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Northwest Sportsman Magazine

Unwelcome Mud Snails Found In Lower Chehalis

New Zealand mud snails have turned up in another Evergreen State waterway: the lower Chehalis River.

Local radio reports say the unwanted invasive species was discovered just upstream of Cosmopolis, at a DNR site in tidally affected Blue Slough.

However, none were found at a nearby WDFW boat ramp, KBKW reports today.

It’s been roughly two years since word of the last infestation got out, the discovery of the molluscs in lower Thornton Creek, on Lake Washington north of the UW and Sandpoint.

They’ve also been found in Capitol Lake in Olympia, the Lower Columbia, Kalama and canals and whatnot on the Long Beach Peninsula.

WDFW describes the problems with New Zealand mud snails thusly:

New Zealand mudsnails are small (about 1/8 – 1/4 of an inch long when full grown) that have brown or blackish cone-shaped shells with five whorls. They tolerate a broad range of temperature, salinity, and water quality, and have no natural parasites or predators in the United States. They are able to close their shells, and live out of the water for quite some time. Unfortunately, it only takes one to begin a new population. Females “clone” themselves, producing approximately 230 new female snails each year. Based on a single snail and each offspring in turn reproduces itself, by year two there may be over 52,000  snails, by year three over 12 million snails, and by year four over 2.7 billion snails carpeting the bottom of a river or lake. Preliminary studies indicate that in areas where they have become densely populated they are becoming the dominant invertebrate via displacement and competitive interactions. Large populations may consume up to half of the available food in a stream, starving out insects essential to trout and salmon. A recent study by the U.S. Bureau of Land Management and the Utah State University (Vinson and Baker, 2008) linked these invasive snails to poor condition of trout due to lack of a regular food source and that the snails did not provide sufficient nutritional value when consumed. The snails are very small – between an eighth and a quarter inch in size – and may not be noticed attached to tackle, waders, boats, etc.

 

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