Geography of Invasion in Mountain Streams: Consequences of Headwater Lake Fish Introductions |
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Authors: | Susan B Adams Christopher A Frissell Bruce E Rieman |
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Institution: | (1) Flathead Lake Biological Station, University of Montana, 311 Bio Station Lane, Polson, Montana, 59860, USA; and, US;(2) USDA Forest Service, Rocky Mountain Research Station, 316 East Myrtle Street, Boise, Idaho 83702, USA, US |
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Abstract: | The introduction of fish into high-elevation lakes can provide a geographic and demographic boost to their invasion of stream
networks, thereby further endangering the native stream fauna. Increasingly, remaining populations of native salmonids are
concentrated in fragmented headwater refugia that are protected by physical or biological barriers from introduced fishes
that originate in the pervasive source populations established at lower elevations. Although fish introduced near mainstem
rivers frequently encounter obstacles to upstream dispersal, such as steep slopes or falls, we found that brook trout (Salvelinus fontinalis) dispersed downstream through channel slopes of 80% and 18-m-high falls. Thus, headwater lake stocking provides source populations
that may be capable of invading most downstream habitats, including headwater refugia of native fishes. The extent of additional
area invasible from lakes, beyond that invasible from downstream, depends on the geography of the stream network, particularly
the density and distribution of headwater lakes and their location relative to barriers inhibiting upstream dispersal. In
the thermal and trophic environments downstream of lakes, fish commonly grow faster and thus mature earlier and have higher
fecundity-at-age than their counterparts in other high-elevation streams. The resulting higher rates of population growth
facilitate invasion. Larger body sizes also potentially aid the fish in overcoming barriers to invasion. Trout introductions
to high-elevation headwater lakes thus pose disproportionately large risks to native fishes—even when the place of introduction
may appear to be spatially dissociated from populations of the native species. Mapping the potential invasible area can help
to establish priorities in stocking and eradication efforts.
Received 28 March 2000; accepted 9 February 2001. |
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Keywords: | : invasion dispersal landscape demography conservation nonnative fish salmonids lake stream |
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