Mitten Crab Projects
Center for Lakes and Reservoirs
PO Box 751- ESR
Portland OR 97207-0751
What Are Mitten Crabs?
Photos courtesy CA Dept. of Fish and Game
The Chinese mitten crab (Eriocheir sinensis) reduces the
structural integrity of banks and levees, damages fishing nets, clogs
fish salvage operations, disrupts ecological structure and function,
and is a potential public health concern (summarized in Veldhuizen
and Stanish 1999). Mitten crabs construct burrows up to 30 cm deep
causing accelerated erosion and instability of banks and levees. A
nuisance by-catch for commercial and sport angers, the mitten crab "steals" bait,
damages nets, damages desirable catch, and overlaps its feeding habits
with other more desirable crayfish. In California, the most significant
impact from mitten crabs has been on fish salvage operations where
nearly 1 million crabs were caught in fish collection screens in 1998.
Mitten crabs are aggressive feeders whose habitat and feeding preferences
overlap with other species, possibly increasing competition with more
desirable species. Additionally, the mitten crab is a secondary host
to the Oriental lung fluke causing tuberculosis-like and influenza-like
symptoms in humans who are the final host.
The mitten crab gets its name from the hairy setae on the front claws
of juveniles and adults. Being a catadromous species, mitten crabs
spend most of their lives in fresh or brackish water and migrate downstream
to brackish or salt water to reproduce. This life cycle is opposite
of anadromous species, such as salmonids, who spend most of their
lives at sea and return to freshwater to spawn. It is unclear what
impact this spatial and temporal overlap between salmon and mitten
crabs will have in the Pacific Northwest.
Adult mitten crabs feed on aquatic vegetation and insects and, after
living 1-4 years in fresh or brackish water, migrate downstream to
salt water. Females can produce up to 1 million eggs that will hatch
in spring or early summer. Larvae spend 1-2 months as plankton, suspended
in the water column, until summer or fall when they settle to the
bottom and begin migrating upstream. Upstream migration of 1400 km
up the Yangtze River has been reported (Cohen and Carlton 1997).
In Oregon, a Japanese mitten crab was caught in the Lower Columbia
River in 1997 and an unconfirmed sighting occurred near the Portland
area (Sauvie Island) in 1999. In California, a commercial shrimp trawler
observed a Chinese mitten crab in the San Francisco Bay in 1992. Since
that time, mitten crabs have invaded miles of the Sacramento-San Joaquin
Delta and its associated waterways. Introduction to California was
likely due to deliberate or ballast water release (Cohen and Carlton
Preventing Mitten Crab Spread
a printable version of the "WANTED" poster.
Preventing the spread and establishment of Chinese mitten crabs into
waters of the Pacific Northwest is a primary component of the education/outreach
effort at the Center for Lakes and Reservoirs. Mitten crab "Wanted" signs
alert anglers, managers, and the community about the impacts from
mitten crabs. Signs are posted at boat launches, marinas, and community
centers in bays and estuaries throughout
the Pacific Northwest. CLR staff provide informative presentations
to watershed councils, drainage districts, port authorities, municipalities,
and dam operators. If you believe your organization would benefit
from a similar presentation, please contact Robyn Draheim at the
Center for Lakes and Reservoirs.
Mitten Crab Colonization Potential
The Potential for Mitten Crab Colonization and Management Options on the West Coast of North America.
Mitten crabs (Eriocheir spp.) are invasive species that pose a risk to the aquatic environments of the Pacific Northwest and the economic and social activities that depend upon intact aquatic systems. The recent establishment of a large population in the San Francisco Bay and the potential for introductions from California, Asia and Europe pose a significant invasion potential for estuaries and rivers from California to Alaska as West Coast estuaries exhibit similar climatic, habitat and environmental conditions as San Francisco Bay and European estuaries
The purpose of this project is to investigate the relationship between the environmental and physical habitat of estuaries with established populations and characteristics of mitten crab life history. This information will be used to evaluate risk of establishment and population expansion in selected West Coast estuaries.
Research into the mitten crabs life history, habitat requirements and environmental tolerances suggests that estuaries with limited salinity intrusion and short flushing times reduce the risk of population establishment. Large, stable estuaries, such as the Puget Sound, may support large populations. Large estuaries provide adequate temperature and salinity regimes for optimal larval survival and flushing times of sufficient duration for development within the estuary. One estuary, Coos Bay, has a flushing time similar to estuaries with mitten crabs but below the threshold required for larval development within the Bay. Small or river dominated estuaries, such as the Columbia River, have salinities below the threshold for larval development or have flushing times less than the duration of larval development.
The northern extent of a potential mitten crab population is likely to be limited by low water temperatures. Most estuary systems in Alaska have sufficient estuarine and freshwater habitat, but limited periods where water temperatures are above the mortality threshold for the larval stages and are at a low risk for the establishment of populations.
Mitten crabs may become established in smaller estuaries with short flushing times if they exhibit vertical migration behavior that prevents transport from the estuary. Experiments on vertical migration were performed at the Romberg Tiburon Center of San Francisco State University using vertical tanks and recorded using a time lapse video system. Analysis of the experiments is ongoing and initial results suggest limited vertical migration in early larval stages.
