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1. Fish and aquatic salamander responses to the creation of riparian canopy gaps along forested headwater streams

   https://doi.org/10.1111/rec.14014  

   Swartz, Allison; Warren, Dana (October 2023, Restoration Ecology)

   Many headwaters across temperate North America have uniform mid‐succession riparian forests recovering from historic land clearing. These young riparian stands contrast with late‐succession forests, which have complex structural characteristics including canopy gaps. Canopy gaps provide structural diversity that can be important for terrestrial species, and they are also hypothesized to be important features for aquatic environments. The light patches below gaps create productivity hotspots in streams and therefore create potential for increased stream apex predator abundances through bottom‐up food web drivers. However, increasing light may also affect stream temperature, a consideration for coldwater fish (salmonids). We established an experimental before‐after control‐impact study to explicitly assess how creating canopy gaps in the riparian forest affects the abundance and biomass of coastal cutthroat trout (Oncorhynchus clarkii clarkii) and Pacific giant salamanders (Dicamptodon tenebrosus) in paired reference and treatment reaches at five replicate streams. Gaps were designed to resemble those in old‐growth forests in the treatment reach of each system although wood was explicitly left out of the stream. At four of five sites, we found small and generally consistent positive responses in adult cutthroat trout and total vertebrate biomass to localized increases in light but only 2 years after treatment. Results suggest that opening riparian canopies adjacent to streams via gaps is a viable tool to enhance structural complexity of riparian forests without negatively impacting stream vertebrates; however, a single gap alone had small effects on aquatic vertebrates. More or larger gaps would likely be needed to notably enhance aquatic apex predators. 

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2. Contrasting apex predator responses to experimentally reduced flow and increased temperature in a headwater stream

   https://doi.org/10.1002/ecs2.70293  

   Maffia, Madelyn; Swartz, Allison; Segura, Catalina; Warren, Dana (June 2025, Ecosphere)

   Abstract Changing climate conditions are expected to cause increases in the frequency and severity of drought conditions in many areas around the world, including the Pacific Northwest region of North America. While drought impacts manifest across the landscape, headwater streams are particularly susceptible to droughts due to limited deep‐water habitats and low water volumes that allow for substantial increases in water temperature. While low volumes of water and increased stream temperature will likely affect all aquatic species to some degree, the response of different taxa to these impacts is expected to vary with differences in physiological needs and habitat preferences among species. Using a before–after control‐impact (BACI) experimental design, this study investigates how reduced streamflow and increased stream temperature affect the two dominant apex predators in headwater streams of the Pacific Northwest, coastal cutthroat trout (Oncorhynchus clarkii clarkii) and coastal giant salamander (Dicamptodon tenebrosus). In a second‐order stream in the H.J. Andrews Experimental Forest in OR, USA, experimental flow diversions created decoupled drought conditions of reduced streamflow and elevated temperatures. Low‐flow conditions were created by diverting water around a 100‐m stream reach and this diverted water was passively warmed before re‐entering a downstream channel to create an increased temperature reach. We compared fish and salamander abundances and stream habitat in an upstream unmanipulated reference reach to the two experimental reaches. Relative increases in temperature ranged between 0.41 and 0.63°C, reflecting realistic stream warming in this region during drought events. Trout responded positively to increased temperatures, showing an increase in abundance, biomass, condition factor, and growth, whereas salamanders responded negatively in all metrics except condition. The low‐flow reach diverted approximately 50% of the flow, resulting in a relative pool area reduction of about 20%. Relative to the reference reach, salamanders displayed a net positive abundance response while trout declined in the low‐flow reach. The contrasting responses of these populations to decoupled drought conditions suggest that interactions of flow and temperature changes together will influence drought responses of the vertebrate communities of headwater streams. 

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3. Quantifying the Foodscape for Stream‐Dwelling Cutthroat Trout Reveals Spatial and Temporal Ranges of Resource Exploitation and Energy Intake

   https://doi.org/10.1002/tafs.10360  

   Owens, Hope W.; Keeley, Ernest R. (March 2022, Transactions of the American Fisheries Society)

   Food availability is a primary factor limiting the abundance of wild populations, but quantifying it requires an understanding of when and where prey are vulnerable to predators. Salmonid fishes in streams are commonly thought to forage on drifting aquatic invertebrates during daylight hours. However, past studies also report benthic and nocturnal foraging despite the predominant view of salmonids as diurnal drift-feeding predators. We used instream videography to assess foraging mode and energy intake for stream-dwelling Yellowstone Cutthroat Trout Oncorhynchus clarkii bouvieri. We recorded the foraging behavior of wild fish with a waterproof video camera and estimated energy intake based on fish size, foraging rate, retention rate, and caloric values of prey. Fish captured prey primarily from the water column and surface, targeting drifting invertebrates during daytime hours; however, they also foraged from the stream benthos and during nighttime. Yellowstone Cutthroat Trout foraging rate was most strongly related to foraging location in the stream, diel period, and month. Energy intake was highest from daytime drift-foraging behavior and exceeded a modeled metabolic limit of food intake during October and November. Nocturnal and benthic foraging contributed the smallest proportion of total foraging attempts but was observed over all months of our study and sometimes comprised up to 30% of estimated energy intake. Our results indicate that Yellowstone Cutthroat Trout in streams acquire most of the food intake as daytime drift-feeding predators. 

