LOS ANGELES — There’s more bad news about the effects of oil spills on warm-water predators, including Atlantic bluefin tuna, already one of the most threatened fish in the seas.

Oil spills such as the 2010 Deepwater Horizon disaster in the Gulf of Mexico may cause serious heart defects in developing fish embryos, according to a study by the National Oceanic and Atmospheric Administration that was published Monday in the journal Proceedings of the National Academy of Sciences.

The release of more than 4 million barrels of oil between April and July 2010 coincided with the spawning window for commercially and ecologically important species such as bluefin and yellowfin tunas, mahi mahi, Spanish mackerels and blue marlin.

Much of that oil rose from the wellhead on the ocean floor to the surface, potentially exposing buoyant and rapidly developing fish embryos and larvae to toxic polycyclic aromatic hydrocarbons.

In the laboratory, the researchers found that embryos of bluefin tuna, yellowfin tuna and amberjack exposed to field-collected Deepwater Horizon oil samples suffered defects in heart development resulting in irregular heartbeat, circulatory disruption and pericardial fluid accumulation.

The defects occurred in the fish at PAH concentrations of one to 15 parts per billion – lower than those measured in samples collected from the upper water column of the northern Gulf of Mexico during the spill.

“Losses of early life stages were therefore likely for Gulf populations of tunas, amberjack, swordfish, billfish and other large predators that spawned in oiled surface habitats,” the study says.

Exposure to low levels of crude oil was shown to produce abnormal heart rhythms even in fish larvae that otherwise appeared to be normal, study leader John P. Incardona, an ecotoxicologist at NOAA, said in an interview.

“Larvae exposed to high levels were dead within a week,” Incardona added. “But we still don’t know how long they lived after exposure to lower levels, or how much spawning area may have been impacted.”

The study was conducted by researchers at NOAA, Stanford University, the University of Miami and the University of the Sunshine Coast, Maroochydore, Australia.

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