Last December, scientists announced that dolphins in Louisiana were experiencing lung diseases and low birthrates in the wake of the Deepwater Horizon oil spill that released more than 636 million liters of crude oil into the Gulf of Mexico in 2010. Now, researchers have also found evidence of potentially lethal heart defects in two species of tuna and one species of amberjack — all economically important species for commercial fisheries. This news, published in the Proceedings of the National Academy of Sciences today, comes less than a week after the announcement that BP will once again be allowed to explore the Gulf of Mexico for oil.

To study the effects of the BP oil disaster, scientists recreated the oceanic environment that yellowfin amberjack, yellowfin tuna and bluefin tuna larvae would have encountered in 2010 in the lab. They did so by introducing the larvae to Deepwater Horizon oil samples at environmental conditions that matched those of the spill. Fish are extremely vulnerable during development, so studying fish larvae is the most direct way of demonstrating the effect of noxious compounds.

Slower heart rates, fluid accumulation, and arrhythmia

The researchers found that the fish exhibited a number of heart defects including slower heart rates, fluid accumulation, and arrhythmia — a condition characterized by an irregular heartbeat. In the areas where the oil concentrations were the highest, the oil would have caused the larvae to die of heart failure, says John Incardona, research toxicologist at the National Oceanic and Atmospheric Administration and co-author of the study. Fish larvae that were located further away probably survived, but if these heart defects mean that "they can't swim as fast, so they are either going to get eaten or they won't be able to eat enough," he says. "That leads to reduced survival."

"Reduced survival."

These sorts of results are not entirely new. Previous experiments on the effect of the 1989 Exxon Valdez oil spill that devastated Alaska's waters have yielded similar results. But that spill involved oil coating the shoreline, "so the fish species that were affected were animals that physically deposit their eggs on or near the shoreline," Incardona says. So this latest experiment represents the first time that scientists have been able to demonstrate cardiovascular effects in pelagic fish — fish that hang out in the middle of the water column, instead of at the surface of the water or near the bottom.

Peter Hodson, a fish toxicologist at Queen's University in Canada who did not participate in the study, explained in an email to The Verge that the method the researchers employed was sound, adding that keeping these larvae alive long enough to study the effects of the oil is a "tour de force." Moreover, Steve Murawski, a marine ecologist at the University of South Florida who also did not participate in the study, said in an email that the experiment involved "as near realistic conditions as possible."

But a BP spokesperson contacted The Verge to state that "the paper provides no evidence to suggest a population-level impact on tuna, amberjack, or other pelagic fish species in the Gulf of Mexico," as the "oil concentrations used in the lab experiments were rarely seen in the Gulf during or after the Deepwater Horizon accident."

Disastrously low tuna and amberjack numbers

For now, it's hard to tell what sort of impact these heart defects will have on commercial fisheries or the food web as a whole, but "we will find out eventually," Incardona says. In a few years, fisheries will begin to catch this particular cohort of fish and they might find that their numbers are disastrously low.

As for human health, it's unlikely that the BP oil spill will have an effect, Hodson said, "other than the potential for a high-value, high-protein food item to disappear from our diets." The population of bluefin tuna in the Gulf of Mexico is already under immense amounts of pressure from overfishing and water pollution. So if the impacts of the oil spill affected an entire year's reproduction, he said, the Gulf bluefin tuna could "be driven to virtual extinction."

Update: This story was edited after publication to include a statement by BP spokesperson Jason Ryan.