New Depths

How bonefish reproduction research—in the lab and in the wild—could help save a species in decline

BY ALEXANDRA MARVAR

From two-foot-wide bunks in the bow of a Bahamian lobster boat called the Lady Breanna, to a ribbed inflatable research vessel being nudged in the dead of night by a curious 9-foot tiger shark, to a 25,000-gallon laboratory tank full of bonefish broadcasting eggs and sperm in a historic spawning event, four years of investigation into the mysterious behaviors of bonefish and their offspring have taken scientists to some wild places.

On one recent expedition, a research vessel hovered for days near a cloud of thousands of bonefish. Preparing to spawn, the bonefish looked like a glistening bubble of black ink suspended in the cerulean flats off the Abaco Islands. Suddenly, on day four, the fish mobilized, heading beyond the steep drop of the continental shelf and taking an unexpected dive into the abyss of the Providence Channel. The research team watched, astonished, as the fish plunged deeper than experts had ever thought possible, to 450 feet.

“We knew from previous research that they went offshore at night to spawn, and that they went deeper, but we had no idea that they went that deep,” said Dr. Aaron Adams, a senior scientist at Florida Atlantic University Harbor Branch Oceanographic Institute and Director of Science and Conservation for BTT. He is also one of the lead investigators of the Bonefish Reproductive Research Project, an endeavor bridging fieldwork and lab research to unravel the mysteries of bonefish reproductive behavior. “That they live their entire adult lives in the shallows, and they’re able to dive to those depths to spawn—it’s pretty tremendous,” Adams said. “When the initial foray into research of bonefish reproduction began back in 2010, nobody really expected that.”

And yet, every specimen in this culturally, ecologically and economically critical species, with its combined $700 million-per-year fishery, likely originated on a deep dive like the one Adams’ team observed. With bonefish populations in some areas in decline for reasons yet undetermined and a conservation status of “near threatened,” researchers and conservationists are just as fixated on these new discoveries about bonefish as they are by how much there is to learn.

BTT Board Chair Carl Navarre has been bonefishing since learning from his father in the 1960s. Now a seasoned conservationist and former director of the Peregrine Fund, Navarre believes that wild fish stocks face unprecedented potential environmental threats—from climate change, water quality, pollution, and disease among other things. These threats—separately or in concert—could decimate populations of bonefish in geographic areas very quickly. Instead of waiting for that to happen, and then spending five to ten years learning how to restore the species, Navarre has spearheaded the effort to develop the technology to reproduce bonefish now, so that a species restoration program could be started very quickly before they vanish entirely. “First and foremost, our bonefish reproduction project is an insurance policy against disaster,” Navarre said. “Secondly, there is a tremendous amount of knowledge about bonefish that we are producing which will help us to develop programs to manage the fishery more effectively.”

This is the premise of the Bonefish Reproductive Research Project, funded by BTT and the National Fish and Wildlife Foundation. Since its launch in 2016, it has yielded some groundbreaking advances: a new understanding of bonefish hormone profiles and healthy eggs; close observation of bonefish embryos and larvae; successful bonefish spawns in captivity; and documentation of never-before-seen spawning behaviors in the wild, like the mind-boggling 450-foot dive in the Abacos. Questions remain about how bonefish spawn, where they spawn, and what they need in order to survive—and thrive—in the first hours, days, weeks and months of their lives.

The key, the researchers say, is starting at the beginning—the very beginning.

THE DANCE OF THE LEPTOCEPHALI

Fish biologist Dr. Jon Shenker, a recently retired professor of marine biology at the Florida Institute of Technology and a leader on the project, has been studying bonefish in their larval and juvenile stages for decades. The first time he saw a bonefish freshly hatched, he was mesmerized: “They are these odd little larvae called leptocephalus, Latin for ‘slender head,’” he said. “They’re glass-like ribbons, anywhere from one-and-a-half to three inches long, absolutely, completely transparent, and they are beautiful, beautiful things to look at.”

Leptocephali are the same unusual larvae that occur among eel species, he explained—another creature whose reproductive habits are shrouded in mystery. “They are so different from any other kind of other fish larvae that they just intrigued the heck out of me. And one thing about fish biologists: We’re always developing fascinations for weird things, and anybody who starts working with these leptocephalus larvae tends to become a little crazy about them.” On Shenker’s wall hangs a two-foot-long leptocephalus glass artwork he commissioned from a local glass artist. It isn’t the only piece of leptocephalus-inspired artwork he owns.

The opportunity to study these ghostly little sea ribbons in the very first moments of their lives is not easy to come by: It requires the ability to observe bonefish embryos in real time. So the project team, with help from expert anglers and guides in the Keys, catch broodstock (wild adults collected for spawning) and transport them to tanks at Harbor Branch.

“Then,” Shenker said, “we have to try to convince these fish that they want to spawn in captivity,” yielding leptocephali for study. That’s where the aquaculture expertise of another of the project’s lead investigators, Dr. Paul Wills at FAU Harbor Branch Oceanographic Institute, comes in.

SPAWNING CAPTIVE BONEFISH

The massive bonefish tanks in Wills’ lab are designed to simulate a number of factors in the marine environment, from salinity and water temperature to the hormones in the fishes’ bodies. The more closely the lab can resemble the sea for these wild bonefish, the better chance the fish will do what they naturally do: reproduce.

