Response of anchovies dampens effects of the invasive bivalve Corbula amurensis on the San Francisco Estuary foodweb

Abstract

Introduced bivalves have suppressed phytoplankton production in many aquatic food- webs, but effects on higher trophic levels have been inconsistent. In the northern San Francisco Estu- ary, the introduced clam Corbula amurensis eliminated summer-long phytoplankton blooms starting in 1987, but responses of mesozooplankton biomass and most fish were somewhat muted. I con- ducted a retrospective analysis of the response of the estuarine foodweb to the decline in phyto- plankton due to clam grazing. When C. amurensis invaded, the distribution of northern anchovy Engraulis mordax shifted toward higher salinity, reducing summer abundance by 94% in the low- salinity region of the estuary. Northern anchovy dominates the biomass of fish in the more saline reaches of the estuary, and is genetically affiliated with the subpopulation on the central California coast, which in turn is strongly influenced by climate variability. However, abundance of northern anchovy within the estuary was not related to available estimates of biomass, catch, or abundance of northern anchovy on the California coast, nor was it related to any of 4 variables describing aspects of climate variability with different time and space scales. The shift in spatial distribution appears to have been a direct behavioral response to reduced food. Bioenergetic calculations showed reduced consumption of zooplankton by all planktivores, including mysids, after C. amurensis became abun- dant and the anchovy left the low-salinity region of the estuary. This reduced consumption appears to have mitigated effects of the loss of phytoplankton productivity, making a greater proportion of the zooplankton productivity available to other fish species. Such behavioral accommodations are likely wherever invasions occur in open systems.