Four years after the Exxon Valdez ran aground in Prince William Sound, Alaska, herring stocks collapsed and a marked decline was seen in salmon populations.
But the levels of contamination remaining in the water during the spawning season immediately after the spill were lower than the limits known to cause abnormalities in fish. Linking the fish die-off to the Exxon Valdez spill remains controversial. Now, over 25 years after the Exxon Valdez, scientists have confirmed that those low levels of oil pollution can be linked to failure to thrive -- by breaking fish hearts.
In a report published in Scientific Reports, researchers describe how they exposed herring and pink salmon embryos for a short period to levels of oil contaminants similar to the "safe" levels detected in samples of Prince William Sound seawater after the spill cleanup. They then removed the embryos from the chemical exposure and raised them to adulthood in clean water.
They measured the swimming speed of the fish as an indicator of overall health. The chart shows that even at extremely low concentrations (23 ppb = 23 parts of chemical to 1 billion parts of ocean water), the herring fitness drops off significantly. Salmon are also affected, but at somewhat higher concentrations.
Fish that swim more slowly more easily become prey to predators, so this could explain the collapse of herring and decline in salmon after the oil spill. The same researchers have previously shown that polycyclic aromatic hydrocarbons (PAHs), toxic chemicals known to be present in crude oil, block the formation of a healthy heart during fish embryo growth, but that link was insufficient to explain fully the mysterious drops in fish populations. This study completes the picture, by demonstrating how the chemical exposures damage the viability of the adult fish.