour period following release on spawning nights, some buoys barely cover any ground at all.  At this point it’s unclear what the benefit is of spawning during these eddy currents – perhaps it provides greater opportunity for eggs to become fertilized before they’re carried off by the prevailing southerly currents – but it is clear that whatever the reason, it’s important enough that the Nassau Grouper spawning on Little Cayman have evolved to hold off on their spawning frenzy until the night these eddies begin.

While the behavior of the drifter buoys in the short-term provides insight into the spawning strategies of the Nassau Grouper, the long-term drifter buoy data provides the most pertinent and valuable information with regards to keeping this resource protected and well-managed. The long-term buoys that are released are invariably carried south by the currents, at first suggesting that the larvae will be carried far from their origins in the Cayman Islands. However, about halfway through their 35-day journey, the buoys make an about-face and begin to head north again. About half the time, the buoys are back to within 100km of the Cayman Islands by 35-40 days post spawning. Again, this is only the passive side of larval transport – it seems likely that once in the vicinity of the Cayman Islands the grouper larvae will navigate to reef areas to settle though behaviors and responses to environmental cues such as reef sounds.

So, what do these findings mean?  We believe it shows that the population of Nassau Grouper in the Cayman Islands is largely self-recruiting: the grouper spawned on Little Cayman end up on Little Cayman. By implication, overfishing Nassau Grouper spawning aggregations in the Cayman Islands would mean destroying the local supply of grouper in the future.  Protecting the grouper spawning sites into the future will therefore protect the overall health of the Nassau Grouper population in the Cayman Islands by ensuring a continued supply of juvenile grouper now and in the future.