The Chinese mitten crab (Eriocheir sinensis), so named for the dense patches of hairs on the claws of larger juveniles and adults, is native to the coastal rivers and estuaries of the Yellow Sea. It was accidentally introduced to Germany in the early 1900s and spread to many northern European rivers and estuaries. In San Francisco Estuary, the mitten crab was first collected in 1992 by commercial shrimp trawlers in South San Francisco Bay and has spread rapidly throughout

the estuary. Mitten crabs were first collected in San Pablo Bay in fall 1994, Suisun Marsh in February 1996, and the Delta in September 1996. As of August 1998, the known distribution of the Chinese mitten crab extends north of Colusa to Hunter's Creek (near Delevan National Wildlife Refuge) in the Sacramento River drainage, east to Roseville (Cirby Creek) and eastern San Joaquin County near Calaveras County (Mormon Slough and Littlejohns Creek) and south in the San Joaquin River to Hiway 165, near San Luis National Wildlife Refuge. The most probable mechanism of introduction to the estuary was either deliberate release to establish a fishery or accidental release via ballast water. In Asia, the mitten crab is a delicacy and crabs have been imported live to markets in Los Angeles and San Francisco.

The mitten crab is catadromous - adults reproduce in salt water and the offspring migrate to fresh water to rear. In the San Francisco Estuary, the mitten crab probably matures in 2 to 3 years, although it reportedly matures from 1 to 5 years elsewhere, depending on water temperature. Males and females grow to a maximum carapace width of approximately 80 mm (3 inches) in the estuary. Mating and fertilization occurs in late fall and winter, generally at salinities >20‰. The females carry their eggs until hatching and both sexes die soon after reproduction. A single female can carry 250,000 to 1 million eggs. After hatching, larvae are planktonic for approximately 1 to 2 months. The small juvenile crabs settle in salt or brackish water in late spring and migrate to freshwater to rear.

Young juvenile mitten crabs are found in tidal freshwater areas, and usually burrow in banks and levees between the high and low tide marks. Mitten crabs apparently do not burrow as extensively in non-tidal areas, probably because they are not subject to desiccation during low tides. Older juveniles are found further upstream than younger juveniles, and in China and Europe they have been reported several hundred miles from the sea. We do not understand what cues this upstream migration, although high densities were reportedly a factor in Germany and the upstream migration may be tied to the monsoon season in southern China. Maturing crabs move from shallow areas to the channels in late summer and early fall and migrate to salt water in late fall and early winter to complete the life cycle.

Mitten crabs are adept walkers on land, and, in their upstream migration, they readily move across banks or levees to bypass obstructions, such as dams or weirs. In Germany, large numbers of mitten crabs were reported to leave the water at night when they encountered an obstruction and occasionally wandered the streets and entered houses. In Stockton, 2 adult mitten crabs climbed over a levee and into a swimming pool when they encountered a small dam blocking their downstream migration.

Mitten crabs are omnivores, with juveniles eating mostly vegetation, but preying upon animals, especially small invertebrates, as they grow. In the Delta, adult crabs have been incidentally caught by anglers using a variety of baits, ranging from ghost shrimp to shad. Relatively little is known about the predators of the mitten crab, although white sturgeon, striped bass, bullfrogs, loons, and egrets have been reported to prey upon them in the estuary. We assume that other predatory fishes, including largemouth bass and larger sunfishes, river otters, racoons, and other wading birds will consume mitten crabs.

Based on the impacts of mitten crabs in their native range and Europe, they pose several possible threats. The mitten crab is the secondary intermediate host for the Oriental lung fluke, with mammals, including humans, as the final host. Humans become infested by eating raw or poorly cooked mitten crabs. However, neither the lung fluke nor any of the freshwater snails that serve as the primary intermediate host for the fluke in Asia have been found in the Estuary. It has been noted that several species of freshwater snails which could possibly serve as an intermediate host are present in the watershed.

The burrowing activity of mitten crabs may accelerate the erosion of banks and levees. In Germany, burrows were reported to be up to 50 cm (20 inches) deep and some damage to levees and structures has occurred. Mitten crab burrow densities as high as 30/m2 (2.7/ft2) have been reported from South Bay creeks, with most burrows no more than 20-30 cm (8-12 inches) deep. The highest density of juvenile crabs was approximately 6/m2 (0.8/ft2) in Suisun Marsh and 1/m2 (0.1/ft2) in the Delta in summer 1997. In the Delta large numbers of juvenile mitten crabs were also reported in water hyacinth, which is not found in Suisun Marsh, San Francisco Bay, or it's tributaries.

In China and Korea, juvenile mitten crabs have been reported to damage rice crops by consuming the young rice shoots and burrowing in the rice field levees. Rice fields in tidally influenced areas apparently are most subject to damage.

The most widely reported economic impact of mitten crabs in Europe has been damage to commercial fishing nets and the catch when the crabs are caught in high numbers. The mitten crab has become a nuisance for commercial Bay shrimp trawlers in South Bay, as it is time consuming to remove the crabs from the nets (one trawler has reported catching over 200 crabs in a single tow several times). Shrimp trawlers have also reported that a large catch of mitten crabs damages and even kills the shrimp, making them unsuitable for the bait market. Shrimp trawlers have been able to move to areas with fewer crabs, but, as the mitten crab population grows, this option diminishes.

The mitten crab overlaps in dietary and habitat preferences with the introduced red swamp crayfish (Procambarus clarkii) in South San Francisco Bay creeks and negative interactions between the two species have been observed in the field. In the Delta, the mitten crab may reduce abundance and growth rates of the introduced signal crayfish (Pacifastacus leniusculus), which supports a commercial fishery.

The ecological impact of a large mitten crab population is the least understood of all the potential impacts. Although juveniles primarily consume vegetation, they do prey upon animals, especially invertebrates, as they grow. A large population of mitten crabs could reduce populations of native invertebrates through predation and change the structure of the Estuary's fresh and brackish water benthic invertebrate communities.

In Germany, extensive efforts were undertaken by the government in the 1920s and 1930s to control mitten crab populations in some rivers. Control measures often took advantage of the mitten crab's migratory behavior; traps were placed on the upstream side of dams to capture juvenile crabs as they migrated upstream. At one site, as many as 113,960 crabs were trapped in a single day. It was hypothesized that this population explosion may have coincided with a reduction of predators, especially fishes, in the rivers. In recent years, European mitten crab populations have apparently been stable, although there are occasional reports of "invasions". In 1981, the mitten crab population in the Netherlands increased substantially, resulting in serious damage to fishing nets.

Information on the impacts of the mitten crab in China and Korea has been more difficult to obtain. Although the mitten crab damages rice crops, no control measure have been reported. In some rice fields, they are cultured with fish. Apparently, mitten crabs are stocked at a rate that does not damage the rice crop.