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Crude oil from the 2010 Deepwater Horizon disaster causes severe defects in the developing hearts of bluefin and yellowfin tunas, according to a new study by a team of National Oceanic and Atmospheric Administration (NOAA) and academic scientists.

The findings, published in the Proceedings of the National Academy of Sciences on the 25th anniversary of the Exxon Valdez oil spill, show how the largest marine oil spill in U.S. history may have affected tunas and other species that spawned in oiled offshore habitats in the northern Gulf of Mexico.

Oil near the Deepwater Horizon disaster spill source as seen during an aerial overflight on May 20, 2010. Photo credit: NOAA

Atlantic bluefin tuna, yellowfin tuna and other large predatory fish spawn in the northern Gulf during the spring and summer months, a time that coincided with the Deepwater Horizon spill in 2010. These fish produce buoyant embryos that float near the ocean surface, potentially in harm’s way as crude oil from the damaged wellhead rose from the seafloor to form large surface slicks.

The new study shows that crude oil exposures adversely affect heart development in the two species of tuna and an amberjack species by slowing the heartbeat or causing an uncoordinated rhythm, which can ultimately lead to heart failure.

“We know from the 1989 Exxon Valdez spill in Prince William Sound that recently spawned fish are especially vulnerable to crude oil toxicity,” said Nat Scholz, Ph.D., leader of the ecotoxicology program at NOAA's Northwest Fisheries Science Center in Seattle. “That spill taught us to pay close attention to the formation and function of the heart.”

“The timing and location of the spill raised immediate concerns for bluefin tuna,” said Barbara Block, Ph.D., a study coauthor and professor of biology at Stanford University. “This spill occurred in prime bluefin spawning habitats, and the new evidence indicates a compromising effect of oil on the physiology and morphology of bluefin embryos and larvae.”

Recent studies are increasingly painting a more detailed picture of how oil-derived polycyclic aromatic hydrocarbons (PAHs) act on the heart. Earlier this year, the Stanford-NOAA team showed in a related paper published in Science (Brette et al. 343: 772) that Deepwater Horizon crude oil samples block excitation-contraction coupling—vital processes for normal beat-to-beat contraction and pacing of the heart—in individual heart muscle cells isolated from juvenile bluefin and yellowfin tuna.

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 a suite of morphological abnormalities including fluid accumulation from heart failure and poor growth of fins and eyes. Image courtesy of John Incardona/ NOAA

“We now have a better understanding why crude oil is toxic, and it doesn’t bode well for bluefin or yellowfin embryos floating in oiled habitats.” said Block. “At the level of a single heart muscle cell, we’ve found that petroleum acts like a pharmacological drug by blocking key processes that are critical for cardiac cell excitability.”

This mechanism explains why the team observed a range of cardiac effects in the developing hearts of intact embryos in the present study. “We directly monitored the beating hearts of living fish embryos exposed to crude oil,” said Dr. John Incardona, NOAA research toxicologist and the study’s lead author. “The tiny offspring of tunas and other Gulf species are translucent, and we can use digital microscopy to watch the heart develop.”

The major difficulty facing the researchers was access to live animals. Tunas are difficult to raise in captivity and few facilities exist worldwide with spawning fish. In the open ocean, fragile fish embryos and larvae are mixed with many other types of plankton, and they usually don’t survive the rough conditions in a net towed near the surface. This made it close to impossible to assess developmental cardiotoxicity in samples collected near the Deepwater Horizon surface oil slicks.

To work around this challenge, the international team brought the oil to the fish. Samples of crude oil were collected from the damaged riser pipe and surface skimmers. The samples were then transported to the only land-based hatcheries in the world capable of spawning tunas in captivity.

This approach allowed the scientists to design environmentally relevant crude oil exposures for bluefin tuna and yellowfin tuna at marine research facilities in Australia and Panama, respectively. Luke Gardner, an Australian native post-doctoral associate from Stanford University and co-author on the PNAS paper, was vital in helping the team investigate the bluefin.

