Is There a Problem with Dolphin (Mahi-Mahi)?

A lack of juvenile fish in 2016 could mean anglers won't have big catches like this in the coming years.

A lack of juvenile fish in 2016 could mean anglers won’t have big catches like this in the coming years.

(from Sporting Classics Daily \ Pat Robertson \ Nov. 28, 2016)

Something very unusual—perhaps ominous—is happening with the dolphinfish population off the southeastern United States.

In a normal year hordes of young-of-year dolphin (mahi-mahi) cruise the offshore waters from Florida to the Mid-Atlantic states, but this year few YOY fish showed up and, in some areas, none were seen or caught. Dolphin are extremely fast-growing, and the YOY fish mature in one year into the “gaffer” fish sought by sport anglers and charter captains the following year.

The sharp decline in the number of young dolphin showing up along the southeastern coast has charter captains and scientists wondering if fishing for the highly prized sportfish will be boom or bust in 2017. Marine biologist Don Hammond, who has managed the Dolphinfish Research Program out of Charleston, South Carolina, for the past 15 years since retiring from the S.C. Department of Natural Resources, is concerned that 2017 may be the acid test for Atlantic mahi-mahi.

For Hammond, who tracks dolphin migrations with a tagging program through the cooperation of charter captains from the Atlantic Coast into the Caribbean, there are three burning questions:

  1. Where were the YOY dolphinfish this year?
  2. Will there be a successful fishing season for the highly prized sportfish in 2017?
  3. If there was a sudden population decline, what caused it?

As for No. 1, Hammond is hopeful the lack of YOY fish this year was something as simple as a slight shift in migratory patterns. The answer to No. 2 lies somewhere between No. 1 and No. 3. Hammond is fearful that the answer to No. 3 may turn out to be that something deadly serious has happened to dolphinfish reproduction and survival.

He fears some catastrophic event has killed almost an entire year class of fish, and it may be devastating to dolphin fishing in 2017. He pointed to the latent effects of the worst oil disaster in U.S. history—the 2010 Deep Water Horizon explosion, which spilled more than three million barrels of crude oil into the northern Gulf of Mexico, contaminating the spawning habitats for many fishes, including dolphin.

Both the crude oil and the dispersant used to dilute it have been proven to be toxic to fish, he said. A study by scientists at the University of California Riverside and the University of Miami Rosenstiel School of Marine and Atmospheric Science found that ultraviolet light increases the toxicity of the oil and damages the heart, eye, and neurological functions of fish larvae.

“Ours is the first experiment evaluating the effects of DWH oil on the genetic responses of mahi-mahi embryos and larvae,” said Daniel Schlenk, a professor of aquatic ecotoxicology at UCR, who led the study, published in Environmental Science and Technology. “It is also the first experiment of this nature on a lifestage and species that was likely exposed to the oil. We found that the weathering of oil had more significant changes in gene expression related to critical functions in the embryos and larvae than the un-weathered oil. Our results predict that there are multiple targets of oil for toxicity to this species at the embryonic life stage.”

For their experiments, Schlenk and UM Rosenstiel School scientists caught the mahi-mahi off the coast of Miami, Florida, and exposed embryos to two types of oil: one set of embryos was exposed to slick oil (weathered) from the spill, while another set was exposed to oil that came from the source of the spill.

The researchers carried out the experiment this way because fish in the northern Gulf of Mexico had been exposed during the spill to both types of oil. Their study attempted to understand which of the two oils—slick oil or source oil—is worse for the fish and how oil affects development.

Schlenk said his team found that not only was the heart of the fish the target, but also that the general neurological function was likely to be impaired, as well as the cardiovascular system. It also affected the sensory function, which is important for prey detection and predator avoidance.

The research was funded by the Gulf of Mexico Research Initiative, as well as the Relationship of Effects of Cardiac Outcomes in fish for Validation of Ecological Risk (RECOVER) consortium.

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