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, or PAHs.
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 BP spill, which was caused by the Deepwater Horizon oil rig explosion, was the largest offshore oil spill in U.S. history. The blast killed 11 workers and unleashed nearly 5 million barrels of crude into the environment.
The study was conducted by researchers at NOAA, Stanford University, the University of Miami and the University of the Sunshine Coast, Maroochydore, Australia.
In a response, Jason Ryan, a spokesman for BP America Inc., said, "The paper provides no evidence to suggest a population-level impact on tuna, amberjack or other pelagic fish species in the Gulf of Mexico."
The study follows an earlier research paper published in February showing that PAHs block signaling pathways that allow potassium and calcium ions to flow in and out of cardiac cell membranes and sustain normal heart rates in fish. It also suggests that very low concentrations of crude oil can disrupt these signaling pathways, slowing the pace of heartbeats.
Both studies suggest that PAH cardiotoxicity was potentially a common form of injury among a broad range of species near the oil spilled into the Gulf of Mexico, where the population of bluefin tuna is at a historic low level and petitioned for listing as federally endangered.
The gulf's population of bluefin tuna, which can grow to 12 feet in length, weigh 1,400 pounds and live for 35 years, is a fraction of its abundance in the early 1950s.
Scientists say future research should be extended to include mammals and humans because the signaling pathways of their cardiac cells are similar to those of tuna. PAHs are found in coal tar, creosote, air pollution and urban runoff.
The findings are a source of concern for environmentalists and fishermen in the Gulf of Mexico, including some who participated in the massive cleanup effort there four years ago.
"For a species like bluefin tuna, whose populations have crashed due to overfishing and are fighting to rebuild their former abundance, BP's oil was a shot to the heart," said Jacqueline Savitz, a spokeswoman for the advocacy group Oceana.
Bonny L. Shoemaker, president of On Wings of Care, a nonprofit dedicated to the protection and preservation of wildlife and habitat, said the studies "are providing answers to important questions: What are the long-term effects of PAHs on ecosystems, and on humans?"