“It is challenging to maintain bluefin in culture and we were privileged to have successfully tested the crude oil in Australian facilities, the only on-land hatchery that has bluefin tuna in culture. This gave us access to tuna embryos and allowed us to study the developmental toxicity of oil,” said Gardner. The pioneering effort to develop new testing methods was also led by Martin Grosell, Ph.D., at the University of Miami.

The new research adds to a growing list of fish that are affected by crude oil. “This fits the pattern,” said Incardona. “The tunas and the amberjack exposed to Deepwater Horizon crude oil were impacted in much the same way that herring were deformed by the Alaska North Slope crude oil spilled in Prince William Sound during the Exxon Valdez accident.”

Crude oil is a complex mixture of chemicals, some of which are known to be toxic to marine animals. Past research has focused in particular on PAHs, which can also be found in coal tar, creosote, air pollution and stormwater runoff from land. In the aftermath of an oil spill, PAHs can persist for many years in marine habitats and cause a variety of adverse environmental effects.

Developmental abnormalities were evident in bluefin and yellowfin tunas at very low concentrations, in the range of approximately one to 15 parts per billion total PAHs. These levels are below the measured PAH concentrations in many samples collected from the upper water column of the northern Gulf during the active Deepwater Horizon spill phase.

Yellowfin tuna in a tank at the Achotines Laboratory in Panama. Image courtesy of John Incardona/ NOAA

Severely affected fish with heart failure and deformed jaws are likely to have died soon after hatching. However, the NOAA team has shown in previous work that fish surviving transient crude oil exposures with only mild effects on the still-forming heart have permanent changes in heart shape that reduce swimming performance later in life.

“This creates a potential for delayed mortality,” said Incardona. “Swimming is everything for these species.”

The nature of the injury was very similar for all three pelagic predators, and similar also to the response of other marine fish previously exposed to crude oil from other geologic sources. Given this consistency, the authors suggest there may have been cardiac-related impacts on swordfish, marlin, mackerel and other Gulf species. “If they spawned in proximity to oil, we’d expect these types of effects,” said Incardona.

The research was funded by NOAA as part of the on-going Natural Resource Damage Assessment for the Gulf ecosystem following the April 20, 2010 Deepwater Horizon oil spill. Contributing to the findings in addition to NOAA and Stanford University were researchers from the University of Miami’s Rosenstiel School of Marine and Atmospheric Sciences and the University of the Sunshine Coast in Queensland, Australia.

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The first and second phases of the BP trial involving the Deepwater Horizon oil disaster are behind us. The judge is now deciding how to rule on the issues of gross negligence and amount of oil released into the ocean, while a third and final phase is set for Jan. 2015. Yet, while the end of the courtroom drama is in sight, the genie BP let out of the bottle almost four years ago has not gone away, and questions abound. For example, how is lingering oil affecting the food web? Which impacts will remain hidden? And how long will recovery of the environment take? Answers to these questions are frustratingly elusive, especially since the results of government studies assessing environmental damage are still mostly confidential.

With the fourth anniversary of the BP disaster nearly upon us, we can look back to the March 1989 Exxon Valdez oil spill in Alaska for insight into the types of impacts seen four years after that oil spill and what they might mean for Gulf recovery. As Alaskans reach a significant milestone today—the 25th anniversary of the Exxon Valdez disaster—the successes and setbacks in coastal Alaska’s recovery are instructive. These insights put the Gulf’s recovery into perspective and tell us that science is the foundation of a decades-long restoration effort, and it must not be shortchanged.

Lingering Oil

Four years after the tanker Exxon Valdez belched 11 million gallons of oil into Prince William Sound, the water’s surface was largely oil-free. However, patches of asphalt-like deposits remained on area beaches and pockets of relatively “fresh” oil could be found below the surface. Residual BP oil persists, embedded in marshes, beaches and offshore sediments. When disturbed, as was the case following Tropical Storm Karen, it re-oils sensitive habitats. Lingering oil poses risks to the species and can slow recovery, either through direct re-exposure or indirectly through ingestion of contaminated prey. To this day, lingering Exxon Valdez oil is monitored on sheltered beaches in the Gulf of Alaska. The Gulf of Mexico needs a similar long-term monitoring effort to track remaining reservoirs of submerged BP oil and its food web impacts.

