A Chinese mitten crab was first collected in South San Francisco Bay by a shrimper during the winter of 1993. MSI brought one of these crabs to the California Academy of Sciences where it was identified by Dr. Bob Van Syke. The discovery of the mitten crab in San Francisco Bay caused a big stir. Although mitten crabs had previously been found elsewhere in the U.S., San Francisco Bay was the first place where the crab could feasibly reproduce and increase its numbers.

Mitten crabs are native to mainland China and coastal areas along the Yellow Sea. They are catadromous, meaning they grow and develop in freshwater and then travel to saltwater to reproduce and die. Mitten crabs were introduced into German waters during the early 1900's, most likely via ballast water carried on large ships coming from China. The crabs did very well in German waters and quickly expanded their range outside of Germany. Mitten crabs were a big nuisance in Germany and elsewhere during the peak of their invasion. Large numbers of burrows excavated by the crabs caused problems with bank erosion and levee damage. Mitten crabs were also a nuisance to commercial fishermen. Because Mitten crabs were so abundant, they were frequently captured in fishing nets, damaging the nets and the fish within.

In December of 1995 the Marine Science Institute conducted the first "Great Mitten Crab Survey" to document the distribution of mitten crab juveniles along the banks of South Bay sloughs and creeks. The survey was repeated on June 8, 1996, December 13, 1997, March 18, 2000, and March 17, 2001..

Over 20 volunteers (who were given prior instructions) broken up into 5 to 7 groups, participated in each survey.  Each group, led by a MSI staff member, was assigned to survey selected sites.  Due to time, weather and site accessibility constraints, the sampled sites changed slightly in location from year to year.  The sites were, however, ecologically similar having banks of clay and some vegetation present.  The area encompassed and the survey locations will soon be posted on the "South San Francisco Bay Map" with the following data from the surveys.  Stay tuned!

The following outlines some of the data recorded in our 1995, 2000 and 2003 surveys.  Please note that these surveys were conducted at different times of the year; therefore it is not possible to make comparisons between years due to the fact that the  seasonal differences reflect different points in the crabs life cycle.  It is interesting though to note some of the quantitative results from these surveys.  Three quantitative measures are reported for this purpose:

                                        1) Crab burrow density    2) Crab population density    3) Crab sizes

DECEMBER 1995 SURVEY DATA

Survey site	Average burrow density
Alameda Creek	1.1 crabs/meter2
Alviso Slough	6.2 crabs/meter2
Coyote Creek	2.6 crabs/meter2
Guadalupe Slough	1.4 crabs/meter2

MARCH 2000 SURVEY DATA

Survey site	Average burrow density
Alameda Creek at Alvarado Rd.	0.07 crabs/meter2
Alameda Creek at Ardenwood Rd.	0.77 crabs/meter2
Alviso Slough	8.90 crabs/meter2
Permanente Creek (south bank)	0.07 crabs/meter2
Permanente Creek (north bank)	0.17 crabs/meter2
Sulphur Creek	0.00 crabs/meter2
Calabazas Creek	2.06 crabs/meter2

MAY 2003 SURVEY DATA

Survey Site	Average Burrow Density
Alameda Creek at Alvarado Road	0.00 crabs/meter2
Alameda Creek at Ardenwood Road	0.00 crabs/meter2
San Tomas Aquinas Creek at Tasman	0.34 crabs/meter2
Calabazas Creek	not surveyed - water too high
Stevens Creek at Crittenden Rd.	0.00 crabs/meter2
Coyote Creek at Dixon Landing Road *	0.00 crabs/meter2

* Six juvenile mitten crabs were found in a passive crab trap outside the transect area.

  As you can see the numbers of Chinese Mitten Crabs are down. No one knows the reason for this.We will continue to update this page.  Please stay tuned for additional data! Contact Marilou Seiff at (650) 364-2760 ext. 11 or e-mail her at marilou@sfbaymsi.org for additional information.

Our surveys thus far have indicated that mitten crabs are well distributed in the South Bay and occur up to 10 miles inland from the Bay in both tidal and non-tidal areas. Concrete lined flood control channels and limited supplies of freshwater in some areas seem to confine the distribution of this species. We have found that the crabs are most abundant in freshwater areas with clay banks lined with tule. Our most recent survey indicated that the crabs have spread into new areas. We have thus far found crabs in Alviso Slough/Guadalupe River, Coyote Creek, Guadalupe Slough/San Tomas Aquino Creek, Alameda Creek, the Alameda Creek Flood Control Channel, Sulfur Creek, and Permanente Creek. Population numbers do fluctuate in mitten crab areas world wide. For more information about our recent findings contact the Institute.

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