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Distribution, ecology and potential impacts of the Chinese mitten crab (Eriocheir sinensis) in San Francisco Bay
Rudnick, D.A.; Halat, K.M.; Resh, V.H. (2000). Distribution, ecology and potential impacts of the Chinese mitten crab (Eriocheir sinensis) in San Francisco Bay. Technical Completion Reports, 206. University of California Water Resources Center: Riverside. ISBN 1-887192-12-3. 74 pp.
Part of: Technical Completion Reports. University of California Water Resources Center, more

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Keywords
    Eriocheir sinensis H. Milne Edwards, 1853 [WoRMS]; Marine; Brackish water; Fresh water

Authors  Top 
  • Rudnick, D.A.
  • Halat, K.M.
  • Resh, V.H.

Abstract
    The arrival of the Chinese mitten crab to the San Francisco Bay-San Joaquin Delta (Bay- Delta) ecosystem has been a source of widespread concern. This crab has spread from its native range, in China, to coastal ecosystems throughout Europe and, most recently, into North America. The Chinese mitten crab population in California has exploded within the last decade to cover hundreds of miles of the Bay-Delta and its tributaries. The Chinese mitten crab is a large, catadromous crab, moving from freshwater habitats where it spends its juvenile years to saltwater habitats in order to reproduce. In other countries into which this species has been introduced, the abundance and behavior of the crab has caused detrimental impacts to fisheries and loss of bank stability in areas where it burrows. Through a grant from the Water Resources Center, we set out to examine the ecology, distribution, and economic and ecological impacts of the Chinese mitten crab in the fresh and saline waters of San Francisco’s South Bay. The Chinese mitten crab offers excellent opportunities to: 1) study the population dynamics of an invasive species; 2) examine differences and similarities for this organism between its native and new environments; and 3) use research findings to make recommendations for understanding and control of this organism. We surveyed the tributaries and main body of South San Francisco Bay in order to examine the ecology and impacts of the Chinese mitten crab. We studied distribution and abundance of juvenile crabs by establishing 72 monitoring sites throughout the salinity gradient of several major South Bay tributaries. These sites were used in 1995, 1996 and 1999 to collect population and habitat data. Adult crabs in the South Bay were monitored using otter trawls over the same three-year period. Population parameters of mitten crabs, including size and sex ratios, were examined. We quantified habitat preferences of the mitten crab by examining stream characteristics including substrate type, vegetation type, and salinity. Gut contents were analyzed in order to examine dietary habits and dietary shifts with age, and frequent behavioral observations of crabs were made to confirm dietary and habitat preference data. In order to examine impacts to banks and levees, we examined burrowing by juvenile crabs and quantified sediment removal from burrowing by estimating burrow density and sediment removed per burrow. The potential impacts of Chinese mitten crab on two species of freshwater crayfish were studied using a combination of behavioral observations, laboratory experiments and surveys of the commercial crayfish industry. We found mitten crabs to be broadly distributed throughout the freshwater tributaries of the South Bay, with the distribution spreading over the three years we monitored these sites. Adult mitten crabs were also found in the main body of the bay, with gravid (eggcarrying) females appearing between November and May of each year. Juvenile mitten crabs preferred intertidal sections of streams that had banks with high clay content and abundant vegetation overhanging or growing on the banks. Gut content analysis of mitten crabs revealed a high proportion of vegetative matter, with low amounts of invertebrates, regardless of the size of the crab or the habitat from which it was collected. v Abundance of Chinese mitten crab also increased over time, as seen by a continuous increase in burrow densities: densities reached a high of 18/ m2 at one site in 1995 and 1996, and densities exceeded 30 burrows/ m2 at two sites in 1999. Densities exceeding 30 burrows/ m2 are considered to be damage-causing levels in other areas into which the crab has been introduced. Research into the impacts of Chinese mitten crabs on crayfish populations produced mixed results: based on behavioral observations, crabs and crayfish in the South Bay were found to co-occur; crayfish industry surveys revealed concern regarding potential competition of crayfish and mitten crabs; and experimental interactions suggested possible habitat competition between adults of crayfish and mitten crabs. We present these findings in coordination with studies occurring in the North Bay, the Delta and throughout California which show that the Chinese mitten crab is quickly spreading throughout all of northern California, and has recently reached southern California through the state’s aqueduct system. In addition, the sheer abundance of the crab has created significant impacts on the state’s water projects. In particular, efforts to minimize impacts to fish from water project turbines and pumps have been seriously impeded by the crabs clogging these systems during their fall migration. The results of our research include important findings for the spread of the organism, similarities and differences between its ecology here and in its native range, and impacts on physical and biological characteristics of San Francisco Bay. We have shown that the mitten crab population has exploded in the South Bay as well as throughout the Bay- Delta region. The highest densities of mitten crab burrows are currently restricted to intertidal segments of the banks and levees; however, at some locations within these areas, bank slumping and erosion already are significant. We believe that there is cause for concern for impacts on freshwater crayfish, although more research needs to be done to quantify these impacts. Given the widespread distribution, abundance, and lack of specialized predators for the Chinese mitten crab, we believe complete control of this species would be extremely difficult and costly. A more effective strategy will be to focus control in specific regions where the crab is producing costly and damaging effects, such as the fish protection programs at state water projects. State and private parties have discussed commercialization of the mitten crab for consumption here or in Asia, and this option could provide an effective, though controversial, method of control. Our findings suggest that the mitten crab is here to stay as yet another member of San Francisco’s evolving non-indigenous communities. Key words: 0170, 0770, 0885, 2300,

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