The Devastating New Zealand Mudsnail: Is the Smith River Next?

Editor’s note: One of Siskiyou Land Conservancy’s ongoing projects is to research and document the “State of the Smith River.” The Smith River remains the healthiest major river in California. As such, it provides pristine habitat for rare and endangered species throughout much of the watershed.The most significant exception is found at the Smith River estuary, which has been besieged by loss of flood plain and off-channel habitat, simplification due to levees and reduction of riparian cover, and the highest concentrations in California of certain highly toxic pesticides.

Now a new threat has emerged at the Smith River estuary: New Zealand mudsnails. This pernicious exotic species has spread rapidly throughout the American west, devastating the food chain of several streams. Last year Wendell Wood, a wildlands interpreter with Oregon Wild, discovered New Zealand mudsnails in the Smith River estuary.The mudsnails were already well established in nearby Lake Earl, but the Smith River estuary had apparently been spared until recently. The California Department of Fish and Game has yet to survey the Smith River to determine the extent of New Zealand mudsnail distribution, though DFG may do so this summer.

Following is a report by Doug Simpson, Siskiyou Land Conservancy’s intern for estuary research, on mudsnails in North Coast streams.  Simpson recently graduated from Humboldt State University with a major in Natural Resource Planning, emphasizing marine resources and climate.

Growing Concerns Over Invasive New Zealand Mudsnails in Humboldt and Del Norte County Waters

Siskiyou Land Conservancy research intern Doug Simpson seeks New Zealand mud snails in the Smith River estuary, March 2012. Photo by Greg King.

By Doug Simpson

The North Coast of California is blessed with an abundance of natural and scenic splendors. Among my favorite places in California are the lagoons and estuaries located along our coastline.  These brackish environments provide a range of ecological communities where fresh water meets the sea. They provide wonderful recreational opportunities for anglers and kayakers, and they support several important fisheries.

However, as more and more people use these waters – locals and tourists alike – impacts are increasing on pace. One troubling management issue is how to deal with invasive species, which can be directly correlated with higher use. In waterways, aquatic hitch-hikers are proving to be more than just a nuisance – both globally and locally. Many of us have already heard about the invasive Zebra Mussel, which has devastated the Great Lakes and many other areas in the United States. A comparable, but less known invasive is the New Zealand mudsnail. It has appeared only in the last decade in local lagoons and rivers and poses a real threat to fisheries and aquatic ecosystems.

When it comes to mudsnails, it’s not size that counts: it’s tiny. It ranges in size from less than one millimeter to nearly a centimeter, but virtually all Mudsnails found locally will be 2mm or smaller – that’s about the width of a quarter. What is alarming is their ability to propagate in astoundingly high numbers. According to a USGS fact sheet they’ve been reported in densities higher than 300,000 per square meter. In other words the ground can literally be crawling with mudsnails.

New Zealand mud snails have spread rapidly in North America. They were first discovered in the Snake River, in Idaho, in 1987. For about a decade their numbers remained relatively low, as seen in this graphic that shows mud snail distribution in 1995.

The New Zealand mudsnail is most likely to travel across watersheds by hitch-hiking on fishing and boating equipment. It can also travel across and within a watershed via the guts of fishes and birds. Fish and waterfowl will feed on benthic invertebrates such as the New Zealand mudsnail. However, many species can’t digest the mudsnails. Instead they travel through the digestive tract intact and are passed in the feces, still alive, sometimes several miles away from where they were originally consumed.

The mudsnail’s indigestibility poses significant problems. One is that they travel much faster and farther than they would otherwise be capable of. The other is that the trout, salmon, duck, or other species that eat the mudsnails often glean very little nutritional value from

them. If mudsnails begin to overwhelm and push out other native macro-benthic invertebrates, as they have been recorded to do, this could pose a serious threat for the many species of waterfowl and fish that feed on small bottom dwelling snails.

New Zealand mud snail distribution, 2001.

