Otolith microchemistry reveals partial migration and life history variation in a facultatively anadromous, iteroparous salmonid, bull trout (Salvelinus confluentus)

Migration of fishes between habitats influences population dynamics and ecological interactions. Some “partially migratory” populations include both migratory and non-migratory individuals, adding complexity to these dynamics. For partially migratory fishes with diadromous life histories, freshwater and marine habitats can differ greatly in availability of prey and physical conditions conducive to growth, predation risk, and exposure to fisheries and to contaminants. Therefore, understanding patterns of migratory behavior can inform population biology and conservation. Using otolith microchemistry, we describe observations of partial anadromy in a threatened, iteroparous salmonid species, bull trout (Salvelinus confluentus), in the Skagit River basin of Washington State, USA. We found that 59% of the fish sampled (> 338 mm fork length) in the river had not been to marine water, despite easy access. The other 41% had migrated to salt water, typically every year beginning at age 2 or 3. We also observed overwintering in marine waters by some individuals, and extended time in fresh water between otherwise annual migrations to marine waters in others. Additionally, there was no obligatory relationship between anadromy in mothers and their offspring. The facultative nature of migration in this species, and the lack of tight connection between maternal and offspring life history patterns are consistent with studies of other Salvelinus species but contrast with the more rigid controls on migration in semelparous salmonids.