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

The Ecological Analysis


Robyn Draheim, Jamie Goen, Florian Wegelein

Assignment 2C
May 20, 1998
SMA 510: Ecological Concepts for Decision Makers




Ecological Analysis
Overview of Nonindigenous Species
Mitten Crab Life History
Chinese Mitten Crab Ecological Model

Scientific Uncertainty

Management Model Evaluation






The Chinese mitten crab (Eriocheir sinensis H. Milne-Edwards, 1854) [pic], a catadromous, burrowing crab native to Korea and China (Cohen and Carlton 1995), is an aquatic nuisance species that may pose a threat to Washington State waters (Puget Sound Exotic Species Task Force unpub.). Now established in San Francisco Bay as well as Europe, Great Britain and Asia, the mitten crab may well be poised for an invasion of the estuarine and fresh waters of Washington State. In the summer of 1997, an adult mitten crab was caught in the Columbia River (Seattle Times 1998). This is the northernmost appearance of the crab to date and evidence of the growing threat. Current management is primarily aimed at reducing the undesirable outcomes of a mitten crab invasion by limiting their introduction into US waters. The Chinese mitten crab Management Model developed in assignment 1 of this project details the structure of these regulations. However, the management model assumes a simplified ecological system, and does not adequately address secondary invasions by, or the impacts of, established mitten crab populations.

The task of assignment 2 is to develop an Ecosystem Model outlining the ecology of the crabs in terms of their potential impacts and mechanisms favoring invasions of other watersheds. This knowledge is then evaluated at each stage in the life cycle with its correlated habitat location. There are uncertainties inherent both in this model and in the understanding of the mitten crab life history that must be acknowledged.

In addition, areas of scientific uncertainty will be addressed in assignment 2. These findings will be compared to the previous Management Model developed in assignment 1 to assess the validity of the ecological assumptions underlying the Management Model and to evaluate the existence of management. Finally, assignment 3 will take both the Management and the Ecosystem Model into account while addressing recommendations for further scientific research, regulation revisions, future courses of action, and management options.



Ecological Analysis

Successful management of the Chinese mitten crab requires an understanding of its life history as well as its impacts. In order to address the appropriate issues, the manager should have a basic understanding of nonindigenous species (NIS), and the ecological principles that drive their introductions and establishment.


Overview of Nonindigenous Species

In general, we cannot accurately predict the success of introduced species (Williamson and Fitter 1996), but there are life history traits that enhance the probability that a non-native species will establish itself if introduced into a new location. Often what makes for a good invader are traits that allow the species to be highly opportunistic. Invasive species are often very physiologically tolerant, capable of mass spawning as well as rapid range expansions (either through population growth or emigration (Groscholtz 1996)), and utilize food sources that are rarely limited (Moyle and Light 1996). Aquatic NIS possessing morphologically different life stages predisposed to transport through a wider variety of vectors.

Being in the right place at the right time can also be advantageous for invasive species. Cohen and Carlton (1998) investigated two factors theorized to make regions more susceptible to invasion: exposure to transportation vectors and disturbance of native assemblages.

The Chinese mitten crab is one such opportunistic species. Assuming that transportation is one of the only limiting factors for the crab, it may prove to be a hardy invader if introduced into Washington State waters. It can withstand salinity gradients from 0 ppt to upwards of 25 ppt, as well as moderate temperature variations (these parameters are not extensively tested, but rather inferred from the variety of global invasion locations and the habitats in which mitten crabs have been found). It need not remain submerged in the water, but can exist on land for up to 38 days in moist areas (Kaestner 1970 as cited in Nepszy and Leach 1973, Peters and Panning 1933). It is an omnivore, known to scavenge for whatever plant and animal materials are available. In addition, mitten crabs utilize a semelparous breeding strategy, mating only once in their life but releasing up to a million larvae each.


Mitten Crab Life History

As a catadromous species, the Chinese mitten crab has a unique life history. The mitten crab is primarily a freshwater species as it spends most of its life in rivers and streams, migrating into the brackish waters of estuaries to reproduce and die. In the spring, mitten crab larvae escape from their eggs and disperse throughout the water column. The larvae (or zoeae) slowly develop through five planktonicstages. During this development the animals move gradually from the salt to the freshwater near the river mouths. The crabs are too small at first to be able to make their way upstream against the strong currents.

