Identification and Characterization of Migratory and Nonmigratory Bull Trout Populations in the St. Mary River Drainage, Montana

Abstract

Populations of migratory and nonmigratory bull trout Salvelinus confluentus were identified in the St. Mary River drainage, Montana. Migratory fish were most conspicuous because they were caught in traps operated near creek mouths or moved between creeks. Capture of postspawning bull trout in traps during four consecutive years suggested that most migratory fish had spawned by late September. Migratory bull trout reached maturity mainly as age-5 fish that were typically 300 mm or larger in total length. Estimates based on the recapture of tagged fish in traps indicated that populations of migratory adult bull trout were significantly larger than the mean values from annual catches of trapped fish. Juvenile migratory bull trout departed natal creeks mainly as age-2 or age-3 fish. The scarcity of age-4 bull trout in trap samples indicated that most migratory age-4 fish were immature and inhabited either the downstream lakes or main-stem river. Conversely, the abundance of age-4 bull trout in electrofishing samples was primary evidence of nonmigratory populations in creeks. The occurrence of age-0 bull trout in electrofishing samples indicated recent spawning and reproduction in five creeks, and annual reproduction was indicated by multiple age-classes of young bull trout. Bull trout had growth rates similar to those of bull trout elsewhere, including marked growth of age-4 fish that ostensibly resulted from their transition to a largely piscivorous diet. Migratory bull trout in the St. Mary River drainage are more common than previously believed and may be more common than in many other regions of the species' natural range.