Metapopulation structure of bull trout: Influences of physical, biotic, and geometrical landscape characteristics

Metapopulation structure of species in fragmented landscapes is ultimately the result of spatial variability in demographic processes. While specific information on demographic parameters is desirable, a more practical approa ch to studying metapopulations in fragmented landscapes may begin with anal yses of species' occurrence in relation to large-scale habitat variability. Here, we analyzed occurrence of stream-living bull trout (Salvelinus confl uentus) in relation to physical, biotic, and geometrical characteristics of habitats. Bull trout occurrence was analyzed at several spatial (10(chi) m ) scales. Data were from nested sampling of 720 sites (10 m), 179 reaches ( 10(2) m), and 81 patches (greater than or equal to 10(3) m) of stream habit ats within the Boise River basin of central Idaho. Based on previous findin gs, patches were defined as stream catchments with suitable conditions for spawning and rearing of bull trout (>1600 m elevation). Patch-scale bull tr out occurrence was significantly related to patch area and isolation (strea m distance between occupied patches). Lack of spatial autocorrelation betwe en patches indicated that isolation effects were more likely the result of limited interaction among habitats (such as dispersal), rather than of corr elated environmental conditions. A third factor, human disturbance in the f orm of roads, was associated with reduced bull trout occurrence at the patc h-scale. Analyses of occurrence among reaches within occupied patches showe d bull trout may select larger (>2 m width) stream habitats. Occurrence of bull trout was not associated with nonnative brook trout (Salvelinus fontin alis) at large (patch), intermediate (reach), or small (site) spatial scale s. Definition of a metapopulation structure for bull trout in the Boise Riv er basin was complicated by uncertainties in the frequency and magnitude of dispersal. From the distribution of patch sizes and isolation among occupi ed patches, we suggest that the metapopulation is a complex mosaic of sever al elements found in conceptual models. This complexity poses a challenge t o empirical and theoretical attempts to study stream-living bull trout. Fut ure work to define the structure of bull trout metapopulations must relate temporal and spatial patterns of patch occupancy with complex patterns of d ispersal that likely interact with habitat spatial structure, life history variability, and the historical context of regional climate changes. Result s of this work suggest that conservation of bull trout should involve prote ction of larger, less isolated, and less disturbed (as indexed by road dens ities) habitats that may serve as important refugia or sources of recoloniz ation. Bull trout populations in smaller, isolated, and more disturbed habi tats may be at risk of extinction. Finally, metapopulation structure implie s the existence of suitable, but presently unoccupied habitat, which should be managed carefully to facilitate potential natural recolonization or rei ntroductions of bull trout.