The heat is on: a look at the ability of cold-water fish to tolerate climate warming

Whether you are sitting lakeside or beach-side on a sweltering summer’s day, there is no doubt that you will start to feel a little sluggish as the temperature creeps upwards of 30°C. As the sun sits high overhead, you may even begin to nod off, only to wake minutes later searching clumsily for your (once) cold drink and a bit of shade. You head inside, put some ice in your drink and bask in the beauty of AC until your next venture outside – after all, we only get a few weeks of this in Canada. You reached what’s known as your upper thermal limit, and luckily you are able to seek refuge and cool off with a cold-one.

All species have an optimal temperature range in which they are able to efficiently forage, reproduce, and take part in other behaviours necessary for survival. This is known as an organism’s thermal window, which, for many species of fish is heavily influenced by water temperature or their environment’s thermal regime. As long as the local environmental thermal regime lines up with an organism’s thermal window, there’s no problem. It’s when these become farther apart that problems can occur. Cue climate warming.

As water temperature increases, fish react in a number of ways: they seek deeper pools, shade, and eventually decrease the frequency or duration of any behaviour that expends energy (such as feeding, reproducing, or humouring your fly). These aren’t necessarily dangerous in the short term, but can be devastating to a population or species in the long run (and your fishing). As climate warming is expected to increase water temperatures anywhere from 0.7 to 7.4°C over the 21st century (Rouse et al. 1997; Heino et al. 2009; Qin et al. 2014), we need to better understand the tolerance of fish species, particularly cold-water ones and use this information to create precautionary management strategies.

Two studies have come out of Trent University over the past few years (McDermid et al. 2012; Stitt et al. 2014) that have looked atthe critical thermal maximum (CTmax) of three distinct brook trout populations in Ontario; essentially, the temperature at which these brook trout can no longer maintain upright equilibrium, and instead go belly up. They had three main questions: is the thermal tolerance of each population of brook trout the same? Does their CTmax change if they’re allowed to acclimate at a higher temperature beforehand? If so, does this vary by population?


McDermid et al. (2012) used fish that were raised at the same hatchery but whose ancestors originally came from three different lakes: Hill’s Lake, Lake Nipigon, and Dickson Lake. They put fish from each population in water that was increased from ambient temperature over 1.5 hours to 26°C and then they measured how long it took for them to lose equilibrium.

In addition, Stitt et al. (2014) examined how acclimation affects the CTmax of the three different fish populations. They allowed subsets of fish from each population to sit at cold, cool, and warm water temperatures over four weeks and then they used the same test to determine their CTmax but at a slightly higher temperature of 27.5°C.  Compare it to joining your friends in a hot tub. On a hot day, you might be able to jump right in to the hot tub, however on cold days, even though all your friends are laughing at you, you probably lower yourself into the hot tub slowly allowing each body part to stop stinging before going further.

McDermid et al. (2012) found that of their three populations, the Northern-most one had the lowest CTmax (an average time of 87 minutes before they lost equilibrium), while the Southern-most population had the highest (an average time of 127 minutes). Essentially, fish whose ancestors were used to slightly warmer waters were better able to tolerate water at the edge of their thermal window, while those whose ancestors were used to colder waters were not; interestingly, this relationship with latitude has been maintained after multiple generations of hybrid wild/captive spawning.

Stitt et al. (2014) found that, if the fish were acclimated at a higher temperature, they had a higher thermal tolerance.  Interestingly, they found that at the lower acclimation temperatures, population of origin still mattered for CTmax, with the fish from the warmest lake on average having the longest CTmax. However, when the fish had been sitting at the highest acclimation temperature of 20 degrees Celsius there was no difference between the different populations.

So what’s the takeaway here? Simply put, fish are complicated and diverse. In the face of climate warming, cold-water species such as brook trout are particularly vulnerable, even though these fish are known to be highly plastic in the face of environmental change. If temperature increases are slow enough, the acclimation time may allow some populations to adapt. However, thermal tolerance has a limit, and although that limit may vary slightly population to population, it is more likely to be a combination of climate warming and lack of refugia or some other random event that will put an individual at risk. In order to maintain healthy populations, we need to provide these fish some sort of relief: avoid fishing on hot days, or if you feel you really need to, make sure to stick to streams with lots of refugia (deep pools etc.). Also, keep the time you’ve spent angling a fish in mind, and allow them time to acclimate before release.

These studies show the huge amount of variation found within a single species of fish, and shed light on the complex issues that fisheries managers face when attempting to target and rescue populations at risk. Understanding differences between seemingly identical populations is difficult yet important, and may result in different management strategies for, in this case, populations at different latitudes.

 Zachery Wells

MSc Candidate, Concordia University


McDermid, J. L., Fischer, F. A., Al-Shamlih, M., Sloan, W. N., Jones, N. E., & Wilson, C. C. (2012). Variation in acute thermal tolerance within and among hatchery strains of brook trout. Transactions of the American Fisheries Society, 141(5), 1230-1235. Request Full Text

Heino, J., Virkkala, R., & Toivonen, H. (2009). Climate change and freshwater biodiversity: detected patterns, future trends and adaptations in northern regions. Biological Reviews, 84(1), 39-54. Full Text

Qin, D., Plattner, G. K., Tignor, M., Allen, S. K., Boschung, J., Nauels, A., Y. Xia, V. Bex, & Midgley, P. M. (2014). Climate change 2013: The physical science basis (p. 1535). T. Stocker (Ed.). Cambridge, UK, and New York: Cambridge University Press. Full Text

Rouse, W. R., Douglas, M. S., Hecky, R. E., Hershey, A. E., Kling, G. W., Lesack, L., Marsh, P., McDonald, M., Nicholson, B.J., Roulet, N.T. & Smol, J. P. (1997). Effects of climate change on the freshwaters of arctic and subarctic North America. Hydrological Processes, 11(8), 873-902. Full Text

Stitt, B. C., Burness, G., Burgomaster, K. A., Currie, S., McDermid, J. L., & Wilson, C. C. (2014). Intraspecific Variation in Thermal Tolerance and Acclimation Capacity in Brook Trout (Salvelinus fontinalis): Physiological Implications for Climate Change*. Physiological and Biochemical Zoology, 87(1), 15-29. Full Text

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