The Chinese mitten crab, Eriocheir sinensis, native to coastal rivers and estuaries of China and Korea along the Yellow Sea (Panning 1938), was first discovered in South San Francisco Bay in 1992 and quickly spread throughout the estuary during the next several years. Mitten crabs were first collected in San Pablo Bay in the fall of 1994, in Suisun Marsh in February 1996, and in the Sacramento-San Joaquin Delta in August 1996 (Hieb 1997). The current known distribution of the Chinese mitten crab in the Delta extends north up the Sacramento River to the Port of Sacramento, east to Stockton (Fourteenmile Slough), and south to Fabian and Bell Canal. The crab is also distributed throughout Suisun Marsh. We expect the known distribution to expand this fall as emigrating adult crabs continue to be incidently caught by fishermen.

This summer was a pilot year for implementing an annual monitoring program for juvenile mitten crabs in Suisun Marsh and the Delta. The 45 adult crabs collected last fall and winter in the Delta, Suisun Marsh and Suisun Bay indicated the population in the northern estuary was large enough to be detected by monitoring. Because the juvenile crab’s diet is comprised mainly of vegetation, capturing them with baited traps was not feasible. Instead, juvenile crabs were excavated from the burrows they dig for protection from predators and desiccation during low tide (Panning 1938).

After surveying the Delta and Suisun Marsh for potential sites in late June and early July, 15 monitoring stations were selected based on several criteria: sites had to be tidally influenced, contain adequate expanses of unrocked bank exposed during low tide, and be accessible by vehicle. We attempted to select stations to achieve an even distribution throughout the Delta and Marsh, but due to large expanses of riprapped bank or inaccessibility, portions of the Delta may be under represented.

Currently, there are 4 stations in Suisun Marsh and 11 in the Delta, of which 8 are core stations and 3 are peripheral stations. Core stations are sampled twice a year separated by 4 weeks. Peripheral stations are sampled once a year, and represent the upstream limit of where juvenile mitten crabs can be expected to burrow.

Each station was surveyed during low tide when the bank was exposed. We searched for mitten crabs along a 5 meter transect paralleling the bank and extending from the water line to the high tide line or to the top of the bank. The transect height was measured at 1 meter intervals and the average height was used to determine the total area of the transect. For core stations, the second transect was placed within 0.25 miles from the first transect, preferably adjacent to the original. Transect searches involved excavating all cavities, such as burrows and rotted root tunnels, and examining all debris, driftwood, rootwads, and ponded water for mitten crabs.

Carapace length and width at the widest point were measured for all crabs. Crabs larger than 9 mm were sexed and all were returned to the same location where captured. Additional information recorded included vegetation and soil types, bank profile, water salinity and temperature, and tidal phase.

Sampling began in late July and continued through early September. Average densities were highest at the Suisun Marsh stations (Figure 1). Of the 4 stations surveyed in the Marsh, Denverton Slough had the highest average density of 3.07 crabs/m2 and Montezuma Slough had the lowest average density of 0.55 crabs/m2. During the second survey, densities increased at the Montezuma and Denverton Slough stations and decreased at the Suisun and Hill Slough stations. The mean carapace width (cw) was 15.3 mm for both surveys (n=25, survey 1, n=36, survey 2). The salinity ranged from 4.4 to 7.2 ‰, with the highest salinity at the Denverton Slough station on both surveys.

Crabs were found at only 4 of the 8 core stations in the Delta (Figure 1). Average densities were relatively low, ranging from 0.31 crabs/m2 in Middle River near the railroad tracks on Jones Tract to 0.13 crabs/m2 in Fabian and Bell Canal at Tracy Oasis Marina. The mean size was also 15.3 mm cw (n=11); all crabs were collected from freshwater. Densities at all 4 of these Delta stations declined to zero on the second survey. No crabs were found at the peripheral stations.

Juvenile crab densities in the Delta and Suisun Marsh are significantly less than those reported in South San Francisco Bay sloughs. Average densities for 1997 in South Bay sloughs ranged from 3.38 crabs/m2 to 6.31 crabs/m2 in July and from 5.02 crabs/m2 to 15.87 crabs/m2 in August (Diana Theriault, UC Berkeley, personal communication). Previously, a maximum density of 30 crabs/m2 was reported (Halat 1997).

Both the State and Federal water project pumping plants collected the first juvenile mitten crabs this summer. The Skinner Fish Facility (DWR) caught one crab of 29 mm cw in August. The Tracy Fish Collection Facility (USBR) captured juvenile crabs in the holding tanks beginning in late June. Mean size of age-0 juveniles was 15.0 mm cw in June (n=1), 17.6 mm in July (n=21), 23.2 mm in August (n=13), and 31.0 mm in September (n=7). Additional crabs, which were determined to be age-1 or age-2, were collected in July, August and September. These totals represent only a small fraction of crabs located at the pumps. Only the crabs caught during the periodic 10 minute counts were saved; others were salvaged with the fish without being counted. Also, the fish facility crews reported finding crabs in the floating debris caught against the screens and among the louver structures.

The IEP will continue to monitor juvenile mitten crabs in the Delta and Suisun Marsh annually. Sampling will occur in July and August, the peak migration period of juvenile crabs to brackish and freshwater rearing areas. We plan to add several more stations in the Delta, including additional core stations in Central and West Delta and a peripheral station in the southeast Delta.

Acknowledgments:

I thank Anna Holmes and Jennifer Osmondson of CDFG for their assistance in field collections and developing sampling procedures; Kathy Hieb for the opportunity to collaborate on the design and implementation of a biological study; USBR personnel at the Tracy Fish Collection Facility and CDFG personnel at the Skinner Fish Facility for crab collections; Kathleen Halat of Wetlands Research Associates and Diana Theriault of UC Berkeley for guidance on sampling protocol and sharing data; Dave Feliz of Grizzly Island Wildlife Area and Bay-Delta Division personnel who provided valuable advice on locations of potential monitoring sites; and Paul Raquel, Tracy Oasis Marina, and Windmill Cove for access to their property. This work is part of the Green and Mitten Crab Studies, which is funded and supported by the Interagency Ecological Program.

Literature Cited:

Halat, K. M. 1997. The distribution and abundance of the Chinese mitten crab (Eriocheir sinensis) in southern San Francisco Bay, 1995-1996. M.S. Thesis, University of California, Berkeley, 80 pp.

Hieb, K. 1997. Chinese mitten crabs in the Delta. IEP Newsletter, Vol. 10, No. 1-Winter 1997.

Panning, A. 1938. The Chinese mitten crab. Annual Report Smithsonian Institution, pp 361-375.

Figure 1. Average densities of juvenile Chinese mitten crabs in the Delta and Suisun Marsh, summer 1997.