DOI: 10.5061/dryad.5qfttdzh5
title: Determining the upper thermal tolerance of Athabasca rainbow trout (Oncorhynchus mykiss) across naturally varying stream temperatures in the Athabasca River watershed

description: Stream temperature is a key driver of physiological function in
 ectothermic fish, and fish have clear upper and lower limits to thermal
 habitat use. Stream temperature increases from climate change are a major
 threat to coldwater taxa like Athabasca rainbow trout (Oncorhynchus
 mykiss; ARBT), listed as Endangered under Canada’s Species at Risk Act. To
 better understand their vulnerability to climate change and prioritize
 recovery locations, ARBT were collected in August 2021 from three streams
 in the upper Athabasca River, Alberta, Canada, across a gradient of
 thermal regimes ranging from cold to warm. Individual thermal tolerance
 was measured using two physiological metrics: agitation temperature (Tag)
 and critical thermal maxima (CTmax) using a portable streamside
 laboratory. Tag is a behavioural indicator of a thermal avoidance
 threshold, whereas CTmax is a physiological response that can be
 interpreted as the upper thermal limit where long term survival is
 reduced. Results provide strong evidence that CTmax is a plastic metric in
 ABRT, however Tag is not. This suggests that ARBT likely have a moderate
 to limited thermal plasticity and will be forced to adapt or find new
 habitats under warming stream temperature regimes.
description: Upper thermal tolerance experiments were conducted in a mobile
 streamside laboratory trailer to determine CT<sub>max</sub>
 and T<sub>ag</sub>. Large heating tanks were filled to a
 predetermined volume with stream water ensuring the experimental starting
 temperature reflected resident thermal conditions. Throughout the
 experiment, fish were individually housed in small acrylic chambers with
 mesh ends (n = 6). Individual chambers functioned to reduce social and
 physical interactions between study fish while permitting continuous water
 exchange between the chamber and tank. Water temperature in the tank was
 controlled by five submersible heaters (TH-0300S titanium heaters, Finnex,
 Chicago, IL, USA) to gradually increase the temperature at a standardized
 heating rate of 0.3 °C/min (Becker & Genoway, 1979) for both
 CT<sub>max</sub> and T<sub>ag</sub> trials, with
 temperature monitored to ensure an accurate heating rate. Two circulation
 pumps and air stones were also placed in the tank to ensure uniform water
 temperature and saturated dissolved oxygen levels. Two experimental trials
 were conducted daily using identical replicate tank systems to increase
 fish throughput. On the morning of each experiment, six
 fish were removed from the instream holding bags where they were placed
 the day before and moved into individual chambers within the heating tank.
 Fish were acclimated for 2 h at a stable temperature to reduce potential
 stress associated with handling and a new environment. Once the
 acclimation period at ambient stream temperature was complete, the dynamic
 heating period began. Fish movement was carefully monitored for
 behavioural cues that define the onset of agitation.
 T<sub>ag</sub> was always observed before the onset of
 CT<sub>max</sub>, and was defined as 5s of consistent burst
 swimming or aggressive turning inside individual confinement chambers
 (McDonnell & Chapman, 2015). CT<sub>max</sub> was
 defined by a loss of equilibrium (inability to remain upright) and/or
 disorganized locomotory movement (Beitinger et al., 2000). Fish were
 immediately removed from the warm experimental tank when
 CT<sub>max</sub> was observed and placed in a cool recovery
 tank. Fork length (in mm) and wet body mass (in g) measurements were
 recorded, and a fin clip was taken from the left pelvic fin of each fish
 for genetic analysis (n = 120). Once fish had regained equilibrium and
 recovered, they were released back into the stream at the site of
 capture.  Rainbow Trout of hatchery origin were
 historically stocked throughout the Athabasca River watershed (COSEWIC,
 2014). To ensure that all fish used in the data analysis were genetically
 pure, fin clips from all assayed fish were genetically analyzed for
 introgression with stocked Rainbow Trout. All protocols closely followed
 those of Geraldes and Taylor (2021). Tissue samples were digested and DNA
 was extracted and amplified to estimate the ancestry coefficient between
 wild and hatchery RBT groups, expressed as the proportion of the genome of
 each fish that was of wild origin (i.e., QH). Although five tissue samples
 had insufficient genetic material for analysis, the remaining 115 samples
 were all genetically pure with an introgression coefficient Qi &gt;
 0.95, indicating genetic introgression was rare in experimental fish
 caught in McPherson, Anderson, and Wampus creeks. Based on these results,
 it is reasonable to infer that all experimental fish (n = 120) were
 genetically pure ARBT. Long-term acclimation
 temperature prior to measurement of thermal tolerance calculated as the
 mean water temperature at each site for 7 days preceding the experiment
 based on observed temperature logger data. However, 2021 temperature data
 was missing from the upstream site in McPherson Creek, and was
 subsequently inferred based on the water temperature relationship between
 up and downstream sites of McPherson Creek measured the following
 summer. Data from thermal tolerance experiments was
 excluded if behavioural observations were deemed inaccurate or ambiguous.
