New Zealand Mudsnail

History

Although the precise origin of New Zealand mudsnails in the U.S. still debated (some suggest it was via aquaculture facilities in Idaho as well as perhaps ship ballast water in the Great Lakes), the substantial risk they pose to trout waters is not. The organism, Potamopyrgus antipodarum, was first discovered in the Snake River, Idaho in the 1980s, and has since spread throughout many western states.  They have an extremely high reproductive potential (populations consists of parthenogenic clones, genetically identical egg-laden females) and can survive across a wide variety of temperature regimes, forming large colonies within the substrate, and displacing established communities of native aquatic organisms. 

Threat
In some of the most highly impacted waterways, the snails can reach densities of several hundred thousand per square meter of substrate, and may comprise as much as 95% of the biomass within a river or stream, substantially altering the primary productivity within entire systems.  When the reductions in the diversity and/or abundance of other insect species become great enough, the resident fish community — which depends on this food base — are themselves put at risk.  While trout can and do consume New Zealand mudsnails, they have proven to be a poor substitute for the traditional food base, yielding as little as 2% of their nutritional value.

Management Implications
As with many introduces aquatic species, there is no known way to entirely remove the organism once it has become established in a river or stream.  New Zealand mudsnails have no known predators or parasites within the U.S., no species-specific chemical treatments currently exist, and the actual physical removal of colonies posses a very real risk on making this situation worse by increasing the potential for large number of eggs to be released.  As such, management efforts are focused on limiting the spread of the infestation to other aquatic systems.

Accomplishing even this, however, is proving to be a challenge.  Given their high densities and exceptionally small size (approx. 5 mm across), the organism can easily be transported via stream sediment, something that is a particular concern in system that see many anglers or other recreational users who directly enter the water.

Closing severely infected waters to recreation, promoting conscientious cleaning of boating and wading equipment, and increasing public awareness of the potential threats posed by aquatic exotics generally may be the best approach for containing the mudsnail.

Sources:

Montana State University

National Park Service

U.S. Geological Survey

National Park Service - Yellowstone National Park