Oil spill 'unexpectedly deadly' for Pacific herring
By Lewis Smith
December 30 2011 Lewis Smith
Fish larvae samples collected by researchers
An oil spill in San Francisco Bay had a much worse than expected impact on the Pacific herring population, scientists have said.
The 54,000 gallons of bunker oil that spilt from the Cosco Busan, a container ship that collided with the San Francisco-Oakland Bay Bridge in November 2007, killed virtually all the herring larvae in shallow waters.
Pacific herring is an important commercial species, one of the last urban fisheries, and the spill took place a month before the spawning season in San Francisco Bay.
Previous oil spills, particularly the Exxon Valdez disaster in Prince William Sound, Alaska, in which 32 million gallons of crude oil were spilt in 1989, had shown that nursery fish suffer but there was surprise that almost the entire 2008 generation died.
Scientists now believe that the nature of the oil – bunker oil is a heavy fuel oil used by ships as a cheap fuel and often contains unknown contaminants – combined with ultraviolet sunlight to make the spill especially lethal to the embryonic fish. Larvae was placed in cages in both shallow and deep water by researchers.
They found that while all the larvae suffered heart defects from exposure to oil, almost all of those that were in sunlight died. Those that were kept deeper down in the water mostly survived. Naturally spawned Pacific herring larvae is found in shallow waters.
As well as the high death rate among larvae in shallow waters the scientists found that the flesh of the embryos was disintegrating much faster that would have been expected normally.
“These embryos were literally falling apart with high rates of mortality,” said Gary Cherr, one of the researchers and director of the University of California Davis Bodega Marine Laboratory. The quantity of oil split from the Cosco Busan would fill less than a tenth of an Olympic swimming pool.
“Based on our previous understanding of the effects of oil on non-embryonic fish, we didn’t think there was enough oil from the Cosco Busan spill to cause this much damage,” he said. “And we didn’t expect that the ultraviolet light would ramatically increase toxicity in the actual environment.”
Pacific herring are an important forage species for larger animals such as salmon and humpback whales, and travel from San Francisco Bay to the Bering Sea.
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