Following on from this post, inspired by discussion on the flystream forums, here I would like to talk about temperature tolerances of both brown and rainbow trout. While this is a seemingly simple question, like anything once you start looking at it in detail it gets a little more complicated. Firstly, there trout have an optimum or preferred temperature range as well as a range of temperatures that they tolerate but don’t thrive in. The upper limit of persistence is often determined using either the critical thermal maxima (CTM) or upper incipient lethal temperature (UILT). While both methods aim to measure the same thing, the methodologies are quite different. CTM is measured by slowly exposing the fish to dynamic temperature change up until a pre-defined sub-lethal end point, usually determined by the loss of equilibrium or muscle spasms. In contrast UILT is measured by dunking fish acclimatised to a certain temperature into water of a different temperature and recording mortality over time and an estimate of the upper temperature tolerated by greater than 50% of fish is determined. Both methodologies have their drawbacks, but it could be argued CTM is a more ecologically important measure as it measures something that is more likely to happen in natural environments. That said neither measure is likely to be perfect and I’m not a fisheries biologist, so not fully versed in the respective weaknesses.
That aside, the temperature tolerances of salmonoids are widely studied, so getting an answer should be easy right? So what are the conclusions of those studies? In general, the optimum temperature range for brown trout is between 10-19 degrees, 15 often being cited at the optimum temperature for growth. The optimal temperature for rainbow trout is very similar, but slightly higher between 10-22 degrees. The CTM or UILT for brown trout, the temperature at which their ability to tolerate a temperature is generally between 22-27 degrees. For rainbow trout the CTM or UILT is generally between 24-29 degrees. Lethal temperatures in those ranges are the most common amongst studies, however higher and lower CTMs have been observed in both brown and rainbow trout depending on the thermal/genetic history of the fish (for example tolerances of up to 32 degrees have been observed in rainbow trout).
So why all the variability? Of primary importance is likely to be genetic variation. Temperature resistance is likely to be genetically controlled and there is likely to be differences between populations of fish which is likely to explain some of the variability seen in the studies of trout thermal maxima. For example, while optimum growth temperatures for brown trout are most often between 10-19 degrees, in a 1970s study Brynildson observed optimal growth of brown trout between 18.3-23.9°C. Genetic differences and local adaptation are the most likely cause of this large difference in optimum temperature tolerances and is also likely to explain some of the variability in thermal tolerances.
Of course, there are also numerous other considerations when talking about temperature tolerances in trout. For example the duration of exposure to high water temperatures is important. Fish may be able to survive for short periods of time in very warm waters, but long term exposure to those same elevated temperatures may eventually be detrimental and eventually lethal. For example, young steelhead are able to tolerate temperatures of around ~30 degrees short periods, however high mortality is seen when water temperatures are greater than 25 long periods.
Of course in complex natural stream environments the dynamic may also be different. The maximum temperature in a stream may get above the thermal maxima and trout may still continue to persist in that stream. There may be multiple reasons for this. Firstly, trout may be using cold water refuges, avoiding the hottest areas in the stream and thus avoiding habitat that is outside the physiological persistence parameters of the fish. Secondly stream temperatures vary throughout the day. So while stream temperatures may reach a maximum point above some theoretical thermal maxima, trout may be able to continue to persist if stream temperatures return to more comfortable temperatures quickly enough.
Other factors are also likely to change the thermal tolerances of trout. For example, socially stressed rainbow trout have lower thermal tolerances compared to their dominant siblings. That is social stress reduces the ability of rainbow trout to persist under high temperature conditions. This is likely to hold for other types of stress, such as poisons, oxygen levels competition and handling by catch and release fishermen. It is widely agreed that stress levels increase in trout in waters above ~20 degrees. A base level of heat stress is likely to reduce the ability of trout to handle other forms of stress and the opposite is also likely to be true, i.e. other forms of stress will compromise their ability to handle heat stress.
Coming back to a real world view of the thermal tolerances of trout, a stream is a complex habitat. It will likely have cold water refuges and will often allow fish to move to colder higher elevation waters as temperatures increase. These factors may mean trout can persist in streams that get above the theoretical thermal maxima of trout. There is also the chance that streams that experience regular high temperatures have produced trout populations locally adapted to high water temperatures, trout that can persist in outside the “normal” thermal tolerances of trout. At some point though there is likely to be a limit to how much trout population can adapt. For example Firehole river trout, live in a warm thermal stream in Yellowstone national park. During summer the stream often reaches temperatures close to or above the thermal tolerance for trout. It appears that trout persist in Firehole river by using cold water refuges. However, despite the fact that the river experiences regular high temperatures Firehole trout show no difference in CTM to hatchery raised fish, suggesting they do not have the genetic tools available to them to increase absolute temperature tolerances. At some point most populations of trout will hit that point, they will run into a physiological brick wall so to speak, where they can no longer adapt to higher water temperatures. From that point on, if temperatures continue to rise, trout will not be able to adapt and will disappear from sections of rivers where temperatures are too high. Their distribution will be restricted to cooler, higher elevation streams that stay within the physiological parameters necessary for the persistence of trout. Globally there is a lot of concern about the potential effects climate change will have on the distribution of salmonoids. In the medium to long term (20-100 years), rising temperatures are likely to severely restrict the distribution of many salmonoids globally. Australia is no exception.
Lastly, just because trout can survive at a certain temperature doesn’t mean they will be competitive with other fish species at that temperature. As temperatures approach the thermal tolerances for trout, they are likely to become increasingly susceptible to being outcompeted by other fish species. In terms of Australian waterways, the most likely competitors are redfin and carp as both species that have higher temperature tolerances than trout and are some of the most abundant fish in lower elevation environments. It is of interest that in the trout surveys done by the DEPI in February, at the lower survey sites on both the Howqua and Jamieson rivers, where very few trout were caught, carp, redfin and roach dominated the fish sampled. It is possible that at lower temperatures, trout outcompete both carp and redfin, but that at higher temperatures the competitive advantage is reversed. For an example of this dynamic, brook and brown trout outcompete creek chub when water temperatures are lower but are outcompeted by chub as temperatures approach their thermal maxima for both trout species. This work suggests there is a “turnover” of the species composition, from trout to non trout in those streams happens somewhere between 22-25 degrees, before streams reach the thermal maxima for trout, when the competitive advantage trout have in colder waters if flipped by increased water temperatures. While in no way conclusive the survey results from the Howqua and Jamieson rivers do suggest that a similar dynamic could potential be at play in the lower sections of those rivers, however that just me wildly speculating and a lot of work would be needed to confirm it.
Summing up: while the temperature tolerances of trout is widely studied and easy enough to assess, at least in some cases, genetic variation may allow some populations to survive in waters that fall somewhat outside the “normal” thermal range of the species. Generally though, water temperatures become lethal between 22-27 degrees for brown trout and between 24-29 degrees for rainbow trout and fish start experiencing stress above about 20 degrees. Added to this, numerous other factors may affect trouts ability to tolerate temperatures at the edge of their thermal tolerance. Pollution, poisons, competition and social stress may all lower the temperature tolerances for trout, meaning it may not always be safe to assume that trout will persist in waterways where temperatures get high but don’t go into the lethal range for trout if other stressors are present, e.g. competition from redfin and carp, pollution or oxygen stress.
Finally, I threw this together pretty quickly, its in no way definitive on the subject, after all, I’m no fisheries biologist 😉