New Zealand Mud Snails

By Dr. John Nickum

Question:  Why is there so much concern about the New Zealand mud snail?  Can a little snail that is only about 1⁄4 inch long really cause much damage to trout fishing?  If so, what can anglers do to help?

The “Old Professor” replies:  You have presented several interesting questions, and thanks for asking about what you can do to help.

There are several reasons for all the concern about New Zealand mud snails, but four factors stand out above the rest.  1)  They are a non-native species.  2)  They form unusually large and extremely dense populations.  3)  They feed on encrusted algae and detritus, consuming up to 75 per cent of the photosynthetic, primary production available on the bottom of the water bodies where they are found.  4)  We really cannot predict accurately what the long term effects of the New Zealand mud snails will be.

Much of the present day “conventional wisdom” dictates that all non-native species are potential threats to natural ecosystems.  This belief may or may not be scientifically valid in specific situations; however, when the non-native species adapts to its new environment and colonizes rapidly in large numbers, it becomes highly publishable news and the media responds.  Everyone is inundated with alarm messages.  Also, we have been conditioned to believe that natural ecosystems are perfectly balanced, so much so that any change must be considered a potential catastrophe. 

Given these basic realities and the uncertainty of not knowing what the long term effects of New Zealand mud snails will be, it is little wonder that there is widespread concern. At this time we just cannot predict what their long term effects may be on aquatic ecosystems and, especially, on populations of our preferred fish.  It appears to be human nature to be uncomfortable with unpredictable situations.  We do not like uncertainty.  However, in biology and especially ecology, uncertainty is quite normal.  Scientists in these fields learn to live with it and to “hedge our bets” by referring to the estimated probability that a particular result will occur.  In opposition to this scientific reality, many will use worst case scenarios to “predict” the future and to attract attention to situations that may cause serious, undesirable effects.  These combinations of actions and publicity campaigns by supposedly knowledgeable individuals and groups are bound to raise concern and confusion in the average citizen.

What do we really know about the New Zealand mud snail and do these facts lead us to predict serious damage to Colorado’s trout fisheries?  As I stated above, we know that these tiny, conical-shaped snails are not native, that they form very dense populations, that they reproduce asexually (parthenogenesis), and that they can consume most of the basic food base in the waters they inhabit.  We know that just one female snail can establish an enormous colony of snails.  (One female can have a million descendants in just one year.) 

We do not know how they got to the United States, or how they got to Colorado.  We do not know if they will persist in our waters, or undergo a population crash to much lower numbers.  This is quite common with colonizing species.  We do not know if they will totally disrupt the food supply for our favorite fish, or simply force the system into a detritus system fueled by the decomposing flesh of the snails.  Some very productive fisheries are supported by detritus based food webs.  The “bottom line” of all this is that we have a potential problem that may be quite serious, but we cannot predict the end result with certainty.

In such circumstances, it seems prudent to try to limit the spread of the New Zealand mud snails to additional waters and to search for practical means for controlling the numbers of the snails.  New Zealand mud snails have limited powers of movement.  They “speed” along at about 3 feet per hour and they must have moist environments.  They cannot survive desiccation, especially at high temperatures (> 105F), nor can they survive freezing.  This means that they have to depend on other animals to get from one body of water to another.  Water birds, mink, raccoons, and anglers provide “rides” to new waters.  Anglers appear to be the main means of long distance movement.  It seems probable that anglers brought the snails to the middle Snake River, where they were first found in the United States, more than 20 years ago.  We do not know whether or not they came from New Zealand, or from Europe where they became established more than 100 years ago. 

Anglers must remember that these little “hitchhikers” may be attached to things other than muddy wading boots.  They have been found clinging to boats, motors, paddles, float tubes, fly rods, lures of all types, and even lines.  Check everything that has been in or near the water before you leave for home, or your next fishing destination.  Drying clothing and all equipment at temperatures over 100 F, or subjecting them to freezing temperatures should be adequate to kill any snails that are “hitching a ride” on your gear and equipment.  If we can slow the spread of the snails, we can improve the probabilities that methods for controlling them can be developed.  Even though the snail population in the middle Snake River has not crashed in 20 years, the possibilities for such crashes increase if we can keep them contained in a relatively small area.  In New Zealand and Europe, parasites that destroy the reproductive organs (parasitic castration) seem to be the main limiting force on the New Zealand mud snails.  Several predators also serve to keep populations in check.  We do not have many snail-eating predators in North America, especially in trout streams.

For Colorado and the West, in general, the primary strategy for now is containment.  Anglers should do what they can to avoid spreading the snail, but don’t panic.  Not every little conical snail is a New Zealand mud snail.  There are several species of tiny native snails.  Snail identification is a job that needs to be left to the experts; however, if you see very small conical snails, get in touch with the local fishery biologist.  She, or he, will take it from there.  In the short term we have a problem, not a crisis.  We simply have to find ways to manage around these tiny pests.    

About The Author

Send your questions for the Old Professor to “Trout Talk” c/o Colorado Trout Unlimited, 1320 Pearl Street, Suite 320, Boulder, CO 80302.  If your question is used for a future column, we will send you a CTU long-billed cap.