Patterns and processes of biological invasion: The chinese mitten crab in San Francisco Bay

The Chinese mitten crab, native to coastal rivers and estuaries of central Asia, has invaded several European countries over the past century, causing widespread concern because of its periodically extreme abundance and burrowing behavior that causes bank erosion. San Francisco Bay and its tributaries contain the first and, currently, only known established population of the Chinese mitten crab in North America. Discovered in South San Francisco Bay in 1992, the mitten crab has spread rapidly to cover several thousand km(2) surrounding the Bay. Between 1995 and 2001, we monitored distribution, population dynamics, and life history attributes of the Chinese mitten crab in San Francisco Bay and its tributaries. Burrow densities increased from a mean of 6 burrows/m(2) in 1995 to >30 burrows/m(2) in 1999 in tidal portions of South Bay tributaries. Mitten crabs are associated with: tidally influenced portions of Bay tributaries as young juveniles; with freshwater streams (less than or equal to250 km from their confluence with the Bay) as older, migrating juveniles; and with the open waters of the Bay as reproductive adults after migrating from fresh water to reproduce between late fall and early spring. Population size peaked in 1998, with 750,000 crabs counted in fall migration in a North Bay tributary and 2.5 crabs/tow collected from North Bay breeding grounds; abundance subsequently declined greatly (2500 crabs in the same river system, 0.8 crabs/tow). Average size of adult crabs diverged between the North Bay population, which increased in size between 1996 and 2001, and the South Bay population, which decreased. The rapid establishment and spread of this species, its tolerance for a wide range of biotic and abiotic conditions, and its cyclical population dynamics pose challenges for control.