Fish Behavior & Mortality During a Monsoonal Spate in the San Juan River, UT, USA

Authors: James Whitney, Blake Hansen, Amber Bell, Michaela Fishback

Year: 2022

Abstract

The Southwestern United States monsoon season can produce heavy precipitation that results in high flow events. Fish kills can occur when these monsoonal freshets derive from runoff over recently-burned catchments, but less is known concerning monsoonal-induced fish kills occurring in unburned riverscapes. Here we report on a fish kill unrelated to wildfire that occurred following a monsoonal spate in the San Juan River, Navajo Nation, Utah, United States, which we observed while camped on an island (latitude = 37.12657, longitude = -109.10227) on 24-25 July 2021. During this event discharge increased by 152 cubic meters/second (5,368 cubic feet/second) in 2.5 hours (i.e. 8:30-11:00 PM on July 24). At the peak of the high flow there were numerous fishes that sought refuge in the slack water amid the flooded vegetation of our island. Many of these fishes were lethargic and gulping for air, although some also appeared to be surface feeding. By the morning of July 25th waters had receded, leaving behind numerous dead fishes. We inventoried all the dead large-bodied (i.e. total length ≥ 100 mm) individuals we could find along a 70 m search path on the eastern side of the island, where we located 14 Channel Catfish, Ictalurus punctatus, 5 Bluehead Sucker, Catostomus discobolus, 4 Colorado Pikeminnow, Ptychocheilus lucius, 3 Flannelmouth Sucker, Catostomus latipinnis, and 1 Razorback Sucker, Xyrauchen texanus. We also observed dead small-bodied (i.e. total length < 100 mm) individuals that were too numerous to count, including Channel Catfish, Fathead Minnow, Pimephales promelas, Red Shiner, Cyprinella lutrensis, Speckled Dace, Rhinichthys osculus, Virile Crayfish, Faxonius virilis, and hellgrammites (Order Megaloptera, Family Corydalidae). Finally, we also note that on the morning of July 25th there were living Channel Catfish on our island that were stranded in remaining pools. We suspect that hypoxia and high suspended sediment loads caused organisms to suffocate, but more research is necessary to identify the cause of death.