New Insights into the Ecology of Adfluvial Bull Trout and the Population Response to the Endangered Species Act in the North Fork Lewis River, Washington

Abstract

Like many other salmonids, Bull Trout Salvelinus confluentus migratory life history expressions are becoming increasingly rare. A critical step in effectively refining management and conservation strategies is a robust assessment of the effectiveness of such strategies and key biological information used in monitoring and recovery planning. To address this need, we integrated long‐term monitoring data to evaluate how the demographics (abundance) and vital rates (survival) of a Bull Trout population shifted in response to their listing under the Endangered Species Act (1998). In addition, we employed mark–recapture data to investigate Bull Trout population dynamics and assessed the life history characteristics (aging, growth, spawning migrations, and iteroparity) to improve our understanding of Bull Trout ecology and diversity. Our abundance and survival data, which extends to the early 1990s, illustrated positive responses in survival and abundance following the protection of Bull Trout under the Endangered Species Act. Over this period, we found high interannual variability in both survival and abundance, and adult survival (average = 0.45; SE = 0.04) was surprisingly lower than that of subadult individuals (average = 0.66; SE = 0.04), suggesting limitations at this important life stage. Our mark–recapture data also suggested that the attributes driving the Bull Trout population trend (i.e., recruitment to the adult stage or adult survival) has varied through time, with declining trends in the relative contribution of recruitment. Bull Trout spawning migrations varied with body size, and a considerable portion of smaller adults (<650 mm) did not spawn each year. Additionally, most spawning individuals made only one spawning migration, while <13% made three or more spawning migrations. Our results provide important insights into the effectiveness of the Endangered Species Act listing and new information regarding the life history patterns of adfluvial Bull Trout and can serve as a template to consider factors potentially limiting this and other native salmonid populations.

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

“…While Bull Trout growth in the East Fork Salmon River differed from previous studies, estimates of apparent survival across years (meanΦ = 0.42) were surprisingly similar to previous estimates for a variety of fluvial and adfluvial Bull Trout populations (e.g., Al-Chokhachy and Budy 2008;Al-Chokhachy et al 2019;Beauchamp and Tassel 2001;Howell et al 2016;Hudson et al 2019), which taken collectively have ranged from 0.35 to 0.47 and averaged 0.43. Apparent survival is an underestimate of true survival for Bull Trout because site fidelity and annual spawning rates are generally not 100% for this species (e.g., Barnett and Paige 2013;Rieman and Allendorf 2001), and fish that are actually alive but go undetected are recorded as mortalities, leading to underestimates ofΦ.…”

Managing Freshwater Fish in a Changing Climate: Resist, Accept, or Direct

“…Bull Trout Salvelinus confluentus are among the most thermally sensitive fish species and are expected to show a strong negative response to climate change (Kovach et al 2017). In the United States, Bull Trout are listed as a threatened species under the Endangered Species Act and no harvest is allowed, except in a few designated waters (Al‐Chokhachy et al 2019). Third, regulations that protect a segment of the population may help prevent climate‐associated declines in fish populations.…”

Evaluating the success of a conservation reintroduction: The case of bull trout in the Wallowa River

“…Migratory bull trout in the Wallowa River watershed were likely adfluvial. Fortunately, the growth and age characteristics of fluvial and adfluvial populations of bull trout can be similar (see Al‐Chokhachy, Doyle, & Lamperth, 2019; Erhardt & Scarnecchia, 2016; Harris, Newlon, Howell, Koch, & Haeseker, 2018; McCubbins, Hansen, DosSantos, & Dux, 2016). Thus, to make these evaluations, we used relatively recent and robust information from populations that migrate to large rivers or reservoirs.…”

“…This suggests they originated from spawning around 2009. Finally, there is typically a positive, curvilinear relationship between the size of bull trout and their probability of survival (see Al‐Chokhachy et al, 2019; Harris et al, 2018; Howell et al, 2016). Given this, for the reintroduced bull trout, we estimated relatively large fish could have experienced negligible mortality, but it is improbable that relatively small fish would have survived from 1997 to 2010.…”