In a study published in the scientific journal PNAS, NOAA scientists and their collaborators reported Pacific herring embryos in shallow waters died in unexpectedly high numbers following an oil spill in San Francisco Bay, and suggest an interaction between sunlight and the chemicals in oil might be responsible.

In November 2007, the container ship Cosco Busan released 54,000 gallons of bunker fuel, a combination of diesel and residual fuel oil, into the San Francisco Bay. The accident contaminated the shoreline near the spawning habitats of the largest population of Pacific herring on the West Coast.

In this study, scientists found that herring embryos placed in cages in relatively deep water at oiled sites developed subtle but important heart defects consistent with findings in previous studies. In contrast, almost all the embryos that naturally spawned in nearby shallower waters in the same time period died.

When scientists sampled naturally-spawned embryos from the same sites two years later, mortality rates in both shallower and deeper waters had returned to pre-spill levels.

"Based on what we know about the effects of crude oil on early life stages in fish, we expected to find live embryos with abnormal heart function, so it was a surprise to find so many embryos in the shallow waters literally falling apart," said Dr. John Incardona, a toxicologist with NOAA's Northwest Fisheries Science Center and lead author of the study.

"The study has given us a new perspective on oil threats in sunlit habitats, particularly for translucent animals such as herring embryos. The chemical composition of residual oils can vary widely, so the question remains whether we would see the same thing with other bunker fuels from around the world."

Two decades of toxicity research since the 1989 Exxon Valdez oil spill has shown that fish embryos and larvae are particularly vulnerable to spilled oil. Most catastrophic spills, such as the Exxon Valdez, involve large volumes of crude oil.

However, residual oils used in bunker fuels are the leftovers of crude oil refining, and are not as well studied as crude oils. Bunker fuel is used in maritime shipping worldwide, and accidental bunker spills are more and more common and widespread than large crude oil spills.

The study, "Unexpectedly high mortality in Pacific herring embryos exposed to the 2007 Cosco Busan oil spill in San Francisco Bay," was jointly undertaken by scientists with NOAA, the Bodega Marine Lab (University of California at Davis) and Washington State Department of Fish and Wildlife.