Bull Trout Life History, Genetics, Habitat Needs, and Limiting Fact in Central and Northeast Oregon. Annual Report 1999.

An Evaluation of Redd Counts as a Measure of Bull Trout Population Size and Trend

“…We used PIT tag recaptures to determine annual frequencies of upstream migration and gonadal maturity for individual fish. To estimate the number of females trapped during 1998-2001 prior to our use of ultrasound, we multiplied the total trap count of 300-mm or larger bull trout (see below) in those years by the mean fraction of females trapped in 2002-2007. A telemetry study indicated that most fluvial adult bull trout in the Mill Creek drainage overwintered downstream from the dam; however, a few overwintered upstream of the dam (Hemmingsen et al 2001b(Hemmingsen et al , 2001c. In 2003-2007, bull trout captured in the trap were marked with a caudal punch to facilitate a mark-recapture (resight) estimate to account for any fluvial adults that had overwintered upstream from the dam and thus were not included in the counts at the ladder trap.…”

“…The diver recorded the number of marked and unmarked bull trout (300 mm or larger) that were observed. Trapping data for the previous 5 years indicated almost all of the migratory fish returning to spawn in the population were 300 mm or larger (Hemmingsen et al 2001b(Hemmingsen et al , 2001c. We estimated the total number of unmarked, 300-mm or larger bull trout by incorporating the number of marked and unmarked bull trout observed while snorkeling and the number of marked bull trout released upstream of the trap at the time of snorkeling into Bailey's modification of the Peterson mark-recapture formula (Ricker 1975), which accounts for replacement of marked fish into the population,…”

“…Endoscopy has been shown to be a highly accurate technique for determining maturity and sex in small salmonids (Swenson et al 2007). Similar work we previously conducted in Silver Creek (Powder River subbasin, Oregon) indicated that mature resident females were 140-200 mm (Hemmingsen et al 2001b). Therefore, to be conservative, we inspected every captured bull trout that was 130 mm or larger.…”

“…Redd counts.-We conducted three to four redd counts at 2-3-week intervals during September and October in Mill Creek and its tributaries upstream of the diversion dam to the upstream limits of fish distribution. Data for Mill Creek indicated that the spawning duration was from the last week of August through October (Hemmingsen et al 2001b) and that 94% of the redds remained visible during that survey interval (Hemmingsen et al 2001a). Thus, the spatial extent, frequency, and timing of our redd counts should have provided essentially a complete count or "census" of the number of detectable redds (after Gallagher et al 2007;Jacobs et al 2009).…”

“…This study was conducted in the upper Mill Creek watershed (Walla Walla River subbasin) in northeastern Oregon and southeastern Washington (Figure 1). Previous redd surveys (Hemmingsen et al 2001b(Hemmingsen et al , 2001c indicated that 99% of the migratory bull trout population spawned upstream of a 2.5-m-high diversion dam, which was equipped with a fish ladder, at river kilometer 41. The Mill Creek watershed above the diversion dam is 8,798 ha (NPPC 2004).…”