Life History and Demographic Characteristics of Bull Trout in Mill Creek (Walla Walla Subbasin)

We completed a 14-year study of the life history and demographic characteristics of bull trout populations in the Mill Creek watershed to better understand the biology of fluvial forms and aid the recovery of the species.  Bull trout that spawned in the main stem of Mill Creek were primarily larger (≥300 mm), migratory adults; however, about 21% of the population of large fish was strictly or intermittently resident, remaining in the spawning areas year-round.  A demographically distinct apparent resident population of small adults (<200mm) occurred in Low Creek, a Mill Creek tributary.  Migratory adults and subadults were distributed throughout 115 km of Mill Creek and the lower Walla Walla River and potentially in adjacent reaches of the Columbia River.  Most of the subbadults (<270 mm) migrating downstream were captured in a screw trap during April-June; catch peaked as discharge was declining and temperatures were increasing.  The subadults captured were mostly 126-175 mm and age 2-3.  Subadults were first recaptured as adults returning to spawn primarily 1-2 years after tagging with average growth rates of 144 and 96 mm/yr, respectively.  Upstream movement of adults was concentrated in two pulses: mid- July and late August/early September.  Smaller runs in 2005-2010 were later and accompanied by shifts in fish size and timing relationships than larger runs in 1998-2004.  Mean FL of migratory adults was 432 mm, and mature females were ≥289 mm.  Median growth rates of adults ≤400 mm and >400 mm were 48 and 36 mm/yr, respectively, and declined with increasing age.  Recapture and maturity data indicated more than 90% of the migratory adults likely spawned in successive years.  Migratory adult abundance declined more than 50% during 2006-2010 mostly due to reduced subadult-to-adult survival.  Subadult survival rates averaged more than 13% during 1999-2003 and then declined to less than 1% during 2004-2010.  Adult survival rates were relatively high during 1999-2005, averaging 48%, and then declined to an average of 30% during 2006-2010.