Resource partitioning as a factor limiting gene flow in hybridizing populations of Dolly Varden char (Salvelinus malma) and bull trout (Salvelinus confluentus)

Dolly Varden char (Salvelinus malma) and bull trout (Salvelinus confluentus ) hybridize in areas of secondary contact in northwestern North America but maintain their genetic integrity in spite of gene flow. We examined juveni le stream ecology and adult reproductive ecology of these species in sympat ry to test for specializations to alternative niches that may act as a basi s for natural selection against hybrids. Juvenile Dolly Varden occupied dee per, faster water and foraged more during daytime in the drift, whereas bul l trout used shallower, slower water and made more nighttime foraging attem pts towards the substrate. The species showed extensive diet overlap (Schoe ner's index = 0.7-0.9), and we found evidence for density compensation betw een species among sites. Our results suggest no obvious basis for selection against hybrids at the juvenile stream-rearing life-history stage. Bull tr out, however, are adfluvial, whereas Dolly Varden are permanent stream resi dents. Bull trout are also much larger at maturity (50-80 cm vs. 12-15 cm) and build redds in faster, deeper water using larger substrate sizes. Conse quently, hybrid genotypes may be selected against in these alternative life -history niches or during reproduction owing to their intermediate size at maturity and size-dependent spawning habitat use and mate choice.