Life Histories, Demography, and Distribution of a Fluvial Bull Trout Population

AbstractTo describe the life histories and demography of a fluvial population of Bull Trout Salvelinus confluentus, we PIT-tagged and radio-tagged Bull Trout captured in Mill Creek, a tributary of the Walla Walla River (Washington–Oregon), during 1998–2009. Adult abundance declined 63% during 2006–2010, driven primarily by a 10-fold reduction in subadult-to-adult returns. Larger subadults and fall–winter emigrants survived at higher rates, but they were a small proportion of the subadult migrants. The survival rates of larger, generally older adults were also more than 40% greater than those of smaller adults. Changes in abundance influenced other characteristics of the population. For example, adult upstream movement into spawning areas during 1999–2005 peaked in late July, whereas the smaller runs observed during 2006–2010 peaked in early September, and the relationship between fish size and migration timing shifted. Unlike many adfluvial populations, more than 90% of the adults in Mill Creek spawned an...