 Three T<sub>ag</sub> data points were excluded when fish
 either expressed agitated behaviour from the start of the experiment, or
 if fish failed to express any agitated behaviour prior to
 CT<sub>max</sub>. Five CT<sub>max</sub>
 observations were excluded when fish demonstrated full recovery of
 equilibrium and activity immediately after being placed in the recovery
 tank, as this indicates a behavioural misinterpretation of loss of
 equilibrium, and that fish were prematurely removed from the
 experiment.
description: # Determining the upper thermal tolerance of Athabasca rainbow trout
 (Oncorhynchus mykiss) across naturally varying stream temperatures in the
 Athabasca River watershed.
 [https://doi.org/10.5061/dryad.5qfttdzh5](https://doi.org/10.5061/dryad.5qfttdzh5) Contact Sarah Hnytka with any questions: [sarah.hnytka@dfo-mpo.gc.ca](mailto:sarah.hnytka@dfo-mpo.gc.ca) ## Description of the data and file structure Streamside thermal physiology experiments were conducted on Athabasca Rainbow Trout during July and August 2021 near Hinton, Alberta, Canada. Fish were collected from three streams using electrofishing and transferred to experimental tanks. The water temperature in the tanks was gradually increased at a rate of 0.3°C per minute, while the fish were observed for signs of agitation (Tag) and critical thermal maxima (CTmax). Upon reaching CTmax, the fish were immediately transferred to a colder recovery tank. Genetic fin clips, length, and weight measurements were recorded, and the fish were released back into their respective streams once they had fully recovered. ### Files and variables #### File: ARBT.CTmax.Data.FACETS.csv **Description:**  ##### Variables * Experiment Date: (DD-MMM-YY) * Site: Site Code: WP = Wampus, MP = McPherson, AN = Anderson, 1 = Upstream site, 3 = Downstream site * Stream: Wampus, McPherson, Anderson * Starting Time: hh:mm * Starting water temperature (C): Water temperature in the tank when the experiment began, measured using YSI. * Trial: The trial is one round of the experiment  * Tank: We used 2 tanks, the left one was 1, the right tank was 2. * Fish ID: number given to each fish * Total Length (mm): Total length of the fish, measured in mm * Fork Length (mm): Fork length of the fish, measured in mm * Weight (g): wet weight of the fish, measured in grams * Tag (C): Agitation Temperature, the temperature the fish became agitated (5 seconds of burst swimming, or aggressively turning around in chamber) * CTmax (C): Critical Thermal maxima, the temperature the fish reached CTmax  (unable to maintain equilibrium) * Taw (C): Window, the difference between Tag, and CTmax.  * TSM (C): Thermal Safety Margin. The difference between CTmax, and the highest water temperature recorded at the corresponding site. * Max site temperature (C): The highest temperature recorded from our temperature loggers at each site in the 2021 season, using a HOBO pendant logger, MX2202. **NA description:** * Tag: NA values for Tag imply that the fish either did not become agitated at all before CTmax occurred, or if it exhibited an agitated behaviour throughout the entire experiment, suggesting the agitation was from external stress not related to temperature. These values were removed from our analysis. * Taw: If Tag values were NA, the corresponding Taw were also NA, as Taw is calculated using Tag. ## Access information Other publicly accessible locations of the data: * [https://figshare.com/articles/dataset/ARBT_raw_data_FACETS_xlsx/24921345](https://figshare.com/articles/dataset/ARBT_raw_data_FACETS_xlsx/24921345)