Spawning Demographics and Juvenile Dispersal of an Adfluvial Bull Trout Population in Trestle Creek, Idaho

Abstract

We utilized a screw trap, trap-box weir, remote passive integrated transponder tag (PIT) detection weir, and otolith microchemistry to evaluate (2000–2004) spawning demographics and migration patterns of adfluvial bull trout Salvelinus confluentus in Trestle Creek, Idaho, a tributary to Lake Pend Oreille. Annual repeat spawning was more common than alternate-year spawning. Annual growth, estimated from adult bull trout PIT-tagged in 2000 and recaptured in 2001, averaged 28 mm. Peak diel movement of adult bull trout occurred after sunset. Emigration of juveniles occurred in two pulses, one in spring that was associated with snowmelt runoff and increasing water temperatures and a second in fall as stream temperatures dropped and fall rains began. Juvenile bull trout moved a mean rate of 455 m per night in 2001 and 423 m per night in 2002. Each year we trapped large numbers of age-0 bull trout that appeared to be emigrating with high spring flows. Based on otolith microchemistry, most of the 47 adults in the sample had emigrated at age 3 or age 4, and none had emigrated at age 0. This suggests that age-0 emigrants are not making a significant contribution to adult returns.