Rare deep-water spawning discovered in shallow-water bonefish

January 11, 2021

Bonefish gather in huge schools near the surface before diving to spawn. (FAU/Tom Henshilwood)

U.S. scientists for the first time documented how bonefish, an economically important fish that lives in waters no deeper than six feet, spawn at remarkable depths as low as 450 feet, revealing important information for conserving the near-threatened species.

Their study, published in Marine Biology in November, found that bonefish, which contribute to hundreds of millions in annual recreational fisheries revenue around the Caribbean Sea and western North Atlantic Ocean, have unique spawning habits for shallow-water inhabitants, reaching unprecedented depths.

The study focused on the 150-million-year-old Albula vulpes Caribbean species found in shallow-water flats habitats, which live at three feet for almost their entire lives. Then there’s their spawning migration, a three-point event that’s unique among coastal marine fish and happens offshore at night.

“This is the only fish that we know that has such a high disparity between its daily lifestyle and the water that it inhabits and the waters that they’re spawning in,” said Steven Lombardo, the study’s first author and a PhD candidate at Florida Atlantic University’s Harbor Branch Oceanographic Institute in Fort Pierce.

During their spawning migration, bonefish dive to 450 feet, then move below 325 feet for two hours before spawning in a quick ascent to 220 feet, the study showed. They are likely spawning at pycnocline density gradients and thermocline temperature gradients.

Other coastal shallow-water fish generally won’t go far offshore to spawn; perhaps to the reef edges, where depths reach 60 to 120 feet.

“Effectively, evolutionarily, [bonefish] haven’t changed at all, and so those conditions at those deep depths are a lot more stable than at the surface,” Lombardo said. “I think that’s what these fish are seeking out.”

Even without precise knowledge of their unusual spawning habits, bonefish hold great cultural weight wherever they’re found, including in Cuba, Mexico and the Bahamas, where they are featured on the nickel. The annual economic impact of the bonefish, tarpon and permit catch-and-release flats fishery exceeds $169 million in that nation, $465 million in the Florida Keys and $53 million in Belize, and a single bonefish is estimated to carry $7,000 in annual economic impact. 

For anglers fishing in the flats, the sought-after fish are renowned for their fight.

“A lot of these countries that these fish are found in are fairly impoverished,” Lombardo said. “The sport of fly fishing has really brought in lots of economic value.”

The new study’s findings are the culmination of about two decades of work — a little over half of which was spent actively tracking bonefish.

In 2013, two of Lombardo’s co-authors, the University of Massachusetts, Amherst’s Andy Danylchuk and Aaron Adams of FAU and Bonefish and Tarpon Trust, attempted to track the spawning fish. Their acoustic telemetry tags, smaller than a pinky finger, didn’t work below 50 meters. But using a directional hydrophone, a sort of wand that receives pings from the tags, researchers could still show the fish reached 164 feet.

Five years later, with support from Harbor Branch’s Bonefish Reproduction Project, which studies reproductive biology and ecology to understand why the Florida Keys population has been declining since the late-1980s to mid-1990s, the group scored a fully captained and crewed on-site research vessel courtesy of the Fisheries Research Foundation, which Lombardo credited with enabling the findings.

They revisited the work with tags that could periodically emit frequencies for up to 300 meters, but lost the fish on their first attempt as it dove below the directional hydrophone. 

Then in 2019, pointing the hydrophone straight down to keep track of bonefish during their abrupt, steep dive, Lombardo witnessed “a perfect storm.”

On the last of a 10-day trip to the southern shore of Abaco, the Bahamas, where scientists were regularly watching for 15 to 18 hours, their subjects finally decided to move offshore from the pre-spawning location. At that spot, where the groups can number 10,000, researchers had surgically implanted the battery-operated tags into four fish.

Around 1 a.m., the bonefish began to dive, and the team watched on tenterhooks as they descended on their three-point journey. 

Two of the subjects were eaten. Another’s tag died. The same should have been true of the surviving fish, as its tag was the first of the quartet to be attached and turned on.

But the tag spawned out at 6:05 a.m., as indicated by steady movement of the depth readings toward the ocean floor. 

“The captain was calling us to come back to the yacht the next morning at about 5 a.m.” to pull anchor and go back to Florida, Lombardo said. “But he was nice enough to give us an extra hour — and an hour and about three minutes later, the fish spawned the tag.”

“It was all absolutely last-second perfection,” he said.

Scientists can now glean a better understanding of how the bonefish they observed are connected to other populations across the Caribbean and Florida Keys. There are no known pre-spawning aggregation sites in the Keys, where five to seven decades ago it was not uncommon to catch 15-pound animals, but the researchers are trying to find one.

“Those fish aren’t around anymore,” Lombardo said, as a rise in boat traffic and decline in water quality led to substantial habitat degradation.

For the second part of his dissertation, Lombardo is modeling where the bonefish larvae end up, which is expected to reveal more about why the fish spawn where they do — for example, if they’re seeking out certain temperatures or water densities.

“In order to manage any species — in this case, fish — you really need to understand the fish, the whole reproduction and survival process,” he said. “This kind of lays the groundwork toward really fully understanding and capturing how these fish are contributing to the population at large.”

The study “Novel deep-water spawning patterns of bonefish (Albula vulpes), a shallow water fish,” published Nov. 23 in Marine Biology, was authored by Steven M. Lombardo, Florida Atlantic University; Aaron J. Adams, Florida Atlantic University and Bonefish and Tarpon Trust; Andy J. Danylchuk, University of Massachusetts, Amherst; Cameron A. Luck, North Carolina Division of Marine Fisheries and Florida Atlantic University; and Matthew J. Ajemian, Florida Atlantic University.

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