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Tracking The Next Generation
A novel BTT study unveils the mysteries of bonefish larval dispersal throughout The Bahamas.
BY ASHLEIGH SEAN ROLLE
In the vast waters of The Bahamas lies a hidden world bursting with life, where bonefish larvae embark on a journey that seems to defy every known odd. Below the moonlit surface of the Northwest Providence Channel, a plume of bonefish eggs begins to hatch a mere 24 hours after their parents made the perilous migration offshore to spawn in water hundreds of feet deep. These millions of unassuming planktonic particles are giving rise to larvae that will embark on a remarkable oceanic journey. Some will drift and navigate the ocean currents for 41 to 71 days, undergoing one of the longest developmental processes documented across fish species, all in preparation for settling in an environment dramatically different from the abyss in which they were conceived—the coastal sand and mud-bottom shallows of the nursery grounds.
In a pioneering effort to better understand a species that acts as the lifeblood for an entire industry in the chain of islands, Bonefish & Tarpon Trust set out on a series of groundbreaking projects to study the movements of these small, yet mighty larvae. To understand how larvae are dispersed upon hatching is to understand how bonefish populations are interconnected, island to island, and thus provides a roadmap for how best to protect the landing spots of these larvae and secure the future of these fisheries. Ultimately, this project aims to link important bonefish habitats—spawning sites, juvenile habitats, adult home ranges—to guide protection of important bonefish habitats, such as the designation of several national parks in 2015 and 2021, including those supporting popular fishing destinations on Grand Bahama and Abaco.
Until this study, the dispersal of bonefish larvae within The Bahamas was not well understood—the beginning, middle, and end of their journey was unclear. Uncovering the journey’s starting point was a labor of significant time and effort. “It took many attempts for us to figure out how deep spawning bonefish went,” said BTT’s Director of Science and Conservation, Dr. Aaron Adams. “They dive to about 400 feet offshore, which is pretty amazing for a fish that lives its whole life in less than three feet of water.”
The first step in this more than a decade-long process was BTT’s support of Dr. Andy Danylchuk, a BTT Research Fellow, to figure out where bonefish spawn. Adams, Danylchuk, and others built on those early results, and after many years of challenging work, now have a better understanding of spawning and bonefish larvae. All of these efforts were rooted in conservation, keeping in mind the capacity for connectivity through the ultimate product of these spawning events—the larvae. “What we’re trying to do is give the management agencies, whether it’s Bahamas National Trust or fisheries managers, a kind of a spatial outlook of how different habitats are connected,” Adams says, “We’re very interested in identifying habitats and showing the extent to which the habitats and bonefish populations are connected.”
Doing so allows researchers to suggest priority locations to be designated for protection. But how exactly do you track a creature the size of a quarter, as transparent as glass, and as delicate as a petal?
That’s where BTT Postdoctoral Research Scientist Dr. Steven Lombardo comes in. “We were successful in documenting the full bonefish spawning process on Abaco in 2019, which was preceded by some partial successes in 2013 and 2018,” says Lombardo. “Once we checked that box, the next step was figuring out where those larvae would disperse to—connecting Abaco with the rest of The Bahamas. We could charter a ship and use nets to try and capture larvae as they drift in the currents, or travel all over The Bahamas pulling seine nets to capture newly settled juveniles…neither is a sustainable option. So, we turned to computer simulations.”
Lombardo created a virtual ocean using a combination of technology developed by the U.S. Navy in response to the Deepwater Horizon oil spill in the Gulf of Mexico, and French oceanographers interested in fish egg and larvae dispersal. The U.S. Navy produces high-resolution data for ocean currents, temperature, and salinity measured by satellite observations. These data can be stitched together to create the virtual ocean over time, from when the bonefish were observed spawning through to the end of the bonefish larval settlement period up to 71 days later. With some clever programing, the virtual ocean can be used by the software program—developed by the French oceanographers—to model how and where the bonefish larvae were sent.
Not all specks of life in the ocean are created equal. Some fish larvae are excellent swimmers, others are not. The same goes for buoyancy—more oil-rich (fatty) eggs and larvae are lighter than seawater and can float to the surface. These kinds of traits can also be programmed into these larval dispersal models. However, little is known about bonefish larvae development and behavior. Much of what we know has been discovered over the last six years during a joint study by scientists at Florida Atlantic University’s Harbor Branch Oceanographic Institute and BTT. Many of the relevant biological and behavioral characteristics remain unknown, but what has been discovered is that larval bonefish are quite similar to a surprising distant relative—the Japanese eel. Both species are known as “elopomorphs,” which are evolutionarily grouped together by having the same leptocephalus larval form. Combining what is known about the development and behavior of both species, buoyancy changes over larval development, and behavior of larva coming into nursery habitats riding the surface waters of a flood tide could be programmed into the computer model.
