Invasive Alien Species in Northern Ireland

Eriocheir sinensis, Chinese mitten crab

Eriocheir sinensis Edwards
 
Introduction
This oriental crab has slowly spread through Europe and it is likely to become established in Ireland at some stage following an introduction to a large estuary, most probably by a ship. Once established they may subsequently be found in a variety of freshwater habitats including lakes, streams and canals.
 
Description
This crab may attain over 6cm carapace width. The adults of the species have distinctive brown ‘fur–like’ hairs on their chelae (‘claws’). Males have larger chelae and a more slender abdomen than females. The rounded carapace is sandy to brown in colour.
 
Country of origin
Its native range lies between Vladivostok and southern China occurring over a wide range of climate conditions. It is also found in Taiwan.
 
Current distribution
The crab is established in the rivers draining into San Francisco Bay, California, USA. In Europe it was first reported from a German river in 1912 and since this time the crab has spread to the Elbe, Ems, Weser, Rhine and Vidaa rivers, and has been found as far upstream as Prague. It is also reported from the Gironde, Dordogne, Garonne and the Tagus rivers. There have been many accounts of small numbers being found in the Baltic Sea and elsewhere in Europe and in other world regions where they are not thought to be established. It has recently appeared in the Caspian Sea. In Britain the species is established in the Thames and there are indications that it may be established in the Humber, Ouse and Tyne rivers.
 
Location in Ireland
There is only one record from Ireland, a single specimen captured from the Waterford Estuary in 2006. Small numbers are unlikely to provide sufficient evidence that there is an established population. Should they become established they may spread by upstream migration using canals and gain access to other river catchments.
 
Life cycle
When two or more years old, adult crabs migrate downstream on a journey that can take some months. They arrive in estuaries in the spring or summer to reproduce. The released larvae can only develop in salinities of 12–32psu and preferably at temperatures of 15–18°C and they cease growing at temperatures below 7°C and above 30°C. Once developed to the juvenile stage they migrate upriver and are capable of moving up to c.1500km upstream. On their upstream migration, crabs may leave the water to bypass river barriers. Mitten crabs are omnivorous and feed on plant materials, invertebrates, fishes and detritus and molluscs. They mainly forage during the daytime. In Germany there are some years when crabs become very abundant and it is thought that such events may be cyclic. The species can withstand turbid, polluted and low oxygenated waters.
 
Wildlife and habitat impacts
Crabs are avid predators and their foraging is likely to result in predation of salmonid and other eggs, benthic invertebrates and native and introduced plants. Should they attain high numbers during migration the impact on native biota can be great. They burrow into banks to avoid drying–out. Such burrowing causes bank–slumping and a release of sediments. Banks of headraces and levees can be affected in this way.
 
Human impacts
Crabs feed on trapped and gilled fishes. They can cause high mortalities in fish ponds as well as damage fishing gear. Where crabs are present there is a need to maintain banks, levees and other such defensive structures. They can create burrows of 3cm diameter and c.20cm deep at densities of c.35m-2 with resultant sediment losses of 500 to 5,500cm³ m-2 of bank area. When migrating in great numbers they may block, or otherwise cause problems for conduits and abstraction systems, and may clog power plant intake screens. Mitten crabs may act as a secondary host for the human lung–fluke Paragonimus westermanii within its native range in China but this parasite has not been found in European waters. To exist it needs to use a particular family of snails in its first stage of infection while the secondary stage occurs in crayfishes and crabs. The final stage occurs in mammals and when consumed by humans it is known to cause bronchial and neurological illnesses.
 
Key vectors
The most likely vectors for the transmission of this crab are:
  1. Ships’ ballast water carrying planktonic stages
  2. Ships’ hulls carrying juveniles and/or adults on the hull in spaces such as sea-chests.
  3. Other possible modes of transmission include:
    1. The imports of live specimens for direct human consumption
    2. Importation and release to develop a new food source
    3. Specimens used for aquaria
    4. Movements of dredged materials
    5. Natural dispersal by migration.

    6.  
      What you can do as an individual
      Specimens appear from time to time in different rivers and estuaries, however, their presence may not indicate the species is established. Nevertheless any findings should be reported to the Fisheries Division of the Department of Agriculture and Rural Development, The Natural Heritage Division of the Environment and Heritage Service of Northern Ireland and/ or CEDaR, Botany Department, Sciences Division, National Museums Northern Ireland, 153 Bangor Road, Cultra, Holywood, County Down BT18 0EU. Tel: 028 9039 5256, email cedar.info[at]nmni.com.
       
      Advice to specific groups relevant to this species
      Those involved in fisheries or angling activities are likely to be some of the first to recognize the presence of this species in an area. They are likely to be captured in seine and eel nets and anglers bait–fishing may encounter them. When found, the appropriate State authorities should be contacted. The crabs should be removed and not returned to the water.
       
      Management measures
      The trapping of crabs can reduce population numbers. This is most effective if undertaken during migrations.
       
      Management groups
      Fishery managers should develop monitoring programmes to provide an early warning of the presence of this species because it is likely to transform the ecology of rivers and may have serious impacts on salmonid resources. Scouting the shores of estuaries for moulted carapaces and monitoring the rotary screens of water intake areas of power plants may provide useful information on their relative numbers in an area.
       
      Further information
      Links
      In Asia this species is cultivated in ponds for human consumption. There are more than one species of mitten crab in Asia and these are difficult to distinguish. It is possible that some of these other species may become imported with shipping during the coming decades on account of increased trade with China. Any freshwater crab found in Ireland will have been introduced.
      www.waterresources.ucr.edu/wrc/publications/contribution_206.pdf
      www.issg.org/database/species/ecology.asp?si=38&fr=1&sts=sss
      www.nobanis.org/files/factsheets/Eriocheir_sinensis.pdf
      www.jncc.gov.uk/page-1709
      www.fao.org/figis/servlet/static?dom=culturespecies&xml=Eriocheir_sinensis.xml
       
      Literature
      Clark, P.F., Rainbow, P.S., Robbins, R.S., Smith, B., Yeomans, W.E., Thomas, M. and Dobson, G. (1998). The alien Chinese mitten crab, Eriocheir sinensis (Crustacea: Decapoda: Brachyura), in the Thames catchment. Journal of the marine biological Association of the United Kingdom 78: 1215-1221.
      Gollasch, S. (1999). Eriocheir sinensis (Milne–Edwards, 1854), the Chinese Mitten Crab. In: Gollasch, S., Minchin, D., Rosenthal H. & Voigt, M. (eds.): Exotics Across the Ocean. Case histories on introduced species: their general biology, distribution, range expansion and impact. Logos Verlag, Berlin, 78 pp.
      Minchin, D. (in press). First Irish record of the Chinese–mitten crab Eriocheir sinensis (Milne–Edwards, 1854) (Decapada: Crustacea). Irish Naturalists’ Journal.
      Ojaveer, H., Gollasch, S., Jaanus, A., Kotta, J., Laine, A.O., Minde, A., Normant, M. and Panov, V. (in press) Chinese mitten crab Eriocheir sinensis (H. Milne–Edwards, 1853) (Crustacea, Decapoda, Varunidae) population in the Baltic Sea – a supply–side invader? Biological Invasions.
       
      See the Invasive Species Ireland web site for further information -
      http://www.invasivespeciesireland.com/mostunwanted/
       
      Text written by:
      Dan Minchin, MOI, Ballina, Killaloe, Co Clare