Notes: Spawning Behavior of Bull Trout in the Upper Flathead Drainage, Montana, with Special Reference to Hybridization with Brook Trout

We observed spawning behavior of adfluvial bull trout Salvelinus confluentus in a stream in the Flathead River basin where this species coexists with resident brook trout S. fontinalis. Bull trout were much larger in total length than brook trout. During spawning activities of bull trout, smaller paired males or females were displaced by larger individuals. Smaller satellite males, especially brook trout, remained near spawning bull trout pairs for long periods. During these periods, the smaller brook trout were attacked by the paired male as frequently as were larger bull trout satellite males. In one case, a small satellite male brook trout was observed to release sperm after sneaking into a redd during spawning by a pair of large bull trout. This activity may account for some instances of hybridization observed in these populations.