POPULATION ECOLOGY OF AN INVASION: EFFECTS OF BROOK TROUT ON NATIVE CUTTHROAT TROUT

Invasion by nonnative brook trout ( Salvelinus fontinalis ) often results in replacement of cutthroat trout ( Oncorhynchus clarki ) in the inland western United States, but the underlying mechanisms are not well understood. We conducted a four‐year removal experiment to test for population‐level mechanisms (i.e., changes in recruitment, survival, emigration, and immigration) promoting invasion success of brook trout and causing decline of native Colorado River cutthroat trout ( O. c. pleuriticus ). We chose 700–1200 m segments of four small mountain streams where brook trout had recently invaded cutthroat trout populations, two each at mid elevation (2500–2700 m) and high elevation (3150–3250 m), and annually removed brook trout from two streams (treatments), but not the other two (controls). We used depletion electrofishing, two‐way fish weirs, and mark–recapture methods to estimate abundance, movement, and survival of trout. At mid elevation, age‐0 and age‐1 cutthroat trout survived at 13 times and two times higher rates on average, respectively, where brook trout were removed. At high‐elevation sites, recruitment of cutthroat trout failed despite brook trout removals, apparently because of cold water temperatures. In contrast, age‐2 and older cutthroat trout survived at similar rates, whether brook trout were removed or not and regardless of elevation. Summer movement by cutthroat trout was unaffected by removal of brook trout. We conclude that brook trout depress cutthroat trout populations at mid elevation through age‐specific biotic interactions that reduce juvenile cutthroat trout survival, whereas populations restricted to high‐elevation sites by invasion continue to decline because an abiotic factor (low temperature) causes recruitment failure. In comparison, brook trout survived at the same or higher rates than same‐aged cutthroat trout. High immigration by brook trout recolonized depleted segments, and may help sustain invasions in sink habitats where environmental conditions limit recruitment. In streams similar to those we studied, eradication of brook trout is likely necessary to eliminate the threat to native cutthroat trout, but selective removal regimes that capture a high percentage of the brook trout population for least three consecutive years, repeated periodically, may permit cutthroat trout populations to persist with brook trout. To identify underlying mechanisms responsible for successful invasion by mobile, age‐structured vertebrates such as stream fishes, experiments conducted at realistic spatial and temporal scales and including multiple age classes will be required.

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