Overbite Clam Population Spreads Upriver in SF Bay Estuary

The overbite clam, Corbula amurensis (Corbula), often blamed as one of the causes of the decline of the delta smelt, has further spread into the fresher portion of the San Francisco Bay Estuary in recent years.

This invasive species, which thrives in the saline waters of the estuary, arrived in 1986 in the midst of a serious drought, probably after being dumped with ships' ballast water. The overbite clam consumes large amounts of plankton, a major food source for critical fish species - such as the delta smelt - and other aquatic organisms, by sucking in and filtering plankton from the water. The larger the geographic distribution of the clam, the more widespread its effects on the aquatic food web.

Jan Thompson, a U.S. Geological Survey biologist who has been studying the overbite clam since its introduction into the estuary, said that the Corbula amurensis populations in the channels near and upstream of Chipps Island have increased in recent years, and a similar increase has been seen in the nearby shallow waters of Honker Bay. Corbula were seen in this region during the 1987-1992 drought, but the clams were very large in size and moderate in numbers. In recent years, the population is made up of smaller clams in very high numbers.

“Recent, persistent expansion of the clam upstream places it consistently in the zone where native fish feed (as larvae and adults) which potentially increases the clam’s effect on the aquatic food web in this critical region,” Thompson said.

She added that the higher mass of clams in the channels of the upper estuary appears to be the result of a combination of the effects of physical processes and water management practices.

Primary controls on the clam population include both biological and physical processes. Biological processes include recruitment (emigration) of young clams to the area and limitation of clams to water with the right proportion of salt, while physical processes include the movement of water and salt in the estuary. Water flows from upstream to downstream and then back up because of the tides, delivering varying salinity and clam recruits from outside the locale.

“The importance of these controls varies between the channel and the shallows,” Thompson said. In recent research, she discovered that juvenile clams have been showing up in higher numbers upriver than they ever have before.

Juvenile clams are successful when they can settle onto the bottom sediments and burrow in to become part of the local population. Larger population densities of adults increase turbulence at the bottom and make it more difficult for the juvenile clams to burrow into the sediment. Therefore, a high abundance of adults inhibits juveniles. This effect has been seen particularly during drought years.

Salinity also limits the success of juvenile and adult clams. Research has shown that juveniles tend to settle seaward of the area where saltwater and freshwater mix. This low salinity zone in the estuary where the salinity reaches two parts per thousand is known as X2. Its location, measured as the distance from the Golden Gate, varies depending on precipitation patterns, river runoff, and water management. Water management practices of controlling the position of X2 may be contributing to the recruitment of the overbite clam higher up in the estuary.

Prolonged wet periods (high flows) limit juvenile success and increase adult mortality due to prolonged low salinities in the upper estuary. “However, if a prolonged wet year is followed by a dry year, or even a dry fall, higher than normal numbers of juveniles are seen the following year in channels as X2 moves upstream,” Thompson said.

There has been some speculation that extended periods of high freshwater flows from the Delta into the Bay would cause a reduction in the overbite clam population by creating an inhospitable environment for the clams, causing mass mortality. But recent population trends show that high freshwater flow would not be sufficient to eliminate the clams from the upper estuary unless the high freshwater flow continued through fall and persisted every year, which is unlikely. Rather, high freshwater flow one year followed by a reduction the next year could increase the population of the overbite clams as juveniles become established in the adult-free environment the following season.

“We are now seeing Corbula populations persisting upstream similar to what we saw in the 1986-1987 drought,” Thompson said. “They can therefore affect the density of phytoplankton and zooplankton in the water column through grazing in this upper reach of the estuary.”

Bottom line? The overbite clam looks like it’s here to stay.

“Our efforts will not be able to get rid of it,” Thompson said.