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

Abstract

To 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 annually. Bull Trout that spawned in main-stem Mill Creek were primarily larger migratory adults; however, about 20% of the large adults were strictly or intermittently resident, remaining in the spawning area year-round. The downstream extent of individuals' migratory distributions varied greatly-from just downstream of the spawning area to the mouth of the Walla Walla River and potentially hundreds of kilometers into the Columbia River. Despite a large sample size of radio-tagged fish, radiotelemetry substantially underestimated the distribution and range that were evident from PIT tag detections. Life history terms such as "migratory," "resident," and "fluvial" and their associations with body size, movement, and distribution are useful for describing general patterns, but they fail to reflect the diversity and complexity within and among populations. For Bull Trout in Mill Creek, that life history diversity, including small, resident adult forms in the tributaries and a continuum of distribution for large adults, maximizes the use of available habitat and likely contributes to the population's persistence.

Modelling individual variability in growth of bull trout in the Walla Walla River Basin using a hierarchical von Bertalanffy growth model

“…Migratory bull trout in Mill Creek were PIT‐tagged and recaptured during 1998–2010 by the U.S. Forest Service during emigration from and immigration to the spawning and juvenile rearing area (Table , Figure ; Howell et al., ). Upstream migrating adults (≥270 mm FL) were collected in a hoop net as they exited a fish ladder at a diversion dam at the downstream limit of the spawning area; downstream migrating fish (mostly subadults, <270 mm FL) were collected in a rotary screw trap located 0.5 km upstream of this dam.…”

“…Upstream migrants were scanned by ultrasound to identify sex and maturity (Howell & Sankovich, ). The trap for upstream migrants was operated May to October in all years, while the trap for downstream migrating fish was operated March to October in some years and year‐round in other years (Howell et al., ). Data from PIT‐tagged bull trout in Mill Creek were used to examine subadult and adult growth of “Mill Creek Male Migrants” and “Mill Creek Female Migrants.”…”

“…Bull trout demonstrate extensive variation in migration and demographic rates within and among populations (Al‐Chokhachy & Budy, ; Howell, Colvin, Sankovich, Buchanan, & Hemmingsen, ; Rieman & Dunham, ), similar to other salmonids (Billman et al., ; Doctor, Berejikian, Hard, & VanDoornik, ; Zydlewski, Zydlewski, & Johnson, ). Bull trout populations can be exclusively resident (i.e., no subadult emigration) or have a mix of resident and migratory individuals.…”

“…We analysed data from change in length over time of tagged fish and back‐calculated length‐at‐age from fish otoliths to examine individual variability and patterns in somatic growth for two populations of bull trout in the Walla Walla River Basin in Oregon and Washington (Figure ): Mill Creek; and the South Fork Walla Walla River (SFWWR). Each is a mixed fluvial spawning population (i.e., interbreeding residents and fluvial migrants with migration that varies both spatially and temporally; Homel, Budy, Pfrender, Whitesel, & Mock, ; Howell et al., ). Our goals were to quantify the extent of variability in growth among individuals and to produce appropriate growth parameters for future population modelling.…”

Prioritising culvert removals to restore habitat for at‐risk salmonids in the boreal forest

“…and habitat quality (0-1; is the habitat of suitable quality for fish species of interest). Bull trout similarly may travel up to 250 km to access spawning grounds (Burrows et al 2001), but likely average significantly less in watersheds with habitat spaced closer together (Muhlfeld & Marotz 2005;ASRD 2009;Howell et al 2016). In a neighbouring watershed, there is less than a 10% chance of bull trout occurrence when road density is as low as 0.8 km km À2 (Ripley et al 2005).…”