Fringe effects: detecting bull trout (Salvelinus confluentus) at distributional boundaries in a montane watershed

Abstract

Robust assessment and monitoring programs are critical for effective conservation, yet for many taxa we fail to understand how trade-offs in sampling design affect power to detect population trends and describe spatial patterns. We tested an occupancy-based sampling approach to evaluate design considerations for detecting watershed-scale population trends associated with juvenile bull trout (Salvelinus confluentus) distributions. Electrofishing surveys were conducted across 275 stream sites from the Prairie Creek watershed, Northwest Territories, Canada. Site-level detectability of juvenile bull trout was not uniform, and imperfect detection affected modelled occupancy probabilities most in fringe habitats near distributional boundaries in steep reaches and large streams. We show that detecting a 30% change in watershed-level occupancy ≥78% of the time as conservation guidelines suggest, may require three repeat surveys (i.e., temporal replicates) and increased spatial sampling intensity of fringe habitats. Additional sampling effort in fringe sites could be offset by sampling fewer sites in core habitats to optimize designs for detecting demographic shifts in bull trout, while still minimizing risk of non-detection for this cryptic species.

An ecothermal paradox: bull trout populations diverge in response to thermal landscapes across a broad latitudinal gradient

“…Northern populations may possess a compensatory mechanism to grow more efficiently in colder environments during shorter growing seasons (i.e., countergradient variation) (Conover and Schultz 1995). All of the streams in the northern area, where we describe fullyear thermal regimes, have abundant, self-sustaining bull trout populations (Mochnacz et al 2021), yet several of these streams accumulate very few thermal units throughout the annual growing season (i.e., ≤ 800). However, we would not expect populations to occupy these streams if they could not survive under these growing conditions; therefore, they appear to have adjusted (i.e., genetically or via phenotypic plasticity/acclimation) to be successful in these colder thermal regimes.…”

“…Unpaved roads/ trails are present in all three areas but are least extensive in the north. All three stream networks have self-sustaining, healthy bull trout populations in multiple streams (M. Taylor unpublished data; Isaak et al 2015;Mochnacz et al 2021). The southern area has the most diverse fish assemblage with 12 species (Isaak et al 2017a), followed by the central and northern areas which each have four species (Schindler 2000;Babaluk et al 2015).…”

“…Because juvenile bull trout distributional data were available across all three areas (ID, n = 180; AB, n = 185; NT, n = 415) and was collected using similar methods-i.e., two spatial/temporal replicates across sites allocated using a stratified random design (Isaak et al 2017b;Mochnacz et al 2021)we combined these data with mean August stream temperatures to portray the available and occupied thermal niche in each respective area. To provide a consistent means of comparison across the fish survey sites which lacked co-located temperature sensors, spatially explicit temperature models were built for streams in each of the three study areas that predicted mean August temperatures (Isaak et al 2017a;Mochnacz 2021).…”

“…Populations in the USA and Canada have protected status under the Endangered Species Act and Species at Risk Act, respectively (USFWS 1999;COSEWIC 2012), and the species is classified as being highly sensitive to climate change (Rieman et al 2007). Bull trout typically occupies montane watersheds and is patchily distributed in headwater streams that are most conducive to adult spawning and subsequent survival of natal and juvenile life stages (Rieman and McIntyre 1995;Isaak et al 2017b;Mochnacz et al 2021). The occurrence of bull trout populations has been strongly linked with the distribution of the coldest water temperatures within landscapes (Benjamin et al 2016;Isaak et al 2017b;Kovach et al 2017).…”

Occupancy of young-of-year Arctic grayling (Thymallus arcticus) in Barrenland streams

“…The size of the local population at each replicate impacts detection probability, and variation in abundance can be the leading cause of heterogeneity in detection probabilities in occupancy studies (Royle & Nichols, 2003). For juvenile bull tout [Salvelinus confluentus (Suckley, 1859)] occupying mountain streams in the Northwest Territories, detection probability is high (p = 0.78) in core habitat areas, but is greatly reduced (p = 0.48) in fringe habitats near distributional boundaries (Mochnacz et al, 2021), where abundance is likely lower. Most occupied streams surveyed in this study had perfect detection (62.5%), and are likely core habitat areas for YOY grayling.…”