Tuna larva

This image shows a normal yellowfin tuna larva not long after hatching (top), and a larva exposed to Deepwater Horizon crude oil during embryonic development (bottom). The oil-exposed larva shows abnormalities, including fluid accumulation from heart failure and poor growth of fins and eyes. (NOAA)

Tuna eggs exposed to crude oil from a massive BP spill in the Gulf of Mexico in 2010 hatched into fish with heart and fin defects, a new study shows.

Those with severe defects would have died soon after hatching, while others may survive for some time before their heart problems affect their ability to swim after their prey, reported a team led by John Incardona of the U.S. National Oceanic and Atmospheric Administration.

"This creates the potential for delayed mortality. Swimming is everything for these species," said Incardona in a statement Monday, coinciding with the publication of the study in the Proceedings of the National Academy of Sciences.

The paper noted that pink salmon eggs exposed to oil as after the Exxon Valdez spill were 40 per cent less likely to survive to adulthood after hatching.

Following the explosion of BP's Deepwater Horizon offshore drilling rig, the undersea oil well it was drilling spewed 636 million litres of oil into the Gulf of Mexico between April 10 and July 14, 2010. That was a time of year when many fish were spawning in the area, including bluefin and yellowfin tuna, mahi mahi, king and Spanish mackerel, greater and lesser amberjack, sailfish, blue marlin, and cobia – many of which are fished commercially, the paper noted.

At one point, the slicks produced by the disaster covered an estimated 17,700 square kilometres, or nearly the area of Lake Ontario.

Eggs, babies float near surface

Spawning bluefin and yellowfin tuna lay eggs that float near the surface of the water, and hatch into babies that also float for the first part of their life. The area where the spill took place is a prime spawning habitat for bluefin tuna, an endangered species.


Following the explosion of BP's Deepwater Horizon offshore drilling rig, the undersea oil well it was drilling spewed 636 million litres of oil into the Gulf of Mexico between April 10 and July 14, 2010. ((Gerald Herbert/Associated Press))

To find out how the oil might have affected the spawning fish, the researchers collected oil from the Deepwater Horizon spill and mixed it into the water where Bluefin tuna, yellowfin tuna and amberjack embryos were developing inside their eggs at an indoor hatchery.

Fish embryos are known to be particularly sensitive to chemicals in crude oil called polycyclic aromatic hydrocarbons (PAHs.)

Developmental abnormalities appeared when concentrations of PAHs were between one and 15 parts per billion. The researchers noted that these concentrations were lower than many of those in water samples collected in the Gulf of Mexico during the oil spill, suggesting "the potential for losses" of baby fish.

The abnormalities seen in the study ranged from slow or irregular heartbeat to the accumulation of fluid around the heart and deformed fins. A study published by the authors in Science last month showed the oil blocks heart muscle cells from responding correctly to signals that tell them when to contract.

The researchers said the effects they saw were similar in all three fish species in their experiment and were also similar to those seen in previous studies on other kinds of fish, suggesting that many different kinds of fish could be affected, including swordfish, marlin and mackerel.

However, the researchers don't yet know how the potential damage to young fish will affect overall populations of each species in the Gulf of Mexico, as they're still missing some key information.

"We've not answered the question — how much of the spawning ground was oiled," said Barbara Block, a Stanford University biologist who co-authored the study, in an email. "This study is currently underway."

The researchers noted that the developing hearts of all fish embryos seem to respond to crude oil exposure the same way. Because of that, watching for heart defects in the embryos of local fish species when exposed to oil may be a good way of gauging how sensitive certain habitats, such as the Arctic, may be to oil spills in their waters.