Fisheries Applied Research and Technical Assistance

 

Helena Field Office

 

Demographic and genetic response of cutthroat trout to barrier removal

Habitat fragmentation has led to isolation of populations of native fishes like westslope cutthroat trout (Oncorhynchus clarkii lewisi) and bull trout (Salvelinus fontinalis).  Migration barriers may disrupt population structure at larger spatial scales or even restrict movement among complementary habitats within a patch, both of which may be important to buffer effects of disturbance.   Based on this, a major focus of native trout conservation has been to re-establish historical connectivity among habitat patches within stream networks by removing human-made migration barriers.  A significant challenge in some cases is to balance the competing threats of isolation versus invasion by nonnative trout.  Nonetheless, migration barriers on many small streams are being removed throughout the western US, sometimes at great expense, and systematic monitoring of the response of fish populations to these actions may not be occurring.

Perched stream culvert at road crossing in the Clearwater National Forest, Idaho

 

Removal of migration barriers is expected to benefit native trout by restoring or increasing genetic diversity, resilience to disturbance, and resistance to ecological and genetic effects of nonnative trout invasions   Evaluation of fish community response and demographic and genetic benefits to key species that result from barrier removal projects is needed to guide future efforts.  The objective of this project is to critically evaluate whether, and how quickly, the expected demographic and genetic benefits of connectivity accrue in formerly isolated westslope cutthroat trout populations after removal of human-made migration barriers.

Westslope cutthroat trout (~130 mm TL) from unnamed tributary to East Fork Lolo Creek, Lolo National Forest.

 

Migratory westslope cutthroat trout (~350 mm TL) from unnamed tributary to East Fork Lolo Creek, Lolo National Forest.

The initial geographic focus is in the upper Lolo Creek watershed in western Montana where the US Forest Service is removing or replacing a number of fish passage barriers (culverts) on small tributary streams occupied by westslope cutthroat trout.  The study design is a before-after-control-intervention (BACI), where the individual, population, and community responses to reconnection will be measured through time (before and after restoration of fish passage.

	Undersized culvert removed from tributary to East Fork Lolo Creek, Lolo National Forest during summer 2008.
	Contractor replacing undersized culvert in tributary to East Fork Lolo Creek, Lolo National Forest during summer 2008.
	Replacement culvert being installed in tributary to East Fork Lolo Creek, Lolo National Forest during summer 2008.
	Post-installation photograph of replacement culvert that provides passage for aquatic organisms. Photo taken fall 2008.

 The project is expected to provide a targeted evaluation of whether barrier removal promotes the demographic and genetic diversity expected to facilitate persistence of native inland fishes in a variable landscape.  More generally, the results might help biologists prioritize future reconnection projects by understanding where such actions may be most successful.

Funding:  USFWS National Fish Passage Program

Project investigators:  Dr. Doug Peterson (PI-US Fish and Wildlife Service), Dr. Helen Neville (Trout Unlimited); Shane Hendrickson (US Forest Service, Lolo National Forest)

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Updated 9/8/09