Wildlife Survey & Photo Service


Sept. 2013





Putah Study


New Zealand Mudsnails are increasing in certain areas of Putah Creek. Surveys show that NZMS are 83% of the invertebrate population at study sites. Survey 9/18/13.

New Zealand Mudsnail (NZMS) attached to the underside of the water column, Note the young NZMS on her shell.

New Zealand Mudsnails are a tiny (2 -5 mm long) invasive snails that move to new waterways within the boots of anglers and biologists. A study we conducted in Putah Creek in 2005 revealed that every boot used in the 80 angler survey carried at least one New Zealand Mudsnail in the boot. Some boots carried more than 300 mudsnails inside and within the lace folds.

Mature New Zealand mudsnails are about 1/4 inch (5 mm) long. Image show various sizes compared to a dime.

Common Snails in Putah South Canal

New Zealand Mudsnails

NZMS are from 0.5-5mm in length. They are live-bearing. To our current knowledge, there are no male NZMS in the U.S.  They should be identified by a qualified taxonomist as there are native species that resemble mudsnails.

New Zealand Mudsnails on a survey brick give a good idea of the relative density.

New Zealand Mudsnail Surveys & Monitoring

Surveys for mudsnails are conducted similar to benthic macroinvertebrate in-stream surveys using a EPA-approved 500 micron net.


Bricks and other materials are used to monitor NZMS because mudsnails prefer fresh periphyton (algae) as a food source. Anything placed in a NZMS infested waterways will probably attract mudsnails when algae begins to grow on the item be it a CD, beer bottle or survey tile.

New Zealand Mudsnails for some unknown reason prefer the bottom side of rocks durinf certain times of the year

Certain times of the year, mudsnails prefer to congregate under rocks. This can confuse anglers who call me and ask where the mudsnails went. Under the rocks! Unfortunately it can also lead to a lack of concern and the need to ALWAYS clean and dry your boots before entering another waterway.


Mudsnails were discovered in Putah Creek in 2003. Within a couple years, the caddisfly Hydropsyche virtually disappeared from the creek. We do not know why this occurred. The caddisfly has recovered in some areas of Putah Creek. Stream management has also helped alleviate the loss of the caddisfly.

Wading boots primarily used by fly fisherts and biologists carry mudsnails inside the boot and within the folds around the boot tongue.

Putah Creek Boot Study

In November 2005 we conducted a treatment study for New Zealand Mudsnail infested boots. We had 71 volunteer fly fishers wade through a mudsnail infested section of Putah Creek. They walked and waded a prescribed course to guarantee a 100% chance of getting NZMS in their boots. Boots were collected and treated with three different chemical regimes. The study was never published because the statisticians I hired (2) disagreed on the results. What I do know is that all treatment failed, 73% of the boots carried mudsnails, some boots carried more than 65 NZMS, and many of the snails were still alive after 96 hours.

New Zealand mudsnails - for unknown reasons - congregate in massive numbers. At this site the snail density was over 500,000 per square meter. 

Preventing the Introduction of Mudsnails

These tiny snails are frequently moved within the wading boots of anglers and biologists. Prevention requires the decontamination of boots and other materials before entering other waterways. Recommendations  have included harsh chemicals, freezing, and total drying. Mudsnails can survive in damp boots for weeks.


Best Decontamination: Inspect, clean and DRY. Best to have a couple pairs of boots to allow complete drying - inside and out.


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