As the virtual larvae are moved through the virtual ocean by currents and their biology is incorporated, each of the one-million simulated larvae move in unique ways. Their dispersal paths and the ocean conditions they experienced are saved as the history of their development journey to be analyzed after the simulations are completed for each of the spawns observed in Abaco in 2019, 2018, and 2013.
From these data, we catch a glimpse of how fickle the balance of life and death is for these larvae. There is the ever-present threat of being brought into shore too soon, before the fragile larvae can handle the jostling of waves or consume what prey can be found among the sand and mud. “The larvae ride the currents,” explains Lombardo. “Bonefish spawn in a way that maximizes the chances that their larvae come back to the areas where their parents live. To succeed at this 71 days later, at the mercy of the ocean, larvae need to be picked up by gyres—large, stable ocean circulation features—where larvae can exist and grow until they are ready to come to shore.”
Lombardo stresses that these gyres, and the currents themselves, are the foundation of connectivity of bonefish populations on different islands. If spawning occurs when conditions are not aligned with the gyres, the result is a recruitment “bust,” as was the result of the 2013 spawn in Abaco. Greater than 99 percent of the larvae were brought to the shore of Abaco too soon, driven by eastward currents, resulting in probable death for nearly all larvae. This high level of larval death is common in marine fish, who produce millions of eggs in hopes that just a few survive.
In contrast, spawning success supports the future of the fishery. Larvae are spread throughout The Bahamas, and enough arrive at the proper habitats to repeat the process that has been carried out for 150 million years. In 2019 and 2018, west-flowing currents moved the larvae into the Northwest Providence Channel gyre—a counter-clockwise flowing mass of water that forms every winter during the bonefish spawning season south of Grand Bahama and west of Abaco. This is key to the successful recruitment of larvae in the northern Bahamas. Plus, there are many other smaller gyres and currents that shape the connectivity of bonefish populations throughout The Bahamas. The variation in these currents during-winter and across years changes the connections in both strength (number of larvae) and where the larvae end up.
The ultimate goal of this work is to identify connections between the Abaco spawn and the other islands of The Bahamas. Repeated connections are signs of stability in the linkage between populations, and thus conservation actions more likely to preserve the products of those linkages—the larvae and subsequent juvenile bonefish. The foundation of a healthy fishery is healthy nurseries. Designating national park status to bonefish nurseries is an important action that can be taken to secure the future of the fishery.
The 2019 and 2018 Abaco spawns showed consistent and strong linkages to parts of Abaco (self-recruitment), Grand Bahama, the Berry Islands, Eleuthera, and Andros. Locations on these islands where high densities of larvae settled were then compared to the footprints of the existing National Park system to assess whether current park boundaries are sufficient to protect young bonefish. Many of the areas where the model showed that bonefish larvae arrive were already protected by established parks. However, six areas with substantial bonefish larvae influx were not within a designated park. This is the basis for BTT’s suggestion for expanding the existing protected area network. BTT has shared the results of this study with long-time partner Bahamas National Trust, detailing suggestions for new park designations and expansions. Locations of interest include Moore’s Island near Abaco, the central most eastern islands of the Berry Islands, and the northern end of Eleuthera. Existing parks that can be expanded to better protect bonefish habitats include conjoining the Marls of Abaco National Park to Cross Harbor National Park, conjoining Northshore/The Gap National Park to the West End MPA on Grand Bahama, and extending Joulter Cays National Park to include the north shore and creeks of Andros.
Now that the Abaco study is complete, what does the future look like in the way of research that supports conservation?
Adams’ answer is clear: “As we discover more locations, we will conduct further modeling, similar to the work done by Steve, to understand how ocean currents transport larvae. Each spawning location will vary in its ability to provide new bonefish for different islands, so it’s crucial to determine the most significant spawning locations. Over time, we will improve our map of the most important habitats for protection to ensure a sustainable bonefish fishery.”
Ashleigh Sean Rolle is a Bahamian writer who calls Freeport, Grand Bahama, her home. She writes for the site 10th Year Seniors, where she regularly shares her opinion on everyday Bahamian affairs. She is a contributor for Huff Post. Her work has also appeared at CNN.com.
