Effects of Catch-and-Release Angling on Salmonids at Elevated Water Temperatures

Abstract

Few studies have assessed catch-and-release mortality of salmonids at water temperatures of 23°C or above, despite predictions of warming stream temperatures due to climate change. The primary objective of this study was to measure the catch-and-release mortality of rainbow trout Oncorhynchus mykiss, brown trout Salmo trutta, and mountain whitefish Prosopium williamsoni in three water temperature treatments, namely, when daily maximum water temperatures were cool (<20°C), warm (20–22.9°C), and hot (≥23°C). A secondary objective was to assess the catch-and-release mortality of salmonids angled in morning and evening within the water temperature treatments. These objectives were related to Montana Fish, Wildlife and Parks' drought fishing closure policy. Angling (fly-fishing only) occurred in the Gallatin and Smith rivers. All angled fish were confined to in-stream holding cages and monitored for mortality for 72 h. Mortality of rainbow trout peaked at 16% in the Gallatin River and 9% in the Smith River during the hot treatment. Mortality of brown trout was less than 5% in all water temperature treatments in both rivers. Mountain whitefish mortality peaked at 28% in the hot treatment in the Smith River. No mortality for any species occurred in either river when daily maximum water temperatures were less than 20°C. Mortality of rainbow trout peaked at 16% in the evening hot treatment in the Smith River. Mortality rates of brown trout and mountain whitefish were not related to time of day. The catch-and-release mortality rates presented here probably represent fishing mortality given that most anglers in southwestern Montana practice catch-and-release angling. The mortality values we observed were lower than predicted (<30%) given reports in the literature. The difference is probably related to the in situ nature of the study and periods of cooler water temperatures between peaks, which facilitated recovery from thermal stress.