State Water Resources Research Institute Program

Project ID: 2007MT155B
Title: Student Fellowship: The influence of beaver on brook trout invasion and subsequent native westslope cutthroat trout displacement in southwestern Montana
Project Type: Research
Start Date: 3/01/2007
End Date: 2/28/2008
Congressional District: At large
Focus Categories: Conservation, Invasive Species, Ecology
Keywords: cutthroat trout, beaver, brook trout
Principal Investigator: McCaffery, Magnus
Federal Funds: $ 2,400
Non-Federal Matching Funds: $ 0
Abstract: Beaver (Castor canadensis) play a keystone role on the landscape. Their impoundments create lentic habitat in otherwise lotic systems, leading to fundamental changes in channel geomorphology, hydrology and nutrient cycling. Consequently, beaver promote changes in succession dynamics, increase biotic productivity, and enhance diversity of floral and faunal assemblages (Collen, P. and Gibson, R.J., 2001). Increases in water storage capacity through impoundments improve riparian habitat, and potentially augment water supply and late season flows (Fouty, S.C. 2003). These aspects of beaver engineering are of increasing interest to landowners and managers, especially as Montanans continue to deal with drought conditions. In the Pacific Northwest, beaver are actively transplanted into degraded wetlands for habitat restoration (Pollock, M.M. et al., 1995). In Montana, beaver are occasionally being introduced to mitigate water quality (D. Sasse, USFS), increase water storage and riparian habitat quality (C. Riley, USFS) and are naturally moving into restored areas, such as Belmont Creek (J. Christensen, BLM). The utility of beaver as possible agents for watershed restoration is exemplified by current proposals to remedy water shortage problems in the upper Big Hole River watershed in western Montana. Transplantation of beaver into tributary streams of the Big Hole River is one option being considered by the Big Hole Watershed Committee as an alternative to increasing landscape water storage through human dam construction.

Promoting beaver on the landscape, either through natural population expansion or active transplantation is a controversial strategy. Aside from human-beaver conflicts such as timber damage, flooding of agricultural and developed lands, and damming of culverts and irrigation systems1, there is the possibility of negative effects on native fish species, such as barrier creation and warming of coldwater streams (McRae, G. and Edwards, C.J., 1994). Relatively little is known about the effects of beaver impoundments on stream fish assemblages, and the patterns and mechanisms behind how beaver may influence fish community structure, abundance and distribution is a contested issue in the western U.S. and in Montana in particular. The formation of pool habitat may increase water temperatures, prey availability to fish, and juvenile rearing habitat for species such brook trout (Salvelinus fontinalis)(Scruton, D.A., Anderson, T.C. and King, L.W., 1998), as well as providing important winter habitat for many stream fishes including cutthroat trout (Oncorhynchus clarki) and bull trout (Salvelinus confluentus) (Jakober, M.J., McMahon, T.E. and Thurow, R.F., 2000).

In Rocky Mountain streams, brook trout are an exotic species, and their invasion often results in displacement of native cutthroat trout through age-specific biotic interactions that reduce juvenile cutthroat trout survival (Peterson, D.P., Fausch, K.D. and White, G.C., 2004). Thus, understanding both what limits the spread of the distribution of brook trout within a system and what factors influence the outcome of westslope cutthroat and brook trout species interactions is critical for the conservation and management of westslope cutthroat trout in western Montana. Gradual upstream declines in growth rates associated with declining water temperatures may explain the upstream limit for brook trout in some Montanan stream systems (Adams, S.B., 1999). Any factors that result in demographic consequences such as growth rates, age-0 recruitment, and dispersal can influence the spread of an exotic species (Byers, J.E. and Goldwasser, L., 2001). Furthermore, it has been posited that brook trout, which are more pool adapted and temperature tolerant, may have an advantage in beaver ponds, and can use these habitats as "source" populations, enabling them to colonize colder "sink" sections of the stream, thus sustaining invasions across a larger range (Peterson, D.P., Fausch, K.D. and White, G.C., 2004). In addition, beaver ponds may alter the outcome of species interactions between westslope cutthroat and brook trout. If beaver ponds provide habitat that preferentially increases abundances of brook trout in a stream, then their impact on westslope cutthroat may be larger. Also, temperature has been implicated in enhancing the ability of brook trout to outcompete westslope cutthroat trout (Novinger, D.C., 2000). Therefore if beaver ponds increase stream temperature, this may give brook trout a greater competitive advantage.

Actions to improve water retention must work in synchrony with efforts to curtail brook trout spread and maintain native cutthroat, and based on a scientific understanding of ecological mechanisms operating in the system. This work directly complements ongoing in the Rocky Mountain region, including work by B. Shepard (MFWP, MSU) who is examining correlates of patterns of brook trout invasions the landscape. This project will enhance our knowledge as to how beaver activity influences processes related to species invasion in western Montana.

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