Why Are Bonefish in Offshore Waters?

In the minds of anglers, bonefish are synonymous with shallow flats. Their remarkable spawning behavior will surprise you.

Bonefish spawning group
A bonefish pre-spawning aggregation in the Bahamas. Robbie Roemer

Silver tails slicing a flat’s glassy surface at daybreak. A subtle V-wake moving along a mangrove-fringed shoreline. A trio of gray silhouettes gliding silently toward the bow over white sand. These are just a few of the scenes that come to mind when anglers think about bonefish, a species that has evolved over millennia to thrive in the shallows.

The Gray Ghost rarely ventures out of its comfort zone; in tagging studies by Bonefish & Tarpon Trust (BTT), a Florida-based conservation organization, 70 percent of recaptured bonefish were caught within one kilometer of where they were initially tagged.

Bonefish Spawning Behavior is Fascinating

But it’s a different story come spawning season, which spans fall to spring. During full and new moon cycles, bonefish leave the relative safety of their home flats and travel great distances to nearshore areas where they form giant schools called pre-spawning aggregations (PSAs) that number thousands of fish. BTT has documented bonefish migrating as far as 70 miles to reach a PSA site, a remarkable feat for a fish that rarely weighs more than 10 pounds.

Bonefish in the PSA prepare to spawn by gulping air at the surface to fill their swim bladders. Then at night they swim offshore and dive hundreds of feet in water that can be thousands of feet deep. Just think about that for a moment—a fish designed for the flats descending the distance of a football field or more in dark waters inhabited by pelagic species, sharks, and numerous other predators. Talk about being out of your comfort zone.

Just think about that for a moment—a fish designed for the flats descending the distance of a football field in dark waters inhabited by pelagic species, sharks, and numerous other predators.

“In 2019, we documented the first complete track of a bonefish spawning aggregation, and recorded a dive depth of 450 feet,” said Dr. Aaron Adams, BTT’s Director of Science and Conservation. “At that depth, bonefish experience 13 times the atmospheric pressure that they experience on the flats.”

After their deep dive, the bonefish surge back up toward the surface. The sudden pressure change makes their full swim bladders expand, helping to release their eggs and sperm. Once fertilization takes place, the hatched larvae drift in the ocean’s currents for between one and two months before settling in shallow sand- or mud-bottom bays, where they develop into juvenile bonefish.

Genetics and ocean modeling studies by BTT show that some larvae spawned in Belize and Mexico wind up in the Florida Keys. These findings underscore the need to think both locally and regionally when it comes to conserving bonefish populations.

Where do Bonefish Gather to Spawn?

Deepwater bonefish aggregation
At night bonefish swim offshore, where they spawn in deep water. Robbie Roemer

With the assistance of guides and partners, BTT has identified PSA sites along the Belize-Mexico border and in the Bahamas, where the organization helped secure the establishment of five national parks—and the expansion of a sixth—in 2015 to protect bonefish spawning areas, home ranges, and the migration pathways bonefish use to reach PSA sites.

“Identifying and conserving PSA sites is vital to the health of the fishery because they are the source of larvae that supports both nearby and distant bonefish populations,” explained Adams. “Habitat loss or degradation, or disruption of spawning behavior by boat traffic or unsustainable fishing practices, negatively impacts a PSA’s ability to produce the next generation.”

In the Florida Keys, BTT is utilizing state-of-the-art acoustic tags and the knowledge of veteran guides to home in on PSA sites, which to date have not been fully documented.

Tagging bonefish
Bonefish & Tarpon Trust’s Florida Keys Initiative Manager Dr. Ross Boucek (right) tags a bonefish with an acoustic transmitter. Ian Wilson

“There are a couple possible reasons that might explain the lack of known spawning sites in the Keys,” said Dr. Ross Boucek, BTT’s Florida Keys Initiative Manager. “Maybe for a time the size of the spawning school in the Keys shrank to the point that it wasn’t noticeable to us. Or maybe the size of the Keys population became so small that the fish completely stopped spawning for a period of time. Fish won’t spawn if there aren’t a critical number of spawning fish.”

Bonefish Spawning in the Florida Keys

Tagged bonefish release
Dr. Ross Boucek releases a Florida Keys bonefish after tagging it. Ian Wilson

Last spawning season, Boucek made a number of promising advances in BTT’s search for PSAs in the Keys. He and his team tagged 50 bonefish and logged more than 1,400 detections; 965 were at a site where several guides reported a potential PSA. Nine bonefish detected there had been tagged on distant flats, including a fish tagged 50 miles away.

“These data represent significant progress in our multi-year project that will identify bonefish PSAs in the Keys,” said Boucek. “We will continue to analyze the data, and look forward to applying it to advance our search next spawning season.”

So the next time you land a bonefish on the flats, consider the long journey that fish on your line must make into the open ocean to spawn, and do your part to make sure it swims away healthy and strong. 

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