The Exxon Valdez oil spill occurred March 24, 1989. Photo credit: Wikimedia Commons

Finfish Exposure and Recovery

Salmon are the most important finfish, culturally and commercially, to Alaskans. Oil from the Exxon Valdez reached about one-third of pink salmon streams in Prince William Sound. Repeated exposure of fish eggs to relatively less toxic oil slowed the recovery of pink salmon, which was not declared recovered until 1998. Another species, Pacific herring, which once supported a multimillion-dollar fishery in Prince William Sound, was another casualty of Exxon Valdez oil. In 1993, the Prince William Sound herring population crashed, the result of a perfect storm of natural and man-made factors, including Exxon Valdez oil. More than 20 years later, Pacific herring still has not recovered and is the focus of ongoing studies.

In the Gulf of Mexico, fisheries are a $5.7 billion industry. Much is still unknown about the impacts of the BP disaster on finfish or what ripple effects these might have in the ecosystem or fisheries, but new research findings give us a glimpse. Scientists found that when young bluefin tuna were exposed to crude oil from BP’s ruptured wellhead, their hearts were at greater risk of malfunctioning. The BP disaster occurred at the time bluefin were spawning, so it is possible the 2010 class took a hit. Menhaden is also a significant wild card because it is so critical to the Gulf food web. The distribution of this forage fish overlapped with the BP oil spill footprint, but scientists don’t yet know—or aren’t saying—to what degree the species was affected.

Now is the time to model best and worst case oil spill impact scenarios for finfish species of concern, like bluefin or menhaden, using the results to guide recovery strategies and help fishermen plan ahead. Long-term studies for exposed finfish similar to those for Pacific herring in Alaska are also needed.

Wildlife Exposure and Recovery

Wildlife was severely affected by the 11 million gallons of oil that spilled from the tanker 25 years ago. Photo credit: Wikimedia Commons

An estimated 250,000 birds were killed by the Exxon Valdez disaster. Several populations of birds, including bald eagles, had recovered by 1994, but many of those in oiled areas had not. Harbor seals may have declined by as much as 300 individuals following the disaster, continuing a declining trend first observed in 1984. Two pods of killer whales occurring in waters exposed to Exxon Valdez oil lost a combined 22 animals between 1989 and 1994, and neither pod had recovered by 1994. Many of these species have been the focus of oil spill impact studies dating back to 1989, with surveys continuing today because populations have not fully recovered.

In the Gulf of Mexico, thousands of birds representing about 100 species were recovered from the BP disaster impact area. In actuality, the total number is likely many times higher because carcasses are eaten, sink or drift away. Since the disaster, hundreds of sea turtles and bottlenose dolphins have stranded. The dolphin die-off is the longest and worst ever seen in the Gulf. We need to get to the bottom of dolphin deaths to not only arrest the trend if we can and aid their recovery, but also to determine whether conditions for dolphins in the Gulf are changing and why. Scientists need to collect data over many years in order to detect trends and understand ecological relationships. For this reason, long-term health assessments for wildlife species impacted by the BP disaster are high priorities for restoration and gauging recovery.

Recovery Is a Marathon, Not a Sprint

The Exxon Valdez experience taught us that recovery from oil spills can be two steps forward and one step back. Oil spill restoration is like a marathon; the process is long and pacing matters. The best way to prepare for the long haul and make periodic course corrections is to learn as much about oil spill impacts and ecosystem drivers as solid science will tell us, and respond accordingly. Tracking the health of an ever-changing Gulf is as important for restoration as regular checkups are for people, even more so for those recovering from an illness. Without a finger on the Gulf’s pulse and understanding how changes in this body of water affect recovering species, the right diagnosis or decisions about this or that species cannot be made. We need dedicated Gulf-wide monitoring for a minimum of 25 years to track recovery from the BP disaster. If there is one lesson we have learned from the Exxon Valdez disaster, it is that good science is the glove that fits around the hand of restoration.

Visit EcoWatch’s WATER page for more related news on this topic.

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