Assignment #1

Chinese mitten crabs (Eriocheir sinensis) - a threat to Washington State's waters?

The Management Model

  • Introduction

  • The Management Problem
  • Management Model
  • Summary
  • References

  • Introduction

    The topic of this paper is in general terms the introduction of aquatic nuisance species (ANS) into domestic waters and more specifically the threat of the Chinese mitten crab (Eriocheir sinensis [pic], H. Milne-Edwards 1854) for Washington's waters. It is the first of three parts of a term project with the respective objectives of

    The Management Problem

    ANS have their natural habitat with naturally limiting factors like food source and predators in other parts of the world. One danger of introduction of ANS is that native species are removed or outcompeted and ANS become predominant. The goal of managing ANS is therefore to prevent their introduction in the first place, and where already established to limit or reduce their actual numbers.

    The Chinese mitten crab is a catadromous, burrowing [pic ] species native to China’s and Korea’s coasts in the Yellow Sea (Cohen/Carlton 1995). The crab was first collected outside its natural habitat in Germany in 1912, supposedly introduced via ballast water. In the 30s the crab became excessively abundant with devastating effects on dams, levees and river banks (Slack 1996) [pic] fishers claimed that it also damaged their catch and gear (Panning, 1938). Subsequently it spread over the northern parts of Europe, but population explosions like in the 30s have not occurred again.

    In the 80s Chinese mitten crabs were available as a delicacy in seafood markets in Los Angeles and San Francisco (Cohen/Carlton 1997). The crab was subsequently banned in California (Section 671(h)(2) Title 14 CCR) for concerns about its possible physical impacts on dams, as well as its potential threat to human health (Horwath 1989)[details] (note: the crab can be secondary host for the Oriental lung fluke, Paragonimus westermanii [details], which can generate - sometimes fatal - tuberculosis like symptoms). By 1989 US regulation (50 CFR 16.13) listed the crab as an injurious species rendering its importation and possession a crime. However, in 1992 a live Chinese mitten crab was described in the San Francisco Bay area; by 1994 it was well established there (Cohen/Carlton, 1997; Halat/Resh, 1996); and in 1997 it was found as far north as the Columbia river (Seattle Times, A 8, 28 January, 1998). With an established and growing population in the San Francisco Bay (Veldhuizen/Hieb, 1998) (in addition to the possible introduction from Asia or Europe) an invasion of Washington State's waters becomes more and more likely. Therefor, the crab is listed in the draft of the Washington State Aquatic Nuisance Species Management Plan as one of four freshwater species that pose the greatest threat to environment and economy (working draft 1998, unpubl.).

    Management Model

    The management model [ figure 1] describes the potential vectors for the introduction and dispersion of the Chinese mitten crab in US waters to the point where it actually can cause harm and therefore is considered to be a nuisance species. Once the crab has reached the shores, it can invade freshwater systems and eventually establish a population. The adverse impacts [table 1a and description] of the crab on humans and the ecosystem, like dam and levee destruction, fish and gear damage, and the assumed disturbance of the trophic cascade, can occur at any stage of its dispersion in fresh and brackish waters (for impacts in accordance with different lifestages see table 2a).


    Management actions relate to the vectors of the introduction. ANS get into US waters over various avenues, which can be roughly divided into intentional and unintentional introductions. Unintentional introduction occurs when ballast water from vessels is dumped in order to load cargo; some when ANS dwelling on the hull of a vessel are scraped off or when they spawn in US waters; ANS may also come as part of a shipment either directly or in package material for live seafood or other "target species" (pets), which is then disposed in the local waters. Intentional introduction includes scientific research and testing, biological control, i.e. the attempt to reduce another ANS by introduction of a natural predator, species for aquaculture production, and pets. As third category one might consider natural invasion, i.e. the migration of animals (or plants) in different waters where there was previously no possibility, for example through a newly built canal.

    Various theories exist for the introduction of the Chinese mitten crab into US waters [table 1b], but just two are considered to be the likely transport mechanism: ballast water and "intentional private-party inoculation to establish a food resource" (Cohen/Carlton, 1997)(note: the crab was sold in California as a delicacy; mitten crabs were intercepted at airports in Los Angeles, San Francisco and Seattle and petitions for mitten crab aquaculture have been submitted to the legislature, Cohen and Carlton 1997). (Legal) introduction for research and testing - the crab is scientifically interesting because of its salinity tolerance - is limited through the permit system established by federal laws (see below). Natural invasion of other US waterbodies from, for example, San Francisco Bay might happen through the crabs' migration.


