Biological invasions in the San Francisco Estuary: A comprehensive regional analysis

This study describes and analyzes 231 nonindigenous salt water, brackish water and freshwater species, including plants, protists and invertebrate and vertebrate animals, that have been introduced into the San Francisco Estuary since the 1850s. Data is also provided on nonindigenous species established in tributary waters and adjacent terrestrial habitats, and species reported but not established in the Estuary.; The Estuary's nonindigenous species derive mainly from the Western North Atlantic (33% of marine introductions), Western North Pacific (25%) and Eastern North Atlantic (13%), and from North America (53% of continental introductions, mainly fish) and Eurasia (34%, mainly plants). Since 1970 an average of one new species has become established in the Estuary every 15 weeks, more than 3 times the invasion rate before 1970. Since 1850, dominant transport mechanisms have been ship fouling, ballast water, oyster shipments and government activities (mainly fish stocking), with ballast water now dominant. Comparisons with other regional studies of marine and aquatic invasions reveal some common taxonomic patterns.; Invaders in the Estuary often occupy habitats upstream of related native species. The rate of successful establishment of intentionally planted fish and shellfish has not changed over time, suggesting that the increased invasion rate may be due to increased numbers of organisms released, rather than to changing conditions making the Estuary more vulnerable to invasions. Freshwater and anadromous invaders have been more successful than salt water and catadromous organisms; other traits tested do not correlate significantly with successful establishment. Analysis of the parasite loads of freshwater fish does not support the hypothesis that lower parasite loads enable introduced organisms to become established by outcompeting natives.; The impacts of introduced salt, brackish and freshwater organisms on human activities and economies have been little studied and rarely quantified. These impacts have been substantial and mainly negative in the Estuary. The continuous addition of new species has made the Estuary's ecosystem more unstable, less predictable and increasingly difficult to manage, and is a significant factor leading to increased restrictions on the operation of the California water system--with potential impacts on the whole California economy.