Loss of riparian forests from wildfire led to increased stream temperatures in summer, yet salmonid fish persisted

Abstract

Native salmonid fishes-cutthroat trout (Oncorhynchus clarkii) and steelhead/rainbow trout (Oncorhynchus mykiss)-are ecologically, culturally, and economically important species distributed across western North America. These fish are generally considered "cold-water" adapted species. As such, recent studies have speculated about the potential effects of climate change on these native salmonids if stream temperature thresholds exceed 16-20 C during the summer. However, the magnitude of stream thermal responses to the slow but steady increases in regional temperatures associated with climate change remains uncertain and hard to predict. Comparatively, abrupt disturbances, such as wildfire, may produce almost instantaneous and substantial increases in stream temperatures that may persist for multiple years until near-stream vegetation becomes re-established. In the first summer following a severe wildfire in western Oregon, we observed the initial persistence of populations of O. clarkii clarkii (coastal cutthroat trout) and O. mykiss (rainbow/steelhead trout). The fire burned the entire catchment, including the riparian area (~76% of the watershed area burned at moderate or high severity), resulting in stream temperature that regularly exceeded 20 C and represented increases of 6-7 C relative to prefire conditions. However, the mechanisms enabling the persistence of cold-water fishes despite the dramatic increases in stream temperature remain unclear and require further investigation. Nevertheless, wildfires represent acute natural disturbances that can substantially alter stream thermal regimes and provide unique insights that allow us to better understand how native fishes in natural systems cope with projected increases in stream temperatures. K E Y W O R D S cutthroat trout, Oncorhynchus clarkii, Oncorhynchus mykiss, rainbow trout, riparian forest, steelhead trout, thermal tolerance, wildfire

Fire impacts on the biology of stream ecosystems: A synthesis of current knowledge to guide future research and integrated fire management

“…(2014) observed that trout in the burned sites were consuming smaller and less prey than fish in reference sites, which along with increased water temperatures resulted in bioenergetically stressful conditions and decreases in fish biomass. Indeed, all six studies in this review that reported fish length observed an increase in burned sites, which are likely related to warmer water temperatures that lead to faster growth, which in turn can exacerbate competition for limited food resources and lead to lower lipid content (Rosenberger et al., 2015) or lower fish condition (Warren et al., 2022). Regarding riparian predators, two out of four studies reported an increase in the emergence of insects in sites that burned several years ago, which could partly explain the greater density of spiders observed in burned sites in two out of four studies, as well as the greater bat echolocation reported by one study (Harris et al., 2018; Malison & Baxter, 2010; Mellon et al., 2008).…”

Rising water temperature in rivers: Ecological impacts and future resilience

“…Although we have a strong understanding of the direct effects of temperature on physiology under controlled laboratory conditions, many factors including food availability, may modify outcomes and lead to unexpected changes in river ecosystems (Solokas et al, 2023; Warren et al, 2022). Therefore, past research conducted under controlled, experimental conditions may not reflect actual responses to climate change because of the complex interactions between organisms, resource availability, and environments.…”

Fish Blood Response to Ash-Induced Environmental Alkalinization, and their Implications to Wildfire-Scarred Watersheds

“…In addition, the high pH exposure and subsequent rise in blood pH e reduce [H + ] and hinders the process of ammonia excretion as increased proportion of gaseous NH 3 (compared to ionic NH 4 + ) in the environment slows the outward diffusion of NH 3 as well as net total ammonia excretion rate, which ultimately results in a rapid rise in blood total ammonia (Wilkie and Wood, 1991, 1996; Wilkie et al, 1993). The alkalosis-induced disruption to ammonia excretion is likely further aggravated by increased ambient temperature typical of aquatic systems after the wildfire denudes the overhead shading vegetation in riparian habitats (Warren et al, 2022), and this trend is observed in the majority of studies (Paul et al, 2022). Warmer temperature would inevitably elevate basal metabolic demands, increases organismal metabolism, promote faster ammonia production, and decrease the energy budget available for mitigating the IOA-B disturbance (Gomez Isaza et al, 2022).…”