Climate Change May Bring the Nassau Grouper’s Moonlit Orgies to an End

Just before a full moon ascends over the Caribbean, Nassau groupers congregate by the thousands in winter and early spring to lace the water column with eggs and sperm. One such bacchanalia in the Bahamas in 1971 included an estimated 30,000 to 100,000 of these four-foot-long, brown-and-white-striped fish getting their freak on. Bull sharks visit this spawning frenzy too, in the hopes of an easy meal, as do gigantic whale sharks who can’t resist all that fresh caviar.

“Put plainly, I think it may be one of the most absolutely amazing diving experiences anyone could ever hope to have,” says Brad Erisman, a fisheries ecologist at the University of Texas at Austin.

Trouble is, after decades of overfishing, we're down to our last 10,000 mature Nassau groupers or so—scientists aren't quite sure, as fish populations are notoriously difficult to estimate. What is clear is that the population is declining, and their spawning spectacles can’t occur just anywhere. The fish, which can weigh as much as a bulldog, breed only in a narrow band of temperatures, from about 74.2 to 80.6 degrees Fahrenheit (24 to 27 degrees Celsius). According to a study published by Erisman and lead author Rebecca Asch in the journal Diversity and Distributions, climate change could cook the Nassau grouper’s orgies out of existence.

Through computer modeling, Erisman and Asch dialed up all the Caribbean habitat currently conducive to grouper sexy times and then projected how much would remain over the course of the next century if current carbon pollution rates continue as expected. The findings were bleak: Some 82 percent of the Nassau grouper spawning habitat found between 1981 and 2000 could disappear by 2081 to 2100. Most of this lost habitat would be in the waters surrounding Cuba, Hispaniola, Puerto Rico, the Virgin Islands, and Central America’s Mesoamerican Barrier Reef System.

Worse still, this fish doesn’t appear to be very flexible with its mating areas. “Nassau groupers go to the same site every year for 10 to 20 years or more,” says Erisman. The fish can live as long as 29 years but don’t reach sexual maturity until between ages 4 and 8.

So while nine-banded armadillos move north and butterflies can sometimes flutter to higher elevations in response to climate change, some species are simply not as able to adapt to such temperature disruptions. Scientists don’t expect climate change to kill Nassau groupers outright—but if the fishies can’t do the deed, extinction may come calling all the same.

In fact, the Nassau grouper’s conspicuous sex life is what made it vulnerable in the first place. At around 50 pounds a pop, the fish are a popular source of protein for people from Florida all the way down to French Guiana. At some point fishermen realized the easiest way to catch these flaky fillets was to drop some nets in the water when and where the fish spawn. Once people started monitoring the species’ decline, they found that the entire Nassau grouper population had fallen by at least 60 percent over just three decades. The International Union for Conservation of Nature (IUCN) listed it as endangered in 2003.

Fortunately, governments across the Caribbean are taking action against overfishing the grouper. Mexico, Belize, and the Dominican Republic have made it illegal to fish for Nassau groupers during spawning aggregations. In the Cayman Islands, fishermen may take them only by line, not net, during that time. And farther north, the United States and Bermuda have banned Nassau grouper fishing altogether. (According to the IUCN, however, while laws protecting spawning groupers do exist, enforcement varies and is sometimes nonexistent.)

Still, Erisman is hopeful, since this species has already shown that it can bounce back when its mating rituals are protected, evidenced by boosted numbers of Nassau groupers off St. Thomas after the island banned fishing during grouper aggregations in 2005. The fact that each mature female is capable of pumping out more than 700,000 eggs a year sure doesn’t hurt—even if most will wind up feeding the many hungry mouths waiting in the waves.

And that’s part of the reason why losing the Nassau grouper would be so terrible. These fish are top predators on the reef, lunging out of the shadows to inhale wrasse, squirrelfish, damselfish, shrimp, lobsters, and even octopuses whole. They also suck down smaller groupers, such as the coney and graysby, which are themselves skillful predators. All of this makes Nassau groupers crucial members of the Caribbean’s coral ecosystems. Research shows that when Nassau groupers disappear, the effects ripple throughout the food web.

While the fishing bans are a good start, there may be a better way to protect this fish and all that rely on it. According to calculations done by Rebecca Asch, the study's lead author and fisheries biologist at East Carolina University, if the world’s nations came together and reined in their collective carbon emissions, the outlook for the Nassau grouper and its spawning aggregations isn't nearly as dire. It will still lose ground—up to 30 percent, according to the model—but that's far more favorable than the 82 percent decrease projected if we do nothing.

“There are still options for us in terms of how we respond to these things,” says Erisman. “This is why we do this sort of science.”

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