in 1986. In two years it became commonest clam throughout the northern part of San Francisco Bay (Carlton et al. 1990). It has reached very high population densities and altered the water chemistry of the bay and hence has affected many dependent organisms. In 1989, the green crab (Carcinus maenas) invaded the bay and is now spreading throughout California's coastal waters, where its voracious appetite threatens the shellfish and crab industries in the coastal regions. At present, there are 164 known introduced species of plants, invertebrates, and fish in San Francisco Bay, many of which have displaced native species (Cohen and Carlton 1998).

The invasive species are causing direct changes in the food webs of economically important species as well as altering the physical nature of the bay as noted above. For example, the major food item for fish, including juvenile striped bass (itself an introduced species), is the zooplanktonic mysid Neomysis mercedis. Neomysis is being displaced by introduced Acanthomysis.

Changing Nature of San Francisco Bay Fisheries

As might be expected from the ecosystem disruptions noted above, fisheries in the bay have had a tumultuous history (Box 5-4). Commercial fisheries for salmon, sturgeon, sardines, flatfishes, crabs, and shrimp were established soon after the start of the gold rush to support the rapidly growing human populations. These fisheries, and others that developed later, especially for striped bass, annually provided millions of pounds of protein until changes in the estuary severely reduced them. Loss of the fisheries can be attributed to a variety of causes, including overfishing, changes in water quality, and reductions in freshwater input through the bay delta.

Shellfish

The native oyster Ostrea lurida was intensively harvested beginning in the 1850s and gradually declined in abundance. Larger oyster species were imported and cultivated, first from the Pacific Northwest, and subsequently from the east coast (Crassostrea virginica). In 1899 more than 1,100 t of oyster meat was produced in the bay. The increasing pollution in the bay resulted in a decline of production by 50 percent by 1908 and by 1921 no more attempts at cultivating oysters were made. The bay was, however, used for holding imported stock until 1939, when the industry closed. Oyster culture has been moved to cleaner bays elsewhere in California (Leet et al. 1992).

Crustaceans

Bay shrimp, an aggregate of four species with Crangon franciscorum being the primary one, have been harvested commercially from San Francisco Bay



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