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4. Mark-recapture data of the Northern Spring Salamander (Gyrinophilus porphyriticus), Hubbard Brook Experimental Forest, 2012 – present

   https://doi.org/10.6073/pasta/cd5f5a03df194930bf87eb12157b8182  

   Lowe, Winsor H (January 2022, Environmental Data Initiative)

   {"Abstract":["This data set includes spatially explicit mark-recapture data of the\nNorthern Spring Salamander (Gyrinophilus porphyriticus) collected during\nthe summer months (June \u2013 August) from downstream and upstream reaches\nin multiple streams in the Hubbard Brook Experimental Forest. Downstream\nreaches begin at the confluence with the Main Hubbard and extend\nupstream 500 meters and upstream reaches begin at the weir and extend\ndownstream 500 meters. Downstream reaches contain brook trout and\nupstream reaches do not. We used a robust design framework with\napproximately 9 surveys per reach each summer (3 primary occasions with\n3 secondary occasions each). Salamanders were captured by hand and\nmarked with either Visual Implant Elastomer and/or a PIT tag.\n These data were gathered as part of the Hubbard Brook Ecosystem Study\n(HBES). The HBES is a collaborative effort at the Hubbard Brook\nExperimental Forest, which is operated and maintained by the USDA Forest\nService, Northern Research Station.\n These data have been published in the following papers: \n Lowe WH, Addis\nBR, Smith MR, Davenport JM. The spatial structure of variation in\nsalamander survival, body condition and morphology in a headwater stream\nnetwork. Freshwater Biol. 2018;63:1287\u20131299.\nhttps://doi.org/10.1111/fwb.13133\n Lowe, W. H., and Addis, B. R.. 2019. Matching habitat choice and plasticity contribute to phenotype\u2013environment covariation in a stream salamander. Ecology 100( 5):e02661. 10.1002/ecy.2661 \n Lowe, W.H., et al. Hydrologic variability contributes to reduced survival through metamorphosis in a stream salamander. Proceedings of the National Academy of Sciences 2019; 116.39: 19563-19570.\n Bryant, A.R., Gabor, C.R., Swartz, L.K., Wagner, R., Cochrane, M.M., Lowe, W.H. Differences in corticosterone release rates of larval Spring Salamanders (Gyrinophilus porphyriticus) in response to native fish presence. Biology 2022; 11.484. https://doi.org/10.3390/biology11040484\n Addis, B.R., and W.H. Lowe. Environmentally associated variation in dispersal distance affects inbreeding risk in a stream salamander." The American Naturalist 2022."]} 

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5. Differences in Corticosterone Release Rates of Larval Spring Salamanders (Gyrinophilus porphyriticus) in Response to Native Fish Presence

   https://doi.org/10.3390/biology11040484  

   Bryant, Amanda R.; Gabor, Caitlin R.; Swartz, Leah K.; Wagner, Ryan; Cochrane, Madaline M.; Lowe, Winsor H. (April 2022, Biology)

   Invasive fish predators are an important factor causing amphibian declines and may have direct and indirect effects on amphibian survival. For example, early non-lethal exposure to these stressors may reduce survival in later life stages, especially in biphasic species. In amphibians, the glucocorticoid hormone corticosterone is released by the hypothalamo–pituitary–interrenal axis (HPI), as an adaptive physiological response to environmental stressors. The corticosterone response (baseline and response to acute stressors) is highly flexible and context dependent, and this variation can allow individuals to alter their phenotype and behavior with environmental changes, ultimately increasing survival. We sampled larvae of the spring salamander (Gyrinophilus porphyriticus) from two streams that each contained predatory brook trout (Slavelinus fontinalis) in the lower reaches and no predatory brook trout in the upper reaches. We measured baseline and stress-induced corticosterone release rates of larvae from the lower and upper reaches using a non-invasive water-borne hormone assay. We hypothesized that corticosterone release rates would differ between larvae from fish-present reaches and larvae from fish-free reaches. We found that baseline and stressor-induced corticosterone release rates were downregulated in larvae from reaches with fish predators. These results indicate that individuals from reaches with predatory trout are responding to fish predators by downregulating corticosterone while maintaining an active HPI axis. This may allow larvae more time to grow before metamorphosing, while also allowing them to physiologically respond to novel stressors. However, prolonged downregulation of corticosterone release rates can impact growth in post-metamorphic individuals. 

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