New Zealand Mud Snail, Baseline Distribution and Monitoring Study

Two mud snails are dwarfed by a U.S. Dime

New Zealand Mud Snails Resting on a Dime for Scale


Significance of the Resource or Issue to the park - Reduction and containment of exotic species in nationally-important natural areas is a high priority goal of the National Park Service (NPS Strategic Plan, 1997). Based on the overall conservation mandate specified in the legislation creating Yellowstone National Park, preservation of native aquatic communities has been an often reiterated management objective in the park (Statement for Management, 1991), e.g., "a principal objective is the preservation of native fish populations and the associated aquatic life in a natural environment .... management efforts in the future must encourage the maintenance of natural, environmentally regulated ecosystems" (Yellowstone National Park Master Plan, 1974).

In 1994, New Zealand mud snails Potamopyrgus antipodarum were discovered in the Madison River near the park boundary. Subsequent investigations by independent researchers have documented a rapid spread of this exotic species to the Firehole and lower Gibbon rivers. Similar to other invasions of aquatic nuisance species, long-term effects of this exotic species on the indigenous invertebrate fauna are unknown; however, studies conducted on the middle Snake river in central Idaho suggest that native molluscs may be reduced in abundance or eliminated entirely. Although a long-term monitoring study may evolve from information collected in the phase, our current proposal is restricted to development of appropriate sampling techniques, and support of research needed to most efficiently assess watershed-level impacts of this prolific invertebrate on other aquatic species.

Red dots indicate locations where mud snails have been found.

Severity of Resource Threat, Problem, or Needs - Despite their tiny size (1/4"), New Zealand mud snails are capable of rapid population growth. Localized infestations in the Madison River in 1997 have approached a density of 28,000 individuals /sq. ft. Thus, these small molluscs have the potential to "cover the stream bottom," similar to impacts observed with the Zebra Mussel in the midwestern U.S. Preliminary baseline surveys indicate that the mudsnails may be impacting the invertebrate community in the Madison, Firehole, and Gibbon rivers not only through physical displacement or crowding, but also through competitive interactions such as food availability. These streams not only support world famous recreational fisheries, but they also historically contained an abundance of native aquatic insects that form an important part of the aquatic food chain. Reductions in the insect species diversity or abundance could diminish the availability of this critical food resource to fish. Mud snails are a poor substitute for the traditional food base, yielding as little as 2% of their nutritional value when eaten by trout. The rapid spread of the mud snail throughout the Madison River watershed may have been assisted by human transport. Mud snails are able to withstand dessication, a variety of temperature regimes, and are small enough that many types of water users (anglers, swimmers, picnicers, pets) could inadvertently be the mechanism for interbasin transfer of this nuisance species.

Problem definition and Information base - The overall goal of park managers is to prevent spread of the New Zealand mud snail to other Yellowstone Park waters. It is unknown how the mudsnails were introduced into park streams, but human transport is strongly suspected. One of the park's primary tools available for reducing additional expansion of mud snails is an extensive public information campaign detailing the potential problems associated with this exotic invader. Preparation of brochures and interpretive signs in areas of high visitor use is a necessary first step in educating the public. The lack of extensive historical information about the aquatic invertebrate communities in the affected streams and the uncertainty surrounding potential impacts of the mud snail on those communities is a major concern of park managers. Preliminary results from 1-3 year baseline studies by independent researchers reveal that the mud snails are rapidly increasing in some park waters. Furthermore, certain types of aquatic invertebrates (mayflies, stoneflies, and caddisflies) appear to be declining in abundance at sites where mud snails now account for more than 50% of the relative abundance of aquatic invertebrates. Expanded NPS support of these research-level projects is necessary to ensure that information presented to the public is timely and accurate.

Feasibility - Participants at a recent informational workshop indicated that there are few effective treatments to completely eliminate mud snails. Each mud snail population consists of parthenogenic clones, that is, genetically identical egg-laden females. Consequently, attempts at crushing or physical removal of the snails may only exacerbate the problem by spreading eggs to new sites. Chemical treatment would not necessarily be selective for snails only and could eliminate remnant invertebrate populations. Alternatively, expanding NPS efforts to increase public awareness of potential threats to existing aquatic communities may be one of the best tools for containing the mud snail. As a "sociological treatment" of a pest invasion, this approach supports recommendations of the park’s IPM criteria (YNP Resource Management Plan, 1995). However, success at preventing spread of this invader can only be evaluated with a rigorous monitoring program. Selection of long-term standardized protocols, study areas, and initial sampling can all be accomplished in consultation with expert researchers within one or two years.

Problem Resolution - Much of the current knowledge regarding New Zealand mud snails in the park is derived from baseline studies of independent researchers. The Aquatic Resources staff of the NPS has initiated preliminary planning for monitoring the mud snail distribution during summer of 1998. However, staff expertise is limited with respect to the most appropriate mud snail sampling protocols. Additional inventories and research studies by recognized experts are critical to accurately define the scope of the current snail infestation. Only with a thorough understanding of impacts of mud snails on invertebrate communities in YNP can emerging control solutions be effectively initiated. If the new projects yield new methods of control, long-term monitoring by NPS will be necessary to assess the effectiveness of any control efforts.

Transferability - The recent one-day workshop hosted by NPS included university researchers, consultants, and state and federal biologists. All of the participants indicated that they were very concerned about the potential range expansion of mud snails to aquatic systems outside of Yellowstone. Part of this concern arises from the studies in central Idaho, which suggest that mud snails thrive in degraded habitats. However, the very high mud snail densities observed in non-degraded Yellowstone streams indicate that mud snails may be adaptable to a variety of habitats. Information derived from Yellowstone studies on rate of spread of mud snails, changes in aquatic insect communities, and subsequent effects on recreational fisheries will be useful to managers within the affected and contiguous watersheds.

Cost effectiveness - NPS support thus far is primarily logistical; preliminary sampling and consultation with independent researchers. What is needed is adequate funding of graduate projects, parkwide mollusc survey, development of NPS survey techniques and selection of long-term sites.

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