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In a study published today 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. The oil spill was from the container ship Cosco Busan in November 2007 which 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. 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.
The early life history stages (embryos and larvae) of fish can be especially sensitive to environmental contaminants due to the rapid proliferation, differentiation, and growth of embryonic tissues. Polycyclic aromatic hydrocarbons (PAHs) derived predominantly from the consumption of fossil fuels can be pervasive contaminants in rivers, lakes, and nearshore marine habitats. Since the early life history stages of many fish species are associated with sediments or affixed to plants or other substrates, they cannot easily evade exposure to PAHs, as demonstrated by the 1989 Exxon Valdez oil spill in Alaska.
Polycyclic aromatic hydrocarbons (PAHs), also known as poly-aromatic hydrocarbons or polynuclear aromatic hydrocarbons, are potent atmospheric pollutants that consist of fused aromatic rings and do not contain heteroatoms or carry substituents. Naphthalene is the simplest example of a PAH. PAHs occur in oil, coal, and tar deposits, and are produced as byproducts of fuel burning (whether fossil fuel or biomass). As a pollutant, they are of concern because some compounds have been identified as carcinogenic, mutagenic, and teratogenic. PAHs are also found in cooked foods. Studies have shown that high levels of PAHs are found, for example, in meat cooked at high temperatures such as grilling or barbecuing, and in smoked fish.
"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."