The average system with a mitten crab population has an estuary area of 488 km 2 with a salinity intrusion of 86 km and a flushing time of 43 days. The total area of Oregon and Washington estuaries, excluding Puget Sound and the Columbia River, would fit inside the average estuary with a significant mitten crab population. A large stable estuary is necessary to permit the development of zoea within the estuary and maintain population levels. A flushing time of 43 days is greater than the zoea development time at temperatures equal to or greater than 14 o C.
Most west coast estuaries have extended periods of temperatures above 14 o C and would allow for larval development. North of Puget Sound, average temperatures are at or below the threshold necessary to maintain populations. In years with below average temperatures, the potential development period would not be sufficient for larval development.
The examination of the key factors necessary for larval development and habitat and environmental condition suggests that the majority of the PNW is not at risk for the establishment of significant mitten crab populations. Puget Sound and Coos Bay are the only estuaries that have the proper combination of temperature, salinity and retention time for mitten crab establishment. Because of the much larger dimensions of Puget Sound, it may be expected to support larger populations that Coos Bay.
Experiments on larval survival at temperatures between nine and 12 o C would provide a more refined analysis of survival in Alaskan and near ocean waters. The current model allows for survival at temperatures above 9 o C. Mortality at 10 o C would greatly restrict the potential northern range of mitten crabs and reduce the period available for larval development.
The ability of megalopae to return to brackish and freshwater areas needs to be investigated. It is unknown what cues the megalopal stage to return to upper estuary areas and at what rate they can travel. These factors can determine if megalopae flushed into near ocean areas can home in on the estuary mouth and the distance from the estuary mouth the larva must remain in for a return to adult habitat.
Mitten Crab Recruitment Studies
Mitten crab (Eriocheir spp.) recruitment: Factors that influence larval supply and return to settlement sites.
This project investigates mitten crab recruitment dynamics in an effort to provide managers with a tool to predict years of high recruitment. The factors that influence larval supply, a key component of recruitment, will be investigated in laboratory experiments and used to build a model of recruitment. Larval behavior combined with estuary flow characteristics can determine the rate at which larvae are retained in the estuary and the potential to return from near ocean waters. Mitten crab populations experience large fluctuations that are a function of recruitment rates. Years of large downstream migrations have been extremely detrimental to management efforts in the bay-delta region. The objective is to correlate information on temperature, salinity and flow and larval vertical and horizontal migration behavior to elucidate the combination of factors that yield high recruitment levels. Research encompasses vertical migration behavior of the zoea and megalopae stages and horizontal migration of megalopae and juvenile crabs in response to estuarine and parental habitat cues.
A series of vertical migration experiments on zoea I, III and V and megalopae in two-meter columns was completed at the Romberg Tiburon Center in conjunction with Dr. Steve Bollens. Video records of the experiments are being analyzed. Initial results suggest a diel migration cycle with zoea in the upper water column during the day and a descent during the night. Addition experiments were conducted on the megalopae stage to mimic hydrostatic pressure and salinity changes indicative of a tidal change. The experiment did not result in an increase of swimming events or total time spent in the water column in comparison to a control.
Choice experiments on higher and lower salinities and estuarine versus artificial seawater were conducted in a y flume along with vertical experiments on response to tidal cues of a hydrostatic pressure and salinity increase. A full set of experiments was completed on the megalopae and juvenile crab molts I, II and III. Initial analysis suggests that the megalopae do not migrate horizontally in response to estuarine water or lower salinity. Megalopae exhibit a positive response to flow but with a limited swimming ability. Mitten crab megalopae are highly benthic and spend the majority of time clinging to substrate, primarily seaweed. The juveniles during the first and second molts do not migrate horizontally in response to flow, higher or lower salinity. At the third molt, the mitten crab responds positively to flow but avoids both higher and lower salinity changes.
A basic temperature based larval development model for the San Francisco Bay was developed. This model was based upon temperature data collect by NOAA bouy and tide gaging stations. Temperatures favorable for larval development exist from October through August.
Initial results suggest that mitten crab zoea and megalopae exhibit minimal behavior that would enhance retention in estuaries or provide a mechanism for reinvasion of estuaries from near coastal waters. Previous work suggests that mitten crab populations are constrained to estuaries where the retention time is great enough to permit larval development within the estuary. Megalopae and juvenile crabs can occupy a salinity range from 35 ppt down to nearly 0 ppt. The movement of crabs back into freshwater likely occurs at a later stage, probably during the second year.
Experiments on the cues for horizontal migration need to be continued on later crab molts. At some point in development, mitten crabs migrate from estuarine habitat to primarily freshwater habitat. Knowledge of this timing could facilitate management and assessment of population size.
Cohen, A. N. and J. T. Carton. 1997. Transoceanic transport mechanisms:
Introduction of the Chinese mitten crab, Eriocheir
sinensis, to California.
Pacific Science 51(1): 1-11.
Veldhuizen, T. C. and S. Stanish. 1999. Overview of the Life History,
Distribution, Abundance, and Impacts of the Chinese mitten crab, Eriocheir
sinensis. California Department of Water Resources, Environmental
Services Office, Sacramento, CA.