Population Dynamics and Temporal Trends of Bull Trout in the East Fork Salmon River, Idaho

Abstract

Because of their long-term listing under the Endangered Species Act, much interest has been placed on estimating population vital rates for Bull Trout Salvelinus confluentus, but the biotic and abiotic factors that influence the interannual variability in those vital rates have rarely been evaluated. We used mark-recapture data to estimate fish growth, survival, and trends in abundance for fluvial adult Bull Trout in the East Fork Salmon River, Idaho. Over an 8-year period, a total of 1,205 individual Bull Trout were collected at a weir on the East Fork Salmon River (29 km upstream of its confluence with the Salmon River) during June-September, of which 420 were recaptures from prior years. Bull Trout varied in length from 215 to 756 mm and achieved a slightly larger asymptotic length and a slightly lower rate of growth relative to other fluvial and adfluvial Bull Trout populations. Apparent survival averaged 0.42 across all years, which was similar to previous studies estimating apparent survival for Bull Trout. The number of emigrating anadromous salmonid smolts in the upper Salmon River basin positively influenced East Fork Salmon River Bull Trout growth and survival, and survival was higher in years with lower annual discharge. Assessment of population growth via linear regression analysis indicated that the Bull Trout population was increasing during the study period (λ = 1.08; 95% confidence interval = 1.03-1.14). Our findings highlight the ecological link between the abundance of wild and hatchery Chinook Salmon Oncorhynchus tshawytscha, Sockeye Salmon O. nerka, and steelhead O. mykiss smolts and growth and survival of adult Bull Trout in systems where these species occur in sympatry.Bull Trout Salvelinus confluentus is a species of char native to western North America that has been listed as threatened by the U.S. Fish and Wildlife Service under the Endangered Species Act since 1999 (USFWS 1999). Bull Trout are large compared with most salmonids, achieving lengths of 700-900 mm and weights of >10 kg, and they commonly live from 8 to 12 years (Sigler and Zaroban 2018). The diet of Bull Trout changes with age, with younger Bull Trout feeding on invertebrates and older Bull Trout tending to be piscivorous. Bull Trout typically mature at 4-7 years of age, with spawning occurring in autumn when water temperatures are below 9°C, primarily in headwater mountain streams.Understanding the viability of threatened or endangered fish populations or assessing the effectiveness of management strategies or habitat restoration to conserve at-risk

Evaluating streamflow and temperature effects on Bull Trout migration and survival with linear spatial capture–recapture models

“…Our estimates of 14-18-week prespawning survival (x̄ = 0.64-0.83) and 38-week postspawning survival (x̄ = 0.76) are much higher than annual survival reported for other fluvial and adfluvial Bull Trout populations, which averaged 0.43 (Roth et al 2021) and ranged from 0.35 to 0.47 (Beauchamp and Tassel 2001;Al-Chokhachy and Budy 2008;Howell et al 2016;Al-Chokhachy et al 2019). It is possible that our survival estimates were biased by our treatment of mortality (i.e., we assumed that all recovered tags represented mortalities), as fish can expel their tags while still alive (Summerfelt and Mosier 1984).…”

“…Consequently, the actual water temperatures sought by Bull Trout were likely cooler than those represented by our downstream thermograph. Even in warmer years, Rapid River water temperatures likely remain sufficiently cold because the riverscape is at high elevation and is intact, complex, and highly connected (Isaak et al 2016;Roth et al 2021). In intact systems with water temperatures in the preferred range for Bull Trout, studies have confirmed that prespawn fish did not use cooler water than was available (Howell et al 2010).…”

“…For example, we do not know whether the sometimesextreme water temperatures encountered by Bull Trout during migrations are associated with increased mortality, which may create a survival bottleneck (Cooke et al 2008). Additionally, Bull Trout are among the most temperature-sensitive freshwater fishes in North America (Selong et al 2001;Guzevich and Thurow 2017;Thurow et al 2020;Roth et al 2021). Therefore, water temperature, stream discharge (Starcevich et al 2012;Sinnatamby et al 2018), and variation in both summer and winter water temperatures and streamflow during a given year or multiple years will likely affect migration and survival (Selong et al 2001;Meyer et al 2014;Letcher et al 2015;Roth et al 2021).…”

“…Additionally, Bull Trout are among the most temperature-sensitive freshwater fishes in North America (Selong et al 2001;Guzevich and Thurow 2017;Thurow et al 2020;Roth et al 2021). Therefore, water temperature, stream discharge (Starcevich et al 2012;Sinnatamby et al 2018), and variation in both summer and winter water temperatures and streamflow during a given year or multiple years will likely affect migration and survival (Selong et al 2001;Meyer et al 2014;Letcher et al 2015;Roth et al 2021). For example, the onset of prespawning migration from May to August may be triggered by optimal water temperatures for growth or when streamflows reach a certain threshold (COSEWIC 2012;Letcher et al 2015;Sinnatamby et al 2018).…”

Evaluating the evolutionary mechanisms maintaining alternative mating strategies in a simulated bull trout (Salvelinus confluentus) population

“…On average, bull trout has an estimated generation time of 4–7 years (Mogen & Kaeding, 2005 ; Roth et al, 2021 ). Generally, sneaker males are characterized by an earlier age‐at‐maturation than territorial males.…”