    When transported to domestic waters ANS can cause various consequences, which are basis for environmental concerns and (subsequently) management actions. The Chinese mitten crab is a nuisance for humans in the following ways: (Veldhuizen/Hieb, 1998; Cohen/Carlton 1997; Halat/Resh 1996).
    see table 1a and also table 2a

    Its impacts on the ecosystem are not fully understood but it can be reasonably assumed that it affects the food chain in similar ways other ANS do, i.e. imbalancing the trophic cascade as a food competitor and/or a predator without predators.

    Management Actions

    Possible management actions encompass regulation and education with the aim of controlling and eradicating the vectors of introduction. Both intentional and unintentional introduction can be restricted by some legislative act if the necessary enforcement is in place. Since authoritative regulation and enforcement is never a 100% measure and compliance will always be lacking, education is an important additional factor. Education is most desirable on the assumption that individuals are not only willing to comply if they are sufficiently aware of the problem but also contribute to prevention and abatement through voluntary action.

    The introduction of the Chinese mitten crab is generally regulated by the legislation on non-indigenous species (50 CFR 16.13, "Importation of live or dead fish, mollusks, and crustaceans, or their eggs") by which the "importation, transportation, or acquisition of ... live mitten crabs ... is prohibited" except in cases of permissions for scientific, medical, educational, or zoological purposes (50 CFR 16.22). Enforcement of these regulations is delegated to different US authorities [table 1b], thus a permit for an import of Chinese mitten crab for scientific purposes has to be obtained from the Fish and Wildlife Service.

    Washington has legislation in place that covers nuisance species in general, and the Puget Sound Exotic Species Task Force works on a policy proposal that includes measures relating to the Chinese mitten crab more specifically (workdraft 1998, unpubl.).
    In addition to the existing injurious species US legislation mentioned above, there is proposed regulations for ballast water exchange on a voluntary basis and for mandatory reporting and sampling procedures, (Department of Transportation, Coast Guard, 33 CFR Part 151) to comply with the National Invasive Species Act of 1996 (Federal Register, vol. 63 (69), 17782).
    On an international level IMO has adopted measures that propose the exchange of ballast water at sea. These, however, are not legally binding and in the IMO guidelines it is observed that the exchange may be a threat to safety when it has to be done in adverse weather conditions; this added to the assumed unwillingness of ship operators to undertake more than is absolutely necessary, the exchange of ballast water on a voluntary basis is unlikely to be of great effect.

    Publications like Gordy Slack’s article, "Chinese Crabs Discover San Francisco Bay" in the summer issue of Pacific Discovery in 1996, are part of an unstructured educational effort not focused on a particular state. Through articles like this the public awareness can be influenced and perhaps a change in behavior initiated.


    The desired outcome of management action is to reduce or limit the threat which the Chinese mitten crab poses to human health, to fisheries, to riverbanks and dams by closing the introduction vectors. Regulation is the only reasonable measure for ballast water exchange since education alone cannot reach addressees from abroad satisfactorily. Education and regulation may interrupt other vectors, especially where the main actors operate from within the US, i.e. aquaculture and seafood marketing. Regulation works for shipping and mailing standards, as well as for aquaculture. But as soon as the vectors cannot be overseen by official enforcement authorities, education should supplement prevention measures, i.e. once ANS are pass the border controls and in the hands of private individuals, regulations alone are as weak as the addressees and without education (or intense and costly enforcement) they might be less than satisfactory.
    Although legislation renders capture and possession a crime the effect is less than satisfactory: Cohen and Carlton (1997) indicate that intentional private-party introduction happens nevertheless, and a Chinese mitten crab population could establish itself in the San Francisco Bay Area by 1994 despite California's ban from 1987.
    Under the current management model it is assumed that crabs are only introduced from foreign sources (ballast water, shipping/packing), management of established population is not in place and the problem of the Oriental lung fluke is only addressed as part of the mitten crab problem. An overview of the management goals and actions is depicted in figure 2.