This fish lives by the shore but dives deep to spawn, breaking records

Bonefish spend most of their time in shallow water. But to spawn, they travel to the deep sea, diving below 450 feet in an unprecedented feat.

By Douglas Main
Published 21 Dec 2020, 14:12 GMT
Bonefish (Albula vulpes) foragie for crustaceans in shallow waters of the Bahamas. These animals perform unprecedented ...

Bonefish (Albula vulpes) foragie for crustaceans in shallow waters of the Bahamas. These animals perform unprecedented dives to spawn in the deep sea.

Photograph by Shane Gross

It’s not easy to catch bonefish. Wary of humans, these silvery fish—nicknamed grey ghosts—blend into the sandy flats where they forage and are quick to dart away.

Several species of the fish are found in shallow tropical waters worldwide. To capture one of these eluders, skilled fishers spend years learning how to mimic the movements of bonefish prey, such as shrimp, with their lures. “It’s an absolute art form,” says Matt Ajemian, a fisheries ecologist at Florida Atlantic University’s Harbor Branch Oceanographic Institute. When they do take the bait, “these fish hit hard,” and put up a fight, Ajemian says. “They’re real juggernauts—that's why people love them.”

Their nickname refers not only to their evasiveness but also fits their mysterious spawning habits. Bonefish in the Caribbean and western Atlantic hang out in water that’s only a few feet deep. But in the fall breeding season, they disappear as they aggregate and jet out to sea. Until recently researchers didn’t have a good idea of where they were going.

Ajemian and his doctoral student, Steve Lombardo, have been attempting to track the Caribbean bonefish, Albula vulpes, for years off the northern Bahamas, including the Abaco Islands. After a few seasons of refining techniques, in November 2019 the team tagged and shadowed bonefish as they spawned—and were shocked by what they found.

As described in a paper published in the journal Marine Biology, the researchers discovered that Abaco bonefish travelled from the shore to the nearby edge of the continental shelf—and then dove to nearly 450 feet. They also stayed in the deep water for more than two hours.

That’s peculiar behaviour. “No near-shore or shallow-water fish has ever been observed going out into the open ocean and to these depths to spawn,” Lombardo says.

The finding is critical to conservation because “if you don’t know where fish are spawning and how they are doing it and the conditions they need, it’s very difficult to manage a population,” Ajemian says. The Caribbean species is considered near-threatened by the International Union for Conservation of Nature, due in part to overfishing. Though eaten in some areas of the world, most recreational fishers target them for catch and release. They’re called bonefish because they have plentiful and dense bones “that make plucking the meat quite painstaking,” Ajemian adds.

Fleeing shore

Before spawning, bonefish form large schools near the coast at dusk, Ajemian explains. For about 10 minutes, they jump porpoise like at the surface. “Then they just beeline away from shore,” he says.

Sometimes it’s a false start—they begin the journey out to sea, and then for whatever reason head back. But one night in November 2019—when the moon was full—everything aligned. The team had placed an acoustic tracker inside a bonefish, which they tracked using a hydrophone aboard a ship provided by the Fisheries Research Foundation. (He also placed sensors that failed or were lost to predation.)

As the fish swam out, the scientists also kept tabs on them via sonar. Once the fish reached the continental shelf, a horizontal journey of only three miles, they dived down, staying below a depth of 300 feet for more than two hours. They moved up and down a bit, before making a final upward rush—from a depth of 430 feet to 220 feet in less than two minutes—to spawn.

This quick ascent would cause any gas in their body to expand and help them expel eggs and sperm, Ajemian adds. Exactly how bonefish bodies are able to withstand the extreme change in pressure is unknown, and unprecedented for any shorefish, the researchers say.

Spawning in the depths

The finding begs the obvious question: Why do the fish bother? “You’re going out at night, there are sharks circling this school the entire time,” Ajemian says. “The risk is high.”

The payoff must be considerable. “With any cost there has to be a benefit, otherwise it wouldn't exist,” says Alan Friedlander, a fisheries ecologist and chief scientist for the National Geographic Society’s Pristine Seas Project, which wasn’t involved in the paper.

As the researchers tracked the bonefish, they recorded water density, temperature, and other variables. They found the fish spent much of the time at a region where warmer surface water comes into contact with cooler bottom water but doesn’t fully mix.

This forms a distinct layer that might be an ideal place for eggs to hatch and develop, the researchers hypothesise. Little bits of detritus called marine snow collects here, which likely provides food for the hatchlings, Lombardo says. The layer also has a relatively constant temperature and provides some shelter from storms and predators.

It may also act as a type of conveyor belt to usher the eel-like larvae toward shore as they develop into young fish, a phenomenon Lombardo plans to study.

Friendlander agrees the study data suggests the fish are sensing temperature and pressure and “are looking for the right conditions to spawn.”

Primitive fish

Bonefish haven’t changed much in more than 100 million years—and it's likely this behaviour has helped them endure, Ajemian says.

European eels, which are related to bonefish, make even more spectacular journeys to spawn in the Sargasso Sea. But eels make only a one-way journey. (Learn more: epic eel migration mapped for the first time.)

“When eels spawn, they die, they don’t do this repeatedly like bonefish,” which might make multiple trips to breed in a single month, Lombardo says.

While this research focused on bonefish in the western Atlantic, “we do have evidence this behaviour may be occurring with Pacific bonefish species too,” Ajemian says.

This study suggests that to manage bonefish species in decline, larger areas may need to be protected “to take into account that dynamic oceanography” that determines where they spawn, Friedlander says.

"We gotta make sure we can maintain these animals and their natural behaviours,” Friedlander says. If you impair a fish behaviour, including spawning, through human impacts such as overfishing, you could doom a fish stock without even realising it, he says. “You wouldn’t even know [the behaviour] was there, and it might be really important to their survival.”

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