The Chinese Mitten Crab can have a variety of effects where it occurs, and its effects differ among locations and time periods. This crab is reported to undergo large fluctuations in abundance, exhibiting occasional outbreaks of high abundance that have been documented in both the introduced and native range. It is likely that the magnitude of effects also increase during population outbreaks.
For example, San Francisco Bay and its tributaries experienced high abundances of mitten crabs in the mid- 1990’s, affecting fish passage and salvage operations. Specifically, the mass of migrating mitten crabs impaired operation of facilities designed to assist fish migration, resulting in the death of thousands of threadfin shad and other species.
Chinese Mitten Crabs can greatly increase bioerosion in some areas, because they burrow into banks and levees along estuaries reducing the structural integrity. This has been reported in various global regions, including California.
In Asia where they are native, the mitten crab is host to the Oriental lung fluke, which, if eaten, can cause tuberculosis-like and influenza-like symptoms in humans. This parasite hasn’t been reported in San Francisco Bay or other introduced areas; moreover, the parasite poses no apparent risk to humans if crabs are either not eaten or fully cooked.
While mitten crabs are known to feed on a wide range of organisms, from aquatic plants to invertebrates and fish, the effects of the crab on local prey populations is not well understood.
The effects of Chinese Mitten Crabs on the US Atlantic coast are currently unknown. The crabs have only recently been discovered, they do not appear to be abundant at the present time, and there have yet to be any detailed ecological studies of this crab in the region.