Consequences of Actively Managing a Small Bull Trout Population in a Fragmented Landscape

Abstract

Habitat fragmentation, which affects many native salmonid species, is one of the major factors contributing to the declines in distribution and abundance of Bull Trout Salvelinus confluentus. Increasingly, managers are considering options to maintain and enhance the persistence of isolated local populations through active management strategies. Understanding the ecological consequences of such actions is a necessary step in conservation planning. We used an individual‐based model to evaluate the consequences of an ongoing management program aimed at mitigating the anthropogenic fragmentation of the lower Clark Fork River in Montana. Under this program juvenile Bull Trout are trapped and transported from small, headwater source populations to Lake Pend Oreille, Idaho, for rearing, and adults are subsequently recaptured in their upstream migration and returned to the natal population for spawning. We examined one of these populations and integrated empirical estimates of demographic parameters to simulate different management scenarios where moderate (n = 4) and high (n = 8) numbers of age‐2, age‐3, or age‐4 Bull Trout were removed for transport with variable return rates under both demographic stochasticity and environmental perturbations. Our results indicated the risks from removal with no returns increased substantially when removal totals and age of Bull Trout removed from the simulated population increased. Specifically, removing eight age‐3 or age‐4 individuals resulted in 26% and 62% reductions in average adult population size, respectively, across simulations. We found the risks of transport were not likely alleviated with low (3%) or moderate (6%) return rates, and there were considerable risks of declines for the source population even when return rates were extremely high (>12%). Our simulations indicated little risk of declines for the source population with removals of age‐2 Bull Trout, and any risks were alleviated with low return rates. However, we found higher return rates were particularly beneficial in the presence of large, density‐independent perturbations.

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

“…First, across all populations the annual growth rates of Bull Trout are highly variable and, as expected, decrease with size and age (Erhardt and Scarnecchia ; Al‐Chokhachy et al. ; Howell et al. ).…”

“…Understanding age structure provides important information on the life history expressions of fish and on their growth trajectories and is a key aspect of age‐ or size‐structured population models used in directing Bull Trout conservation and management (Al‐Chokhachy et al. ; McCubbins et al. ).…”

A Hidden‐Process Model for Estimating Prespawn Mortality Using Carcass Survey Data

“…One population restoration and reintroduction effort that is particularly sensitive to prespawn mortality is outplanting, which involves trapping and hauling sexually mature or maturing adults upstream of migration barriers or into habitats where reproduction is limited. Outplanting is increasingly being used for different purposes, including to accelerate recolonization of Chinook Salmon into newly available habitat after barrier removal in Shitike Creek, Oregon (Baumsteiger et al 2008); to reintroduce Chinook Salmon above high-head dams in the Willamette River basin, Oregon (Keefer et al 2010); and to allow adult Bull Trout Salvelinus confluentus to spawn upstream of barriers (DeHaan and Bernall 2013;Al-Chokhachy et al 2015). Even moderate levels of prespawn mortality can undermine the effectiveness of outplanting operations because the number of adults outplanted is limited by (1) the number of adult fish available and (2) the high costs of associated infrastructure and personnel time.…”

Temporal Variation in Capture Efficiency Underrepresents Spring Out‐Migrating Bull Trout in a Trap‐and‐Haul Program

“…However, trapand-haul programs are resource intensive and the high degree of human intervention that is involved may have unintended consequences. The extent of these consequences, such as altering behavior, influencing migration timing, and unintentionally imposing selective pressure, may be difficult to fully understand and quantify (Budy et al 2002;Muir et al 2006;Al-Chokhachy et al 2015). Multiple aspects of trap-and-haul programs may impose selective pressures, such as size selectivity of the gear used to capture fish for transport or selection on timing of outmigration because of the timing and duration of trapping seasons.…”

“…2006; Al‐Chokhachy et al. 2015). Multiple aspects of trap‐and‐haul programs may impose selective pressures, such as size selectivity of the gear used to capture fish for transport or selection on timing of out‐migration because of the timing and duration of trapping seasons.…”