“We do have an LED light that we’ve calibrated to the lumens of a full moon that comes on during the full moon,” Wills said. “Most research has shown that that’s generally not a key, but we didn’t want to leave anything behind.”

Two larval tanks called Kreisel Tanks sit in a temperature-controlled water system.

Anthony Cianciotto

Two years ago, the team achieved a major milestone: the spawning of wild bonefish in lab captivity, for which they employed a decades-old method used in farmed salmon and trout called strip-spawning. “That was the first time we’ve seen those early, early stages of the species,” he said, referring to the transformation of embryo to leptocephalus, up close and personal. “Now we know what the larvae look like. We know how they behave in the tanks. Those are all innate behaviors—they’re going to be doing the same thing in the wild.”

Bonefish eggs ready for fertilization.

Anthony Cianciotto

How can this further conservation efforts? According to Wills, a great deal of what’s known about bonefish habitats comes from mathematical flow models that predict how bonefish larvae will be distributed by currents throughout the Caribbean, Florida and the Gulf of Mexico. Thanks to findings by Shenker and colleagues in the 1990s, researchers know bonefish in their larval state may drift for between 41 and 71 days before they settle into a habitat. And in this amount of time a tiny leptocephalus the weight of a feather can make quite a journey.

“A better understanding of how the animal actually behaves helps to improve those mathematical models,” Wills explained. “Up to now, models have just been based on a theoretical sphere of a certain density at the surface. Now, we know they’re only a sphere for a short time before they hatch. Then, they have behavior.” For example, the team has learned that bonefish larvae hang with their head up upwards for a period of time in the water column, he said. “Those things can now be incorporated into the flow models to improve prediction of where they start out and where they’re going to drift to.”

Of the 450-foot dive Adams’ field team observed, Wills noted, “That was the first time we’ve ever been able to collect this data. That wild component allowed us to, in the comfort of a warm laboratory, mimic those conditions. You can see how the two pieces—research in the lab and in the wild—feed into each other for a better understanding of the species,” he said. “Then, we have the ability to better protect it.”

Going forward, Wills and colleagues are slowly eliminating factors one by one to see which factors are key to prompt, beyond strip-spawning, the spawning of bonefish by their own volition in captivity. At the outset, the team wasn’t sure such an event was even possible. But a pivotal breakthrough came by chance: They heard a rumor that bonefish had spawned on their own in an aquarium at a Bahamian resort—by complete accident. “When we found that out through the grapevine,” Wills recalled, “we knew that the actual dive to depth probably wasn’t the trigger—that there was some other factor.”

Bonefish embryos developing.

Anthony Cianciotto

Now, he said, they believe that key factor is a subtle change in water temperature—also a key factor in the spawning of red drum. Applying this theory, this fall Wills’ lab succeeded in inciting bonefish to spawn with minimal interference—rather than stripspawn the fish, after a hormone injection the fish spawned on their own. It was a victory, Wills said, and it paved the way to the next challenge: In the span of a week, primitive bonefish embryos transform, developing functional eyes, a jaw, and a digestive system—and their survival relies on finding food. “The difficult part that we have to get through now is finding something they’ll eat.”

CLUES FROM EELS

Looking to Japanese research on the bonefishes’ close relative, eels, the team is developing a working theory about what bonefish leptocephali eat, along with mechanisms for how to feed it to them. According to Shenker, marine snow is a flocculant aggregation of various sea creatures’ mucus that wafts through the water column picking up bacteria, phytoplankton and more mucus on the way. Shenker hopes baby bonefish will find this delicious.

A five-day-old bonefish larva.

Molly Wightman and Maddy Wheeler

RESTORE, NOT RESTOCK

Shenker emphasized the team has no intention of spawning cultured fish in a laboratory and then releasing them into the wild; this could compromise wild populations in ways that are impossible to predict. Rather, he looks forward to a day when juvenile bonefish from a lab can help teach us more about how they behave in the wild: “If we can get them to spawn in captivity, and we can get them to start producing juveniles on our own, we can do what are called cage studies, where we study caged bonefish and find out how they grow in different habitats,” he said. “One hypothesis is that we’ve impacted the best nursery habitats for the juvenile bonefish in the Keys by building bulkheads and causeways and destroying the sandy habitats. If we can identify suitable juvenile habitats, now we’ve got a target for habitat restoration.”

Indeed, Adams added, putting habitats at the center of bonefish management is the beating heart of the research.

A fully formed bonefish larva.

Anthony Cianciotto

“We’ve learned that a fish population can be overfished, and you can recover from that by changing regulations to reduce overfishing, but once a population loses a sufficient amount of habitat on which to depend, there’s no recovery. You’re done,” he said. “That hasn’t been incorporated into natural resource management for the marine world. Of course managing the fisheries is important, but unless we start including this habitat component in it, fishery agencies can manage all they want, but they’re still going to see fishery declines because habitat is getting destroyed.”

“Thinking big picture,” Adams said, “what drives me and colleagues is being able to learn what makes this fish tick, and getting enough of that information into fisheries management that we can change the paradigm, and shift fisheries management to focus on habitat.”

Alexandra Marvar is a freelance journalist based in Savannah, Georgia. Her writing can be found in The New York Times, The Guardian, Smithsonian Magazine and elsewhere.