Columbia River Fisheries Program Office
Pacific Region

Native Trout Program - Bull Trout Effective Population Size

The Wallowa Valley Improvement Canal (WVIC) is a water diversion in northeastern Oregon that has impacted bull trout and their habitat. The canal was constructed in the 1880s and diverts water from several Imnaha River subbasin streams between Big Sheep Creek and McCully Creek to Prairie Creek in the Wallowa River basin. The diverted water is primarily used for irrigation purposes.

Salt Creek Summit SpillwayBig Sheep Creek is the most upstream tributary that is diverted by the WVIC. Barriers that prevent migration into upper Big Sheep Creek (at the WVIC diversion) and the WVIC (at the Salt Creek summit spillway) have potentially created an isolated population of bull trout in upper Big Sheep Creek. There is a culvert on Little Sheep Creek approximately 200 m above where the WVIC diverts the stream that could impact upstream migration of bull trout and isolate a population above the culvert. The WVIC does not divert McCully Creek. Instead, the WVIC is carried over McCully Creek and some water from the canal is diverted into the creek. It is not likely that much, if any, immigration into McCully Creek is occurring through this diversion given the physical structure being used. In addition, McCully Creek no longer drains into the Imnaha subbasin. The stream bed was shifted in the past so that the creek now drains directly into the Wallowa Valley and provides another water source for irrigation. Therefore, the only potential source of bull trout immigration into McCully Creek would be from the Grande Ronde River subbasin, through a series of irrigation canals that most likely act as temperature barriers for bull trout. Thus, it is reasonable to speculate that the bull trout population in McCully Creek is isolated. Historically, these populations could have been connected by migratory individuals. However, the construction of the WVIC has potentially prevented gene flow or allowed only unidirectional movement for over a century. Despite the existence of these potential isolating mechanisms, small bull trout populations persist in all of these streams above the WVIC.

Big Sheep CreekEffective population size (Ne) is simply the number of breeding individuals in a population given the assumptions of a randomly mating population. Effective population size is typically smaller than the absolute population size. Population genetic theory indicates that Ne = 50 is necessary to prevent inbreeding depression, and an Ne = 500 is necessary for sustainability over ecological time (Franklin 1980, Soulé 1980, Allendorf and Ryman 2002). It is likely that this theory holds true for bull trout, although exceptions do exist (Rieman et al. 1997, Whitesel et al. 2004). Departures from the 50/500 concept associated with Ne should be supported by empirical data that is robust and well described (Whitesel et al. 2004). Potential information that relates effective population size theory to absolute abundance and population genetic variability would provide information toward defining minimum viable population objectives.

The goal of this work is to provide empirical data toward defining minimum viable population objectives that will be used for restoration and recovery of bull trout across the range. The objectives toward this end are to: 1) Determine juvenile and adult bull trout connectivity (movement) between populations; 2) Determine abundance of isolated bull trout populations above WVIC; 3) Determine within and among population genetic variability for the four local populations of the Imnaha Core Area; and 4) Determine effective population size for isolated populations above WVIC.


Cook J.R. and J.M. Hudson. 2008. Effective Population Size and Connectivity of Bull Trout in the Imnaha River Subbasin. 2006 Annual Report.

Cook, J.R., J.M. Hudson, and T.A. Whitesel. 2010. Effective Population Size and Connectivity of Bull Trout in the Imnaha River Subbasin. 2007 Annual Report. (pdf 1.33mb)


Allendorf, F.W., and N. Ryman. 2002. The role of genetics in population viability analysis. Pages 50-85 in Population Viability Analysis, S.R. Beissinger and D.R. McCullough, editors. The University of Chicago Press, Chicago, IL

Franklin, I.R. 1980. Evolutionary change in small populations. Pages 135-149 in Conservation Biology: An Evolutionary-Ecological Perspective, M.E. Soulé and B.A. Wilcox, eds. Sunderland, Mass.: Sinauer Associates.

Rieman, B.E., D.C. Lee and R.F. Thurow. 1997. Distribution, status, and likely future trends of bull trout within the Columbia River and Klamath basins. North American Journal of Fisheries Management 17: 1111-1125.

Soulé, M.E. 1980. Thresholds for survival: Maintaining fitness and evolutionary potential. Pages 153-169 in Conservation Biology: An Evolutionary-Ecological Perspective, M.E. Soulé and B.A. Wilcox, eds. Sunderland, Mass.: Sinauer Associates.

Whitesel, T.A., and 7 coathors. 2004. Bull Trout Recovery Planning: A review of the sciences associated with population structure and size. Science Team Report #2001-01, US Fish and Wildlife Service, Regional Office,, Portland, Oregon.

Last updated: December 7, 2012
Columbia River Fisheries Program Office
Pacific Region Fisheries Resources Home
Pacific Region Home

U.S. Fish and Wildlife Service Home Page | Department of the Interior  |  | About the U.S. Fish and Wildlife Service  | Accessibility  | Privacy  | Notices  | Disclaimer  | FOIA