Effects of water temperature and angling on mortality of salmonids in Montana streams
Boyd, J. W. (James W.)
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In Montana, angling closures are used to protect salmonids from the deleterious impacts of angling at elevated water temperatures (23°C). Catch-and-release angling (CR) studies have reported high levels (30-40%) of salmonid mortality at water temperatures >20°C, but few studies assess CR mortality of salmonids at water temperatures observed in Montana streams during mid-summer (23°C). The primary objective of this study was to measure CR mortality of rainbow trout, brown trout, and mountain whitefish in three water temperature treatments; when daily maximum water temperatures were cool (<20°C), warm (20 to 22.9°C), and hot (23°C). A secondary objective was to assess CR mortality of salmonids angled in morning and evening within water temperature treatments. Based on the literature, mortality of salmonids was predicted to be >30% within the hot treatment and higher in evening than morning. 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 Oncorhynchus mykiss increased to 9% and 16% in warm and hot treatments, respectively. Mortality of brown trout Salmo trutta was (4%) in the hot treatment in the Smith River. Mountain whitefish Prosopium williamsoni had increased mortality in the warm (20%) and hot (28%) treatments in the Smith River. No mortality for any species occurred in either river when water temperatures were <20°C. Mortality of rainbow trout angled in evening was higher than morning in the warm (14%) and hot (16%) treatments in the Smith River. Laboratory results indicated rainbow trout stressed in evening had higher mortality (7%) than those stressed in morning (0%). Angler catch rates were lower for most species in evening than morning angling events; however, catch rates remained high (0.7 fish/h) in several evening angling events. Study results indicate that salmonid mortality rates associated with catch-and-release fly-fishing are higher at elevated (>or equal to 23°C) water temperatures. Although there was a relationship between elevated water temperature and salmonid mortality, most of the mortality estimates were well below the 30% mortality that was predicted.