“Very low levels” of crude oil from the 1989 Exxon Valdez spill in Prince William Sound, Alaska, are a threat to the survival of herring and pink salmon that spawn in the region, according to a study released today by the US National Oceanic and Atmospheric Administration.
The study shows that embryonic salmon and herring exposed to very low levels of crude oil can develop hidden heart defects that compromise their later survival.
That means that the Exxon Valdez spill on March 24, 1989 may have had much greater impacts on spawning fish than previously recognized, according to the study published in Nature’s online journal Scientific Reports Very low embyronic crude oil exposures cause lasting defects in salmon and herring.
“These juvenile fish on the outside look completely normal, but their hearts are not functioning properly and that translates directly into reduced swimming ability and reduced survival,” said John Incardona, a research toxicologist at NOAA Fisheries’ Northwest Fisheries Science Center (NWFSC) in Seattle. “In terms of impacts to shore-spawning fish, the oil spill likely had a much bigger footprint than anyone realized.”
Previous research has shown that crude oil disrupts the contraction of the fish heart muscle cells. Embryonic fish exposed to trace levels of crude oil grow into juveniles with abnormal hearts and reduced cardiorespiratory function.
“With this very early impact on the heart, you end up with an animal that just can’t pump blood through its body as well, which means it can’t swim as well to capture food, form schools, or migrate,” said Mark Carls, toxicologist at the Alaska Fisheries Science Center. “Crude oil is changing basic physiology, or what makes a fish a fish.”
The research builds on earlier work by the Auke Bay Laboratories, part of NOAA Fisheries’ Alaska Fisheries Science Center, which found much reduced survival of pink salmon exposed as embryos to polycyclic aromatic hydrocarbons (PAH) from crude oil.
“Our findings are changing the picture in terms of assessing the risk and the potential impacts of oil spills,” said Nat Scholz, leader of the NWFSC’s ecotoxicology program and a coauthor of the new study. “We now know the developing fish heart is exquisitely sensitive to crude oil toxicity, and that subtle changes in heart formation can have delayed but important consequences for first-year survival, which in turn determines the long-term abundance of wild fish populations.”
The Exxon Valdez spill was the largest in U.S. history, with extensive oiling of shoreline spawning habitats for Pacific herring and pink salmon, the two most important commercial fish species in Prince William Sound.
Herring larvae sampled in proximity to oil were visibly abnormal, and mortality rates were higher for pink salmon embryos at oil spill sites than unaffected regions.
The herring fishery collapsed three to four years after the spill, when the herring spawned in oiled areas reached reproductive maturity.
The paper notes that the contribution of the spill to the herring population collapse, if any, was never determined and remains controversial.
Other studies, however, tend to confirm the findings, including heart problems for fish exposed to the Gulf of Mexico Deepwater Horizon spill and even fish exposed to naturally occurring oil seeps.
The new findings suggest that the delayed effects of the spill may have been important contributors to the declines.
Scientists from the Northwest Fisheries Science Center and Alaska Fisheries Science Center temporarily exposed embryonic salmon and herring to low levels of crude oil from the North Slope of Alaska and found that both absorbed chemicals at similar concentrations in their tissues. The embryos were then transferred to clean seawater and raised as juvenile fish for seven to eight months.
Few of the exposed embryos were outwardly abnormal in any way. However, closer examination of the fish revealed subtle defects that could reduce their long-term survival.
Juvenile salmon exposed to oil grew more slowly, with those exposed to the highest concentrations growing the slowest. For salmon, early survival in the ocean is strongly influenced by juvenile growth, with smaller fish suffering higher loss to predators.
Scientists used swimming speed as a measure of cardiorespiratory performance and found that fish exposed to the highest concentrations of oil swam the slowest. Slower swimming is an indication of reduced aerobic capacity and cardiac output, and likely makes fish easier targets for predators.
Exposure to oil as embryos altered the structural development of the hearts of juvenile fish, potentially reducing their fitness and swimming ability. Poor swimming and cardiac fitness is also a factor in disease resistance.
Earlier studies on the ecosystem-scale crash of the Prince William Sound herring population several years after the Exxon Valdez spill were based on higher levels of exposure to the oil. The new study shows that that cardiac injury occurs in normal-appearing fish that survive even lower level exposures.
The scientists reviewed data on measured oil concentrations in surface water samples collected in Prince William Sound after the oil spill and during the 1989 herring spawning season. Most of the 233 samples contained less oil than was believed to be toxic to herring at the time, based on visible gross developmental abnormalities. However, nearly all of the samples contained oil at or above concentrations shown in the new study to alter heart development.
If the Exxon Valdez spill impacted heart development among a large majority of fish that were spawned in proximity to oiled shorelines, the subsequent losses of juveniles to delayed mortality would have left fewer adults to join the population. Although not direct proof, this provides a plausible explanation for the collapse of the Prince William Sound herring stock four years later, when fish spawned during the oil spill would have matured.
The study concludes that the impacts of the Exxon Valdez spill on near shore spawning populations of fish are likely to have been considerably underestimated in terms of both the geographic extent of affected habitat and the lingering toxicity of low levels of oil. The findings will likely contribute to more accurate assessments of the impacts of future oil spills, Incardona said. “Now we have a much better idea of what we should be looking for,” he said.
That means, according to the study “that the impacts of the Exxon Valdez oil spill on populations of near shore spawning fish are likely to have been considerably underestimated, in term of both the geographic extent of affected habitats and the lingering toxicity of low levels of residual oil.”
The report calls for more studies of the sensitivity of the developing fish heart since the vulnerability “also has implications for other pollution sources in marine ecosystems, including increasing maritime vessel traffic and expanding land-based urban runoff.”
Scientific studies after the Exxon Valdez spill indicated that the vast majority of species recovered following the spill and that functioning ecosystems, similar to those existing pre-spill, were established.
Species for which recovery is not fully apparent, such as Pacific herring, killer whales, and pigeon guillemots, appear to have been affected by other environmental factors or human influences not associated with the oil spill. Insufficient pre-spill baseline data on these species contributed to difficulties in determining the extent of spill effects.
Based on the evidence, the Panel finds that natural recovery of the aquatic environment after an oil spill is likely to be the primary recovery mechanism, particularly for marine spills. Both freshwater and marine ecosystem recovery is further mitigated where cleanup is possible, effective, and beneficial to the environment.
Natural processes that degrade oil would begin immediately following a spill. Although residual oil could remain buried in sediments for years, the Panel finds that toxicity associated with that oil would decline over time and would not cause widespread, long-term impacts.