Some like it hot, trout do not.

April 24, 2017

New real time water temperature gage provides valuable river information in the Upper Clark Fork.

The Clark Fork Coalition recently teamed up with the George Grant Chapter of Trout Unlimited, Montana Fish, Wildlife & Parks (FWP), and the Natural Resource Damage Program (NRDP) to fund a seasonal water temperature monitoring site on the upper Clark Fork River (near Garrison, MT) for the summer of 2017. The data is collected in real time and is currently available on the USGS’s National Water Information System web site.

Although there are currently a number of groups collecting water temperature data in the upper Clark Fork, none of the information was available to resource managers or the public on a real time basis. The only other real time water temperature site on the Clark Fork River is located on the lower river, 170 miles downstream of Garrison near Superior.

The Garrison site is located along an integral stretch of the upper Clark Fork River (from a fisheries perspective) and has experienced drought conditions and warm water temperatures (in excess of 75°F) over the past two summers. These warm water temperatures prompted FWP to close the upper Clark Fork River to fishing during 2015 and 2016. Due in part to ongoing restoration efforts, the upper Clark Fork River has also experienced a substantial increase in sport fishing use (both commercial and non-commercial) over the past 5 years. The new temperature gage will help FWP respond to extreme summer water temperatures and provide outfitters and recreational fishers valuable information before they hit the water.

A long history of mining related impacts to water quality and dewatering of tributaries has all but expatriated native trout from the mainstem of the upper Clark Fork River. The fickle populations of native cutthroat trout and bull trout that remain must survive summer drought conditions and completion from non-native trout species.

Warmer water contains less dissolved oxygen than colder water and as water temperatures rise above a certain level, fish begin to experience stress. These stresses begin to set in well before the temperatures reach lethal limits. Fish that are already temperature stressed will be dramatically more stressed after being hooked and attempting to fight for freedom. In many cases, a fish even properly handled and released under thermally stressful conditions may not survive. As illustrated in the table below, non-native trout have higher temperature thresholds and a competitive advantage over native trout in warm water conditions.

Species Fundamental Niche Lower (°F) Optimum Growth Temperature (°F) Fundamental Niche Upper (°F) Upper Incipient Lethal Temperature (°F)
Rainbow Trout 50.2 55.6 57.4 75.7
Brown Trout 57 62.4 64.2 76.5
Brook Trout 51.8 57.2 59 76.1
Cutthroat Trout 51 56.5 58.2 67.2
Bull Trout 50.4 55.8 57.6 69.6

Table 1: Thermal tolerance of trout species using the acclimated chronic exposure method (ACE) to assess upper thermal limits and growth optima. The “fundamental thermal niche” for fish is defined by Christie and Reiger (1988) as “-3 and +1 °C” around the optimal growth temperature. The upper incipient lethal temperature (UUILT) for each species was calculated as the temperature that was lethal to 50% of test fish as the end of the controlled 60-day experiment. Data sources: Bear et al., 2007; Selong et al., 2001.

In the face of declining budgets, the USGS has been removing monitoring locations, not expanding their network, and relied on outside funding mechanisms to maintain the current aquatic monitoring system. The USGS tracks its stream monitoring sites that have recently been defunded and also those that are currently “threatened and endangered”. Presently, there are 117 stations nationwide that may soon be discontinued by the USGS due to lack of funding (one being a long term gage on the Yellowstone River in MT).

The cost associated with adding real time seasonal temperature monitoring at Garrison for 6 months was $1,400. The CFC would like to thank our contributing partners (George Grant TU, MT FWP, NRDP) and members for their financial support.

And please remember, when it’s hot outside, check the water temperatures before you hit the river. Fishing when water temperatures are over 68°F may detrimentally impact native fish. There are hundreds of mountain lakes in Western MT that are full of trout and remain cool all summer long!


Bear, E. A., McMahon, T. E., & Zale, A. V. (2007). Comparative thermal requirements of westslope cutthroat trout and rainbow trout: implications for species interactions and development of thermal protection standards. Transactions of the American Fisheries Society, 136(4), 1113-1121.

Christie, G. C., and H. A. Regier. 1988. Measures of optimal thermal habitat and their relationship to yields for four commercial fish species. Canadian Journal of Fisheries and Aquatic Sciences, 45:301–314

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