California Department of Fish and Wildlife

Bay Delta Region

Main Office
  7329 Silverado Trail
  Napa, CA 94558
  (707) 944-5500

Stockton Office
  2109 Arch Airport Rd
  Stockton, CA 95206
  (209) 234-3420

Regional Manager:
Scott Wilson

Mitten Crabs on the Move

Kathy Hieb and Tanya Veldhuizen, CDFG

The Chinese mitten crab, Eriocheir sinensis, was first discovered inthe San Francisco Estuary in 1992. Since then, its abundance has rapidly increased and its distribution in the estuary has greatly expanded. The mitten crab is catadromous and is well know for long distance upstream migrations to freshwater rearing areas. In its native range, the mittencrab has been reported to migrate 1400 km (870 miles)up the Yangtze River from the China Sea. The mitten crab was accidentally introduced to Germany in the early 1900s where the population exploded and the distribution rapidly expanded. The mitten crab invaded neighboring countries such as the Netherlands, Belgium, Poland, Denmark and Sweden,and juveniles were reported as far upstream as Prague, Czechoslovakia,which is 700 km (435 miles) up the Elbe River from the North Sea. Juvenile mitten crabs were reported to travel 1 to 3 kmper day during their upstream migration while adult crabs traveled up to12 km per day during their downstream migration. In Germany, the mittencrab also demonstrated its renowned athletic ability, as it readily climbed over or circumvented small dams and weirs which blocked its migration route.

Since January 1998, we received several reports of large numbers of migrating juvenile mitten crabs, which were most noticeable when they concentratedat the base of weirs or other structures in waterways. Mitten crabs were reported climbing over weirs in the Sutter and Yolo bypasses, includingthe Sacramento Weir, in February and March. Thousands of mitten crabs migratedup Mormon Slough and Little John Creek, to the east of Stockton, also inFebruary and March. The USBR fish collection facility also collected largenumbers of juvenile mitten crabs in January and early February.

All of the migrating juvenile crabs were 1 year old, with a carapace width of approximately 25 to 40 mm (1 to 1.5 inches); interestingly,at least 75% of the crabs we observed were males. These crabs probably reared in the Delta last year, where they burrowed into banks for protection from desiccation and predators or sought cover in floating vegetation,especially water hyacinth. We believe, based on their size, that thesecrabs will mature this fall, when they will migrate to brackish and saltwaterto reproduce.

As of the last week of March 1998, the known distribution of the mitten crab extends north up the Sutter Bypass to Meridian, east up Little John Creekto Farmington and Mormon Slough to the Calaveras River, and south up theSan Joaquin River near Vernalis. Although large numbers of downstream migrating adults have been documented in the Delta and elsewhere in the estuary,this is the first time we have seen mass upstream migrations of juvenilesfrom the Delta. Although we do not know what cues these migrations, wehypothesize that increased outflow and overcrowded rearing areas may beimportant factors.