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. 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).
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.
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
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).
Region 1, USDA Forest Service
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)
Gregory, J. S., and B. L. Gamett. 2009.
Cattle trampling of simulated bull trout redds. North American Journal of
Fisheries Management 29:361–366.
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 12:450–459.
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