During the fall and winter months from August to November, maturing adults migrate downstream to reproduce (Panning 1938). Return migration occurs after a several year period (2-3 years in San Francisco, 3-5 years in Germany) spent upstream (Hieb 1997, Nickles 1997, Panning 1938) A

In the following summer after mating, males and females set out for the river banks in the estuaries where they gradually perish. The single breeding period (semelparity) is compensated by an enormous egg production (250,000 to one million eggs per female (Cohen and Carlton 1997)),a characteristic which renders the Chinese mitten crab a potentially successful invasive species.

[more detailed description of life history]


Chinese Mitten Crab Ecological Model

The Chinese Mitten Crab Ecological Model (figure 2a) is a visual depiction of the crab's life cycle with concurrent secondary invasions and impacts relevant to each general life stage (i.e. zoeae, juveniles, and adults). In addition, the model notes where in the habitat these life stages occur by demarking adults as occupying estuarine habitat with a decreasing salinity gradient as the zoeae and juveniles migrate upstream to freshwater in rivers and streams. By evaluating each separate life stage, a more direct analysis of the secondary invasions and impacts specific to each cycle can conducted.

During its different life stages the crabs may enter and, in turn, impact the environment through secondary invasions. Due to the high market value of the crabs, illegal harvest and transport of adults may provide a potential secondary invasion vector despite the prohibition of such activity under 50 CFR 16. Secondary invasions by juveniles include both transmission associated with recreational boating activities and natural dispersal via overland migrations. However, this natural form of range expansion is limited to a watershed by watershed migration and may not pose a much of a threat to interstate dispersal. Even though it falls under the Nonindigenous Species Act of 1996 (NISA), recreational boating on the other hand should still be considered an open vector for secondary invasion. Citing the zebra mussel as an example, unintentional transport from watershed to watershed is possible through the holding tanks and bait buckets of recreational boaters.

The likelihood of successful transportation is enhanced by the ability of the crab to survive for up to 38 days without constant submersal. In this case, it may even be possible for the crab to survive transportation in a boat being trailered from location to location. As a result of this tolerance for desiccation, natural secondary invasions can result from terrestrial migrations from watershed to watershed. Conventional, manmade obstacles to migration such as dams may increase the risk of such range expansions by forcing the crabs to migrate overland and around the impediment. Lastly, zoeae may become established through a secondary invasion via inter- and intra-state ballast water transport, recreational boating, or shipping and packaging material vectors. Zoeae can travel via recreational boating in a similar manner as juveniles as long as they remain submersed and within their temperature range. Larvae may also travel in the waters carried in shipping and packaging materials for live seafood. A critical part for all of these secondary invasion vectors is that they rely on being introduced into a salinity range and environment that corresponds to their life stage and in numbers sufficient for reproduction in order to survive and establish themselves.

Table 2a. details the life history stages of the mitten crab and the impacts which can occur at each stage.
[a more detailed description of these impacts can be found here]

During their long stay in freshwater it is presumed that the mitten crabs have an ecological effect. Although studies are underway in San Francisco Bay (Halat and Resh 1996), no ecological impacts have been reported. There are undoubtedly many ecological interactions that are not yet understood about the mitten crab, including how they might compete within or shift the trophic systems in different habitats. Vitousek (1990) suggests that invasive species can change their new ecosystems if they 1) differ substantially from indigenous species in resource acquisition or utilization; 2) alter the trophic structure of the invaded area; or 3) alter the disturbance frequency and/or intensity of disturbance.

