Are We Loving Our Blue Ribbon Fisheries To Death?

Updated: Jun 7

We're all aware of whirling disease and its negative effects on trout. But, are you familiar with other aquatic invasive species affecting our rivers? One of these invasive species, New Zealand mudsnails (NZMS), has rapidly expanded its range across the United States, and scientists have long suspected that anglers play a key role in its growing prevalence. The embedded video (above) shows the geographic expansion of NZMS between 1987 and 2019. But are anglers actually to blame?

First, why would we even worry?

New Zealand mudsnails rapidly outproduce native river mollusks and negatively impact mayflies, stoneflies, and caddisflies some of the most important food sources for trout. Unfortunately, NZMS bind to waders, shoes, and even dogs, and can then be transported across vast distances to new areas via assisted migration. With the ability to self-replicate (asexual reproduction), it takes only one snail to establish a colony in a new waterway. Additionally, if NZMS are the main or only source of food in a river system, trout can starve since they provide very little nutritional value to fish. In fact, mudsnails have been found to survive the digestive process.

Are anglers really responsible for their spread?

When we were outlining this article, we wanted to find a way to test if anglers could be contributing to the spread, at least in the Rocky Mountain states (ID, MT, UT, CO, NM, WY). Long fishing trips where people bounce between popular stretches of rivers could easily explain the spread if an angler's gear isn't appropriately cleaned and dried. We hypothesized that blue-ribbon or gold medal waters (recommended waters) were more likely fished than the other rivers and streams. For our hypothesis to be true, these recommended waters would show a higher prevalence of NZMS than rivers without blue-ribbon (BR) or gold medal (GM) designations. One way to test this notion is to randomly select a stream, determine if it's recommended and if the river tested positive for NZMS. If we replicate that process many times with different streams, we can identify the proportion of streams with mudsnails that are not recommended, and compare that statistic to the proportion of streams with mudsnails that are recommended. If you're interested, this method is called bootstrapping.

We found that recommended waters had a higher probability of NZMS occurrence than waters not recommended.

Differences in New Zealand mudsnail occurrence between recommended fishing waters (bar on left) and other waters (bar on right). The dashed red line indicates baseline proportions, random sampling regardless of fishing recommendation.

Following these methods, outlined by Epperly & Witt, 2018, we now have statistical support for our hypothesis that angler presence is correlated with New Zealand Mudsnail occurrence in the Rocky Mountain states.

Therefore, it is likely that anglers play a key role in NZMS dispersal and propagation. As a result, mudsnails can be transmitted over great distances via waders, boots, and other wet angling gear. However, the best we can show is a correlation, not causation. Causation suggests one event is the direct result of another, while correlation shows that two events are related to each other. However, correlation does not explain that one event is the result of the other. So, we are confident that recommended fishing waters (blue-ribbon and gold medal designated waters) will continue to face the threat of New Zealand mudsnails, though we have yet to pinpoint the exact mechanism of transference (causation). Nonetheless, we are faced with the responsibility to maintain and protect our freshwater fish and their habitats.

How to prevent the spread:

If you can’t clean and drain your gear fully, rinse it under hot water (120°F) for 5 minutes to kill any unwanted hitchhikers. Freezing your waders has also been shown to work. So take the extra time both the fish and your fellow anglers will thank you in the long run.


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  1. National Management and Control Plan for the New Zealand Mudsnail. 2007.

  2. New Zealand Mudsnail occurrence data. USGS. 2019.

  3. Epperly J*, Witt A*, Haight J, Washko S, Atwood TB, Brahney J, et al. (2018) Relationships between borders, management agencies, and the likelihood of watershed impairment. PLoS ONE 13(9): e0204149.

  4. Colorado Parks & Wildlife. Top Invasive Concerns: The New Zealand Mudsnails.