Fish Management in Wilderness Lakes and its Consequences for Conservation of Threatened Native Fishes in the Northern Rocky Mountains

Christopher A. Frissell and Susan B. Adams University of Montana Flathead Lake Biological Station, Polson, MT

Several formerly widespread native fishes in the northern Rocky Mountains are now threatened by reduced abundance and fragmented distribution and habitat, including the bull trout (Salvelinus confluentus), westslope cutthroat trout (Oncorhynchus clarki lewisi), and Yellowstone cutthroat trout (O. clarki bouveri). These species are threatened by habitat deterioration, coupled with displacement by and hybridization with introduced congeneric species or mongrel conspecific stocks within their historic ranges. Little is known about the specific mechanisms of displacement (or replacement?), but they may include interference competition, restructuring of food webs, predation, behavioral or genetic disruption, and new species or strains of disease carried by stocked fishes. In a handful of cases, introduction of native fishes into historically fishless lakes has established artificial "refuges" from which the native species has been further propagated. However, this function pales in significance compared to the extent of adverse impact across the landscape. Extensive introduction of nonnative fishes into lowland lakes and rivers has contributed to the retreat of native salmonids into isolated enclaves in headwater drainages, often associated with wilderness or roadless areas. Introduction of nonnative fishes into headwater lakes thus can hasten the further demise of remaining natives within such enclaves through primary or secondary invasion. Introduced fishes have directly displaced native species from most of the few headwater lakes that historically supported native fish populations. A much larger proportion of the landscape, however, is affected by scondary invasion of associated stream habitats by emigrants originating from introduced populations in headwater lakes. Headwater lake introductions facilitate secondary invasion of stream segments where native fishes are insulated by physical barriers from invasion from downstream sources. In streams vulnerable to invasion by both downstream or upstream sources, falls typically prevent lake-origin immigrants from returning to their natal habitat, forcing them to pioneer new reproductive habitat within the range of native fishes. Continual or frequent immigration by displaced lake-origin fish may increase the likelihood of successful establishment of the introduced species in the stream. From the standpoint of biotic integrity, most presumably "secure" population complexes of native fishes in the region (e.g., the South Fork Flathead in the Bob Marshall Wilderness) are peppered with headwater lakes hosting reproducing populations of nonnative trout, which serve as insidious, latent or slow-acting threats. Many of the more truly secure habitats for native trout occur in roadless areas outside of designated wilderness and parks, which commonly lack popular recreational lakes and thus have not been subject to headwater stocking and secondary invasion.

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