- Prepared for the Aquatic Nuisance Species Project, Pacific States Marine Fisheries Commission.
- Once a non-native species arrives and survives in an area, its long-term persistence depends on its recruitment success. If conditions are not favorable for recruitment it will ultimately disappear. The European green crab (Carcinus maenas) has a six-year life span and has persisted at low densities in Oregon and Washington coastal estuaries for the past 13 years. After the arrival of the strong founding year class of 1998, significant self-recruitment to the Oregon and Washington populations occurred only in 2003, 2005 and 2006. Warm winter water temperatures, high Pacific Decadal Oscillation and Multivariate ENSO (El Niño Southern Oscillation) Indices for March, late spring transitions and weak southward shelf currents in March and April are all correlated with the these strong year classes (Behrens Yamada and Kosro 2010). Cold winter water temperatures, low Pacific Decadal Oscillation Indices, early spring transitions and strong southward (and offshore) currents in March and April are linked to year class failure. Right now, green crabs are still too rare to exert a measurable effect on the native benthic community and on shellfish culture in Oregon and Washington. However, this could change if ocean conditions were to switch to a high PDO and strong El Niño patterns. For example, green crabs were first documented in New England in 1817, but it took over 100 years for their numbers to build up to a level to decimate the soft-shelled clam industry during the 1950’s when warm ocean conditions prevailed.
Extensive surveys by Fisheries and Oceans Canada found green crabs in all the major inlets on the west coast of Vancouver Island, but so far none have been discovered in the inland sea between Vancouver Island and the mainland nor north of Vancouver Island. Therefore, outreach efforts should continue to help prevent the establishment of this invader in the inland waters via ballast water, shellfish transport or other human-mediated vectors.
Even though green crab abundance in the Pacific Northwest is still low when compared to Europe, eastern North America, Tasmania and California, it is imperative to continue monitoring efforts for two reasons:
1) to elucidate the process of range expansion and population persistence of this model non-indigenous marine species with planktonic larvae and 2) to predict the arrival of strong year classes from ocean conditions and alert managers and shellfish growers of possible increases in predation pressure from this invader.
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