Isolation of headwater streams and removal of non-native trout does not increase the abundance of the threatened cutthroat trout Oncorhynchus clarki pleuriticus, Green River watershed, Bridger National Forest, Wyoming, USA

  • Published source details Novinger D.C. & Rahel F.J. (2003) Isolation management with artificial barriers as a conservation strategy for cutthroat trout in headwater streams. Conservation Biology, 17, 772-781


Isolation of aquatic systems is an extreme conservation technique that has allowed persistence of some native species threatened by introduced non-natives. While there are obvious short-term benefits of minimising threats from non-natives by isolation, the long-term success of isolation depends upon maintaining sufficient ecological and genetic resources to sustain or enhance the isolated population. If sufficient resources are not contained or provided within an isolated population, increased intraspecific competition and high levels of inbreeding could lead to population bottleneck or local extinction. In the USA, the cutthroat trout Oncorhynchus clarki pleuriticus is threatened by non-native rainbow Oncorhynchus mykiss and brook Salvelinus fontinalis trout, which hybridise with, compete with, and possibly predate cutthroat trout. In this study, the effects of isolating headwater streams and of removing non-native fishes from the isolated zone on cutthroat trout is investigated.

Stream isolation, electrofishing & stocking: To eliminate the threats posed by non-native trout to cutthroat trout Oncorhynchus clarki pleuriticus, the headwaters of several first-order streams (1-3 m wetted widths) in the Rocky Mountains of Wyoming, were isolated using migration barriers. These isolated headwaters were then intensively electrofished to remove non-native fishes, and stocked on three occasions with cutthroat trout of hatchery origin to increase population sizes (stocking densities of 18 to 138 fish/100 m of water course).

Monitoring: On four streams (Clear, Irene, Nameless and Nylander Creeks), population monitoring of cutthroat trout was conducted within the isolated area of the streams and in the unprotected area below the barrier as a control. Also, as an additional control, three sites (Spring [2] and Trail [1] Creek) were located on streams without barriers. On each stream, two to four sites within the isolated area and one site immediately below the barrier were selected. Monitoring was conducted in August for 4-7 years between 1992 and 1999. The first year of monitoring was conducted prior to removal of non-native trout.

Trout censuses: Each site was enclosed with block nets (1 cm mesh diameter) positioned 100 m up and down stream. A single backpack unit was used to conduct three-pass, depletion-removal electrofishing. Following each pass, all trout captured were counted, measured (total length to nearest mm), weighed (to nearest gram) and placed in a holding cage outside of the site. This sampling method censused one-year and older trout, with age 0 trout hatching just after the census. Any brook trout Salvelinus fontinalis captured were removed.

Abundances of brook trout: The abundance of brook trout at age >1 year declined by 75-96% following the first year of removal, and they remained at very low population levels over the course of the study in isolated steams. By comparison, brook trout populations in sites below the barrier decreased on Clear Creek (but were still around 15 times more abundant than the isolated stream), remained at similar levels on Irene and Nameless Creeks, and increased on Nylander Creek (becoming around fifty times more abundant than the isolated stream). Furthermore, age 0 brook trout were not observed in Clear and Nameless Creeks after 1995.

Abundances of cutthroat trout: The decline or extirpation of brook trout within isolated streams did not correspond with an increase in the population of cutthroat trout compared to the population below the barrier. Indeed, in Nylander and Irene Creeks, the cutthroat trout population in the isolated stream was about half the population below the barrier.

Condition of cutthroat trout: The was no evidence that cutthroat trout upstream of barriers were larger or heavier than below the barrier.

Stocking of cutthroat trout: Stocking failed to enhance the populations for more than 1-2 years after release. In the year following release, stocked trout decreased by 87 to 100%, with <1% of stocked trout remaining after 3 years. However, stocked trout were found below the barrier at a density of 17 fish/100 m, suggesting that there was downstream movement.

Conclusions: In this experiment, there was no evidence that stream isolation and non-native trout removal increased the abundance of cutthroat trout in headwater streams. However, one benefit of this management was the removal of rainbow trout which can inter-breed with cutthroats. The authors suggest that serious consideration should be given before attempting similar management to determine whether the possible benefits will out-weigh the risks of isolating already threatened populations.

Note: If using or referring to this published study please read and quote the original paper. This is available from Please do not quote as a case as this is for previously unpublished work only.

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