Geography of invasion in mountain streams: Consequences of headwater lake fish introductions

The introduction of fish into high-elevation lakes can provide a geographic and demographic boost to their invasion of stream networks, thereby furthe r endangering the native stream fauna. Increasingly, remaining populations of native salmonids are concentrated in fragmented headwater refugia that a re protected by physical or biological barriers from introduced fishes that originate in the pervasive source populations established at lower elevati ons. Although fish introduced near mainstem rivers frequently encounter obs tacles to upstream dispersal, such as steep slopes or falls, we found that brook trout (Salvelinus fontinalis) dispersed downstream through channel sl opes of 80% and 18-m-high falls. Thus, headwater lake stocking provides sou rce populations that may be capable of invading most downstream habitats, i ncluding 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 upst ream dispersal. In the thermal and trophic environments downstream of lakes , fish commonly grow faster and thus mature earlier and have higher fecundi ty-at-age than their counterparts in other high-elevation streams. The resu lting higher rates of population growth facilitate invasion. Larger body si zes also potentially aid the fish in overcoming barriers to invasion. Trout introductions to high-elevation headwater lakes thus pose disproportionate ly large risks to native fishes-even when the place of introduction may app ear to be spatially dissociated from populations of the native species. Map ping the potential invasible area can help to establish priorities in stock ing and eradication efforts.