It’s a long held belief around here that the oysters native in San Francisco Bay were once abundant, and that their loss may have had a great negative impact on the bay. Our speaker Andrew Cohen believed it, but became curious about one particular question. Why did they disappear?
The native oyster was rediscovered in 1999, and since then people have set about reestablishing their abundance. The only problem Andrew found with this was that it seemed that they had never quite left. Since 1912, the native oyster seems present in oyster census data through the whole century. They’d never disappeared to being with.
But still, the oysters in these counts were still not abundant. The story goes that until the mid 19th century the oyster was there in abundance, that it was the “dominant commercial fishery” (from an NOAA report). There have been various explanations for the lack of oysters: pollution caused the decline, or mining sediments covering up the oyster beds, or perhaps over-harvesting. Cohen set about investigating these explanations.
He looked at the times before and after 1850 (1769-1850 and 1850-1912) , to look at what might have caused the decline. His first thought, as an expert in invasive species, was that perhaps the Spanish or other colonists brought something with them. But as much as he would have liked to have found an explanation there he did not. Pollution as an explanation also fell by the wayside. There were no studies, no specific causes ever listed.
Hydraulic mining sediment also did not seem to be a likely explanation. The sediment peaked in 1890, the sediment was not evenly distributed, and in some places the sediment even decreased.
And in nowhere in the literature of eating after 1850 did anyone ever make much mention of the native oyster. Imports from Japan and the east coast got the lion share of the press. Furthermore, any over-harvesting impacts should have radiated outward from populated areas of the time. But there is no evidence of that.
So in Cohen’s mind, suddenly a different question arose. Were in fact the native oysters ever abundant?
He now dug deeper, looking at contemporary records, harvest records, laws & lawsuits, shell middens, and natural shell deposits. The earliest statement for abundance it turns out came in 1962 with no sources. The original source for “significant harvest” of native oysters seems to have come in 1979. There didn’t seem to be much in the way of contemporary records at all.
And nowhere else did he find any evidence of abundance. Unlike other oyster areas like Washington, there were not laws on the books about native oyster beds and harvesting. There were no lawsuits over ownership.
Finally though, looking at shell middens and shell deposits, he did find evidence for abundance. But it was not what he expected. Oysters were there in abundance, but more than 2000 years ago. The shell middens of the current era are full of clams and mussels. The large abundant oyster deposits were deep with in the bay’s sediments. The data matched each other pretty well. It seems the 1759-1850 abundance of the native oysters was something that was made up.
Andrew Cohen still left us with a mystery (and there are those who dispute Cohen’s evidence) why did they become less abundant 2000 years ago. There is no evidence that they were over-harvested by the Native Americans. Something seems to have changed about the bay, perhaps related to climatic events of the times. But that is perhaps a later talk.
The question for us now, is what about efforts to reintroduce the oyster to San Francisco Bay? Something else is at work to keep the oyster less than abundant, and it is not likely humans, do these little mollusks need our help?
For more information about the San Francisco Bay Estuary, check out “An Introduction to the San Francisco Bay Estuary” (download the PDF).
Andrew Cohen is the Director of the Center for Research on Aquatic Bioinvasions (CRAB) in Richmond. His research includes the science and policy of invasions, and he helped write California’s ballast water laws. He’s received a Pew Fellowship in Marine Conservation, the San Francisco BayKeeper’s Environmental Achievement Award, and in 1994 was named by the Bay Chapter of the Sierra Club as the Best Public Official for the Environment.