Climate warming is expected to have substantial impacts on native trout across the Rocky Mountains, but there is little understanding of how these changes affect future distributions of co-occurring native fishes within population strongholds. We used mixed-effects logistic regression to investigate the role of abiotic (e.g., temperature) and biotic factors (Bull Trout presence, Salvelinus confluentus) on distributions of Westslope Cutthroat Trout (Oncorhynchus clarkii lewisi; WCT) in the North Fork Flathead River, USA and Canada. The probability of WCT presence increased with stream temperature and decreased with channel gradient and Bull Trout presence, yet the effect of Bull Trout was reduced with increasing pool densities. Combining this model with spatially-explicit stream temperature projections, we predict a 29% increase in suitable habitat under high emissions through 2075, with gains at mid-elevation sites predicted to exceed Bull Trout thermal tolerances and high-elevation sites expected to become more thermally suitable for WCT. Our study illustrates the importance of considering abiotic and biotic drivers to assess species response to climate change, helping to guide local scale climate adaptation and management.
Summary Previous molecular diet analysis identified lake sturgeon (Acipenser fulvescens, Rafinesque, 1817) DNA in the gastrointestinal tracts of stream‐resident rainbow darters (Etheostoma caeruleum, Storer, 1845) during the egg incubation, free embryo, and larval drift stages. The objectives of this experimental study were to: (a) quantify levels of predation by rainbow darters on lake sturgeon at the egg and free‐embryo stages; and (b) evaluate whether predation varied as a function of substrate size and rainbow darter body size. We conducted experimental trials in 23‐L polycarbonate tanks 0.41 m (L) × 0.33 m (W) × 0.30 m (D) with a standardized benthic area of 0.14 m2. The tanks were randomly assigned one of two different substrate size classes: large rock (51.35 mm ± 0.91 mm) or small rock (27.68 mm ± 0.57 mm). We stocked individual rainbow darter, which were deprived of feed for 48 hr, with lake sturgeon (133 individuals/m2) in each of 12 replicates per ontogenetic stage and substrate type. The number of surviving lake sturgeon was quantified following a 24‐hr predation exposure period. We used a generalized linear model with a binomial distribution to assess the influence of ontogenetic stage, substrate size, and rainbow darter body size on proportional lake sturgeon survival. Predation on lake sturgeon occurred at both egg (6.25 ± 1.16 individuals, mean ± 2SE) and free embryo (3.08 ± 1.08 individuals, mean ± 2SE) stages. Egg proportional survival was generally lower than at the free embryo stage in both substrate sizes; however, free embryo proportional survival was greater in small substrate trials. Rainbow darter total length did not affect the probability of lake sturgeon survival at either developmental stage. Results demonstrate that rainbow darters prey on early ontogenetic stages of lake sturgeon, corroborating previous results based on genetic diet analysis. Results fill a major knowledge gap concerning the vulnerability of pre‐drift sturgeon to predation by an abundant river resident species that was previously discounted as a predator for early ontogenetic stages of lake sturgeon due to its small body size.
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