Wallin, T. and C.A. Caldwell. Comparative assessment of laboratory-derived thermal maxima of Gila Trout (Oncorhynchus gilae) with current stream temperatures. The Southwestern Naturalist 66(4):317-326.
Gila Trout (Oncorhynchus gilae) represents an iconic species of the American Southwest. The salmonid has survived extensive logging, livestock grazing, and mining, and is currently threatened by climate change, wildfire, and extended drought. Long term conservation and recovery of the species and its unique lineages rely on stocking and translocation into historically inhabited streams. Thus, the fish’s thermal tolerance is essential in determining suitable habitat and potential vulnerability to a warming climate. We compared laboratory-derived temperature metric, critical thermal maximum (CTMax), in hatchery-reared Gila Trout from three lineages (Main Diamond, South Diamond, and Whiskey Creek) to determine if CTMax differed among the lineages. The average initial (28.9, SD = 0.81°C) and final (29.2, SD = 0.74°C) loss of righting responses did not differ across the lineages. We compared the CTMax values with stream temperatures across three drainages representative of extant Gila Trout populations from spring 2015 to fall 2018. The East Fork Gila River drainage was represented by more extreme stream temperatures. The maximum daily maximum temperature (i.e., single highest temperature) varied from 24.1 to 33.4°C, exceeding CTMax in four of the five streams. The maximum daily temperature range varied from 12.1 to 28.6°C, and the mean weekly maximum temperature varied from 19.9 to 31.1°C. Mean weekly maximum temperature in Lower Diamond Creek exceeded CTMax on eleven occasions. Stream temperatures were cooler within the Middle and the West Fork Gila River drainages and did not exceed CTMax. Intermittency loggers within the East Fork Gila River Drainage, which supports Main Diamond and South Diamond lineages of Gila Trout, recorded over 300 days of intermittency during a drought. Continued long term monitoring of stream temperature and comparison to CTMax of Gila Trout could assist with decisions of which streams in the Gila Drainage are suitable for Gila Trout recovery, stocking, and translocation.