Using San Francisco Bay as an example we have attempted to outline potential trophic level impacts [Figure 2c] [Figure 2d] of mitten crabs as zoeae and juveniles. However, it is important to note that inference of ecological impacts in San Francisco Bay may be misleading, and impacts may be difficult to discern due to the fact that it is such a highly invaded system. Juvenile striped bass, although itself an introduced species, is considered to be an indicator species for San Francisco Bay, and its populations are greatly in decline. Striped bass depend upon mysids as their primary food source and the mysid population has been depressed due to the immense water filtering capacity of a more recently introduced Asian clam Potamocorbula amurensis which has depleted the standing stock of phytoplankton (Orsi and Mecum 1996, Nichols 1988). The introduction of large numbers of mitten crab zoeae may act as a food source for planktivorous juvenile striped bass, but they too would be reduced in numbers by the diminished phytoplankton stocks as hypothesized in Figure 2c. Thus any ecological impacts of mitten crab zoeae are masked by the total ecosystem change caused by the Asian clam.

Figure 2d details the hypothesized trophic impacts of juvenile mitten crabs in freshwater systems. As omnivores, juvenile mitten crabs consume both terrestrial and submerged aquatic vegetation, detritivores (worms), insect larvae, carrion and other small crustaceans (Panning 1938). The freshwater system that feeds into San Francisco Bay supports a large population of crayfish (Hieb 1997) whose feeding habits are virtually identical to those of the mitten crab (Huxley 1880). It can thus be surmised that the mitten crabs may be engaged in competitive interactions with crayfish.

Unlike true freshwater crabs and crayfish, mitten crabs have a planktonic larval stage which hatches in vast numbers and this recruitment strategy may facilitate their dominance and spread throughout the may small river systems that feed into the Bay. It is unknown whether or not mitten crabs and crayfish prey on one another but Huxley (1880) noted that invasions of freshwater crabs into Russian streams was leading to a reduction in crayfish populations and surmised that predation might be the cause of this decline. In addition it is possible that mitten crabs population explosions may reduce food availability and force younger crabs to migrate further upstream in search of food (and possibly space) (Panning 1938). This contrasts with Moyle and Light's (1996) statement that the food resources of omnivores and detritivores are usually underutilized in freshwater systems.

Predation on mitten crabs must play a role in the ecological impacts of this invasive species however none of the preliminary investigations of mitten crabs in San Francisco Bay name potential or known predators. Panning (1938) mentions that birds and large fish prey on mitten crabs but notes that "this means of exterminating them is altogether ineffective..."

None of the studies surveyed for this assignment mentioned the trophic role of adult mitten crabs. Though they most certainly are a food item for predatory fish in the estuary, the lack of information available seems to suggest that after mating the crabs overwinter and die of starvation in the late spring early summer but this merely reflects an assumption.



Scientific Uncertainty

Both this model and the management model reflect assumptions and scientific uncertainty (figure 2b). Many of the undesirable impacts are clouded by uncertainty as to the extent of the impact. Generally, experiences in other geographic settings have to be considered with some caution as one characteristic of successful invasive species is their adaptability. Thus the observations from Germany about the life cycle seem to be not directly transferable as crabs do not wander as far, return to the brackish waters earlier, and start their temperature dependent behavior earlier in the year (compare Panning, 1938 and Hieb, 1997). The intensity of the effects is most likely density dependent as is the extent and duration of migrations but the controls over population increases are not understood. Hieb (1997) does predict that the San Francisco population will see a rapid increase in population in the next few years. Monitoring and assessing population sizes with visual surveys , traps and trawls while the population is at low densities is difficult and not aided by the fact that the crabs are primarily nocturnal, may dig extensive burrows, and overwinter deep in the channels (Hieb 1997).

The ecological impacts of the mitten crabs in San Francisco Bay and tributaries are unknown, but currently the subject of studies being run by the California Department of Fish and Game and the University of California at Berkeley (Veldhuizen and Hieb 1998, Halat and Resh 1996). It is possible that the mitten crab may compete with crayfish (the only other large freshwater crustacean) for food resources (Hieb 1997). There is a good deal of uncertainty in the prediction of the impact of mitten crabs on fisheries resources. Other than direct competition with crayfish the impacts on commercial and recreational fisheries is not known. Crabs caught as bycatch or preying upon caught fishes may be the most direct impact on fisheries, although Panning (1938) noted that the sheer numbers alone of crabs swarming in the river channels had a significant impact on German fisheries.

