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Montana Ecological Services Field Office

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Fisheries Applied Research and Technical Assistance

 

Helena Field Office

 

Modeling the potential effects of redd trampling by cattle on cutthroat trout

 High estimated rates of cattle trampling on artificial redds (clay targets, e.g., Gregory and Gamett 2008) within federal grazing allotments in southwestern Montana has raised concern that direct mortality from trampling may contribute to imperilment of native westslope cutthroat trout (Oncorhynchus clarkii lewisi). Our goal was to estimate and model the effects of trampling by cattle on egg-to-fry mortality for stream-resident cutthroat trout and to explore the demographic implications of that mortality. We used results of a study of angler trampling by Roberts and White (1992) to estimate the mortality caused by cattle trampling, and incorporated these estimates into a temperature-driven model of egg-to-fry mortality representative of the developmental stages during which resident cutthroat trout populations in central and southwestern Montana would be vulnerable to the effects of trampling.   The egg-to-fry model was used to characterize the effects of trampling by cattle in streams under two thermal regimes and across a range of empirically-estimated trampling rates from typical cattle grazing scenarios. We linked the egg-to-fry model to a matrix population model to evaluate how trampling affects population growth rates, then considered how trampling may influence persistence in demographically isolated populations.

Conceptual depiction of egg-to-fry mortality and matrix population model for stream-resident cutthroat trout

Figure 1.  Conceptual depiction of egg-to-fry mortality and matrix population model for stream-resident cutthroat trout.  A stream temperature threshold initiates spawning and developing embryos pass through five discrete developmental stages which cumulatively last 500 Celsius temperature units.  Embryos are subject to natural mortality and competing trampling mortality (if it occurs) on a daily basis during each stage.  A weighted average estimate of overall egg-to-fry mortality is calculated across 30 cohorts, which becomes one of the vital rate inputs into the matrix population model (model 2).  Egg-to-fry mortality affects population growth rate through its influence to total life time mortality and adult number.  Recruitment depends on the number of maturing adults at each stage (dashed arrows that point to age-0). (figure by Doug Peterson/USFWS).

 Modeling results indicated the effect on mortality not as dramatic as observed trampling rates might suggest, but these trampling rates were most likely to contribute to declines where the population is marginally stable without the additional impact of trampling.  Additional details can be viewed in Peterson et al. 2010. 

Peterson, D.P., B.E. Rieman, M.K. Young, and J. Brammer.  2010.  Modeling predicts that redd trampling by cattle may contribute to population declines of native trout.  Ecological Applications 20(4):954-966. (pdf)

The models and results are intended to serve as guidance for federal biologists evaluating the potential environmental impacts of grazing (e.g., under the National Environmental Policy Act, or NEPA).

Funding:  Region 1, USDA Forest Service

Project investigators:  Dr. Doug Peterson (Principal Investigator - US Fish and Wildlife Service), Dr. Bruce Rieman (US Forest Service, Rocky Mountain Research Station - Retired); Mike Young (US Forest Service, Rocky Mountain Research Station); and James Brammer (US Forest Service, Beaverhead-Deerlodge National Forest)

Related references:

Gregory, J. S., and B. L. Gamett. 2009. Cattle trampling of simulated bull trout redds. North American Journal of Fisheries Management 29:361366.

Roberts, B. C. and R. G. White. 1992. Effects of angler wading on survival of trout eggs and pre-emergent fry. North American Journal of Fisheries Management 12:450459.

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