Water Temperature and Dissolved Oxygen Measured During a Manipulative Thermal Challenge Experiment for Adult Salmonids, Yukon River, Alaska, 2018

The experimental design required three Chinook Salmon captured each morning by the ADF&G test fishery for nine days, for a total of 27 individuals. All individuals used in this study were captured using 45.7 m long by 8.00 m deep drift gill nets made of double knot multifilament nylon twine with mesh sizes ranging from 6.98 cm to 21.59 cm. All individuals included in this experiment appeared vigorous.

Initial heating of the experimental tanks from ambient river temperature to experimental temperature was done with a submersible liquid propane gas (LP) stock tank heater. The LP heater analog thermostat was manually adjusted as necessary to maintain the desired rate of heating. The most consistent results were achieved by increasing the thermostat until the burner of the heater ignited, letting the heater run for 2–3 min, then lowering the thermostat to extinguish the burner and let the warmed water close to the heater circulate through the tank. When the water in each tank approached the desired experimental temperature, the LP heater was shut off and four electric aquarium heaters per tank were used to finely control and maintain the desired treatment temperature with two digital heater controllers. Temperature controllers and electric heaters were suspended over the water using metal bars secured across the top of the tanks. All electricity for the temperature controllers (3 tanks x 2 temperature controllers per tank = 6 temperature controllers) and electric aquarium heaters (3 tanks x 4 heaters per tank = 12 heaters) was supplied by a single gasoline-powered portable generator, that produced 1800 watts of power.

Water tanks used in this experiment were 587 L (155 U.S. gallon) oval polyethylene stock tanks commonly used in agriculture. A 1.90 cm (0.75 inch) ball valve was plumbed into the drain hole of the tank and connected to a hose running back to the river to allow easier draining. Water was pumped from the river into each tank (n = 3) with a gasoline powered pump prior to each experimental trial and drained back into the river by gravity feed at the conclusion of each day.

Following capture, fish were transferred to polyvinyl chloride (PVC) holding tubes in a live well aboard a skiff. The live well was filled with water directly from the river (~14 ℃). The PVC holding tubes were opaque white, 20 cm diameter by 90 cm long, with a series of 2.50 cm holes drilled along the length of the tube, and 6.35 mm mesh end caps to allow water to flow through. Keeping the fish in labelled tubes allowed for easy identification and reduced further handling when transferring fish into experimental tanks. Due to the size limitation of the holding tubes and tanks, fish that were > 900 mm mid-eye to fork of tail, approximately the 90th percentile in the population, were excluded from the experiment (Jasper and Evenson, 2006). The size of the experimental tanks was limited by the need to fit components into the aircraft. Different transportation methods may allow for larger scale tanks.

Fish were transferred from the live well on the skiff to the experimental tanks immediately upon the skiff’s arrival at the experiment site. The live well was refreshed with fresh river water to maintain oxygen and water temperatures similar to the river whenever the skiff was awaiting an additional fish from the test fishery crew.

River temperature was recorded in the morning and used to determine the temperature of the control tank. Temperature loggers (Onset TidbiT v2, https://www.onsetcomp.com/) positioned near and opposite the LP heater recorded temperature every five minutes. Prior system testing indicated that water circulation pumps were effective at preventing thermal stratification in the tanks.

Dissolved oxygen concentration was recorded every 30 minutes using a YSI Professional Plus multiparameter meter (YSI Incorporated, Yellow Springs, OH, https://www.ysi.com/). Dissolved oxygen was supplemented using battery powered aerators and a submersible pump to cycle water at a rate of 30 L min-1.

The submersible pump outflow was routed through a vinyl hose (1.58 cm inside diameter) to create an approximately one-meter waterfall back into the tank. Manually agitating the water in the tanks using a bucket was also implemented to supplement dissolved oxygen. Water from the tank was repeatedly scooped and poured out of the bucket from a height of approximately one meter for one minute of continual agitation every 30 minutes. The agitation was applied equally across treatments such that any potential stress caused by the disturbance would not be a confounding factor.

To mitigate against potential toxicity from by-products of fish metabolism (Wicks et al., 2002), a known volume of water in the tank was replaced approximately every 30 minutes with fresh river water. The volume of water exchanged was approximately ~10% of tank volume and frequency of water changes was once an hour, and only occurred during the initial heating of the tank to experimental temperature.

All fish were sacrificed at the end of the four-hour treatment period by cranial concussion, cervical dislocation, and exsanguination, followed immediately by tissue sample collection.