The chemical and physical conditions that Northern California has to offer seem to be ideal for the mudsnails. They originated from New Zealand, whose climate is similar to our own. Keith Bensen, resource manager for Redwood National Park, is seriously alarmed at the presence of the mudsnail. Currently, Bensen is

New Zealand mud snail distribution, 2009.

monitoring the speed and densities of the mudsnail’s spread in known present locations, as well as areas that they expect the mudsnail may spread to. Big Lagoon and Stone Lagoon may harbor the largest population of mudsnails in our area. Evidence of their hitch-hiking style can be seen in their proximity to boat ramps and popular fishing areas. I recently circumnavigated Stone Lagoon in a kayak, and I found the mudsnails everywhere I looked. They are also in Redwood Creek, Lake Earl, the Smith River and undoubtedly waiting to be discovered in many other local waterways.

Studies by Dybdahl and Kane in 2005 have shown that mudsnails can tolerate a wide range of conditions. They thrive in disturbed watersheds with high nutrient flows. They can tolerate high water levels by finding shelter under rocks and in eddies. They can grow and reproduce in salinities as high as 15 parts per thousand, but can tolerate salinity as high as 30 parts per thousand for short periods. (The ocean is 33 parts per thousand.) That means they can live in the bilges of ocean-going boats long enough to get dumped into a storm drain and potentially invade a whole new watershed. They also reproduce sexually and asexually, so all you need is one to soon have an entire population.

The U.S. west coast population of New Zealand mudsnails seems to have originated from Idaho – presumably from an angler having traveled from New Zealand, or perhaps from the dumped remains of someone’s aquarium. The entire North American population of mudsnails is clonal, consisting of genetically identical females. One female snail can produce 230 young per year, and it reaches sexual maturity at around 3.5mm, which is usually about 8 months of age. If one hitch-hiking mudsnail lands in a new spot conducive to growth and has 230 young, and each of those young reproduce, within one year that same spot could have a population of more than 50,000.

New Zealand mudsnails reproduce to such high densities that they push out other macro-invertebrates and begin to dominate the food chain level called “secondary production.” A study led by Robert Hall junior found that mudsnails have the highest secondary production rate of all benthic invertebrates. In this way they threaten to become a dominant part of fish and bird diets, which, combined with accompanying low nutrient values, could devastate native populations and completely alter the processes of an ecosystem. Trout have been known to starve on a mudsnail diet.

Preventing the spread of New Zealand mudsnails is essential. This is done primarily through protocols established for use of waterways. After use in the water, all fishing and sporting equipment should be decontaminated. If possible, the same set of gear should be dedicated to a single location. After use, gear should be cleaned with a scrub brush and water, preferably high-pressure. Inspect gear – it should be free of any visible traces of sand, mud, gravel, or plant fragments. In addition to rinsing and scrubbing, there are two follow-up treatments to decontaminate gear. One involves a chemical treatment of what is known as quaternary ammonium compounds. Formula 409® cleaner degreaser disinfectant in a 50% dilution has been proven to kill Mudsnails. Gear should be soaked in the solution for 5 minutes and then rinsed thoroughly with tap water. (This method should only be used where the chemical solution will not run-off into a water body). The second treatment is to expose mudsnails to intolerant physical conditions, including:

1)    Freezing gear for a minimum of 4 hours.

2)    Soaking gear in hot bath water (minimum 120°F) for 10 minutes.

3)    Drying gear before reusing. Gear must be dry for at least 48 hours with low humidity. Places like pockets and boots stay damp longer and may need longer to dry.

The New Zealand mudsnail is an incredibly tolerant and proliferative invasive species. It hasn’t been seen on the North Coast for long, and solutions to its spread, and safe means of eradication will not be easy to develop. But our fragile and often pristine North Coast ecosystems demand that we take up the challenge.

2 Responses

  1. Wow! This seems like a problem of huge proportions. Aside from the suggestions above, what can the average person do?

  2. Nice article, Doug! Did you make the maps? If not, I was wondering if you could send me the reference for where you found them. Please email me ASAP (workin’ with a deadline, here). Thank you!

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