When I first saw the acoustic tag, I couldn’t imagine inserting it under fish’s skin. It was the size and shape of the SPF 45 ChapStick that was melting in my pocket. The tag would soon be surgically implanted into a Nassau grouper, a medium-sized fish, but Dr. Rick Nemeth, and his team at University of Virgin Islands are pros. They had been researching grouper every winter for two decades.

I joined Nemeth and his team of three research technicians on a dive to tag Nassau grouper. We boarded a tiny, open, motorboat and headed south. Waves arched over the boat as I huddled behind the dashboard watching the island of St. Thomas disappear below the horizon. Soon the vessel slowed to a crawl and we arrived at the Grammanik Bank, a reef south of St. Thomas, 25 miles from shore.

Scientists on a grouper-tagging expedition. Credit: Teresa Carey

Nemeth partnered with the Virgin Islands Marine Advisory Service to relaunch the Bring Back the Nassau Grouper campaign in April 2016. The Nassau grouper population experienced a massive decline in the 1970s due to overfishing. The fish are now protected, and it is illegal to fish for them. VIMAS educates the island community on local ocean conservation issues. It provided fishers with data collection kits to help Nemeth research the grouper population. If fishers accidentally catch a grouper in their traps, they save a piece of the fin for genetic studies, and record the size of the fish and where it was caught.

Howard Forbes Jr., Coordinator of VIMAS, says the partnership makes sense. “The fishers are the ones out there every day fishing,” said Forbes who explained that a collaboration between researchers and fishers could strengthen the research that could lead to a revived grouper fishery.

Out on the bank, we spent the day hauling aboard fish after fish. Nemeth’s team moved quickly, making a small incision in each fish’s belly, inserting an acoustic tag, and stitching the fish back up. The tag collects location data by sending out a sound, which is recorded by acoustic receivers planted in a grid underwater, a method called acoustic telemetry.

Scientists measure and tag a grouper before returning it to the water. Credit: Teresa L. Carey

The goal of all this research is to understand how and why the grouper population expands and shrinks. With that knowledge, Nemeth plans to predict how fishing could affect the population, and recommend ways to reduce threats to the Nassau grouper. “Our objective is to bring these populations back to a point where we can sustainably harvest them again,” Nemeth said.

The author, right, and Rick Nemeth, leader of the tagging expedition. Credit: Ben Carey

UVI research analyst Shaun Kadisun stood behind the wheel and navigated us from one fish trap to the next. Kadisun and Nemeth have been working together since 2004. The two make a dynamic team, with Kadisun’s wild blond hair and uninhibited laughter balancing Nemeth’s quiet and serious disposition. “When I first started this research,” she said, “if we saw 20 Nassau grouper a day, we got really excited. Now we are seeing hundreds a day.” The team was focusing its studies on a newly formed spawning aggregation, a yearly gathering of fish that travel great distances for the sole purpose of reproduction.

Grouper live near the seafloor, hiding among the reef from sharks. Having just implanted an acoustic tag into a grouper, the researchers were concerned that if they tossed the fish back into the ocean, a shark might eat it as it swam down through the open water. So, Kadisun suited up in scuba diving gear to escort the fish back to the reef.

Kadisun had witnessed many grouper spawning aggregations through the years. I asked her what a spawning aggregation looked like. “A bar scene,” she said with full laughter as she jumped in and swam down toward the spawning aggregation—a fish rave in full swing.

During the spawning ritual, fish swim in a spiral pattern, rushing toward the surface. At the top of this spawning rush the males spew out plumes of sperm and the females spew eggs. The sperm fertilizes the eggs in the cloudy burst that looks like a milkshake, swirled in a blender of activity. This bizarre ritual, called a spawning aggregation, is like the Tinder app for fish. It is the only time Nassau grouper reproduce, increasing their chance of finding a mate.

Nassau grouper spawn just after each full moon for only four months in the winter. Because they spawn in the same place and the same time every year, fishers know exactly where to find them. Fishers who set their traps in a spawning aggregation could catch a lot in a little amount of time, giving them the impression that grouper are more abundant than they really are.

Kadisun explained how overfishing of a spawning aggregation in the 1970s lead to the demise of the Nassau grouper.

“They completely fished out, extirpated, an aggregation site, and fish disappeared from our island,” said Kadisun. Forty years later, the Nassau grouper is still considered threatened and could become endangered. Now that fishing for Nassau grouper is prohibited, Nemeth’s team believes they may be observing a population comeback.

Many animals migrate en masse, such as the Leatherback sea turtle or the African grey elephant, to feed or reproduce. Scientists often recognize mass migrations as critical opportunities to protect a species.

“If we don’t protect spawning aggregations,” said Kadisun, “then we are going to lose our fish stocks.

The UVI research team believes fishery managers should also consider fish spawning aggregations as an important part of fishery management. The reforming Nassau grouper spawning aggregation could be a second chance to create a sustainable fishery.