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BTT Expands Knowledge of Bonefish Spawning

RIMG3710 Pre-spawning aggregation of bonefish.

BTT has been working with lodges and guides in the Bahamas as part of our ongoing efforts to identify bonefish spawning locations, and to better understand pre-spawning and spawning behaviors and requirements. We rely on guides and lodges to contact us when they see possible spawning, and their help has been instrumental to our work. BTT has already worked with Bahamas National Trust to protect spawning locations on Grand Bahama and Abaco from development through the creation of National Parks. We are working to identify more locations in need of protection.

In late April, the guides of East End Lodge on Grand Bahama Island identified a pre-spawning aggregation of bonefish. They contacted BTT’s Bahamas Initiative Manager, Justin Lewis, and after a brief visit to the site, Justin put science into action. After a few phone calls and quickly arranged travel, Jacob Rennert, of Florida Institute of Technology, joined Justin at the site. Over a four-day period during the full moon phase Justin and Jacob found schools of 500 to 3,000 pre-spawning bonefish located in a small deep protected bay.

Bonefish eggs. Bonefish eggs.

Pre-spawning bonefish typically stage for several days, and gather in pre-spawning schools in protected bays before swimming offshore at dusk. After swimming offshore, they descend to approximately 200 feet, and stay there for an hour or two before quickly ascending. We believe it is during the ascent that they eject their eggs and sperm into the water, fertilization occurs, and the eggs hatch about a day later.

One of the questions we seek to answer is whether they must spawn in deep water or if they are capable of spawning in shallow water. In other words, why do they risk the gauntlet of predators to leave the flats and spawn offshore? We believe the drastic change in pressure from the surface to 200 feet, followed by the drastic decrease in pressure as the fish ascend helps to expel the eggs and sperm.

DSC_0258 (2)To test this, Justin and Jacob captured some bonefish and kept them in a small inflatable pool on a nearby dock. They checked on the fish every hour or so, and over the next few days they checked to see if the bonefish eggs were developing and if the bonefish were able to spawn. None of the bonefish spawned in the pool, but on the third night, several females expelled eggs when gently squeezed. This tells us that bonefish eggs continue to develop in shallow water, but that actual spawning doesn’t occur on its own in shallow water. This suggests that there is something about the descent to depth that is important for spawning success. This further underscores the importance of protecting the shallow bays that bonefish use for pre-spawning and the nearby offshore deep areas they need to spawn.

DSC_0267 (2)This was the first time bonefish have been successfully strip spawned, and was done with two females under no treatment. This is another important step towards a bonefish restoration program for the Florida Keys, and will provide valuable information for BTT’s Bonefish Restoration Research Project. Many thanks to Rob Neher and Cecil Leathen, guides and staff of the East End Lodge for exceptional hospitality and help with the research. This successful trip would not have been possible without all of their support.

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