There is also a good deal of uncertainty regarding the establishment of a parasite population and the transfer of the parasite to human hosts. The primary host of the lung fluke is a freshwater snail, although this particular species has not yet established itself in the Bay Area, there exist many suitable hosts were the parasite to arrive, accompanying either a snail or a crab. This parasite, the Oriental lung fluke (Paragonimus westermanii) is not very host specific and has been found on over 35 species of Japanese crustaceans, including crayfish (Gyoten 1994).

Most dispersal vectors are well understood. A secondary invasion of the mitten crab will most likely be through illegal introductions (intentional or not) or via ballast water (Cohen and Carlton 1997). But the role of natural dispersal whether by water or over land, however unlikely, should not be taken too lightly.

The impacts on agriculture are uncertain as to their probability and extent. In places with rice agriculture, the mitten crab is known to cause damage by eating the rice shoots (N. Vogel 1994; Hieb 1997) the consequences of an invasion by this crab on Washington State's agriculture are not predictable, though levees are certainly at risk as much as they are in other regions.

The largest uncertainty is the one about the ecological interactions: little is known about the actual population sizes and the behavior of the crabs, assessing and monitoring is complicated by the fact that the crabs migrate in deep water and tend to be nocturnal. The impact on the food web is poorly described. As an opportunistic, omnivorous animal the mitten crab has the potential of disturbing the food web on more than one level. In Washington State waters it may compete with crayfish, it may compete with other predators, it may be an additional predator on native species. Much more scientific data is necessary on the crabs' distribution, burrow densities, size frequency, food habits, reproduction, and behavior. In addition, there is not much known about the migrational distances which the crabs can cover and what cues control the start of their migrations.



Management Model Evaluation

The Management Model does not account for the gaps in the knowledge of the life history and potential impacts of the crab. Regulations in place address some of the vectors for NIS in general but do not close them. Thus federal regulations prohibit the illegal harvest and the transport of NIS but they are silent on the transfer of parasites, directly or indirectly. The policies that do apply, NISA and CFR-16, do so in a roundabout way to slow the transport of mitten crabs and prevent the infestation by the crabs. One can see that the "injurious species" listing under CFR-16 has not prevented the introduction of the crab into US waters. Equally, the IMO Resolution and the Coast Guard regulations based on it (or rather the federal Act implementing the IMO resolution) address the vector of ballast water but they do not close it since ballast water exchange is only voluntary.

Currently no measures are being taken to prevent the spread of the crab internally, even though it is a vector that might be the most likely way of invasion into the waters of Washington State. The voluntary ballast water exchange refers only to international vessel traffic, intercoastal traffic from California to Washington is not covered (regulations in place refer only to the Great Lakes). Neither the natural nor the human induced dispersal over land is addressed. No measures are taken to limit the spread within the freshwater system, such as traps at dams. The Management Model does not provide for mitigating or abatement actions where the damage is occurring for example eroding banks and levees. It also does not emphasize the significance of an intact habitat for the natural defense of non-indigenous species. No plan exists to maintain the natural "immune system" of the habitat. Panning suggests as a consequence of river degradation that "many native animals [are deprived of their natural habitat], as for instance ... the predatory fishes which would have been of the greatest importance in fighting and checking these mitten crabs" (Panning 1938, see also Moyle and Light 1996). In short there is a blind spot in Management Model when it comes to "internal" introductions and impacts.

Finally, there are no monitoring or evaluation measures in place to find out about the actual population size, its behavior and development. No educational efforts have been taken to mobilize public support at large which would be important to utilize a "voluntary potential" for the reduction of vectors.

Keeping Hieb's (1997) prediction in mind, that we will see a population explosion within the next three years, one notices that there is no legislation and no state projects in existence to slow or prevent the damage that a thriving crab population might do. With potentially only a few years to go before the population might explode and the threat becomes stronger still, there is little time left to address the issues raised by the model. This is not a grievous situation, for although the mitten crab is a very unique organism and well adapted to invasion, it may be possible for the mitten crab situation to benefit from the new regulations, assessment and management techniques that are underway for controlling the spread of green crabs and zebra mussel. Further discussion of this potential will be discussed in assignment 3.




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