Dissolved Oxygen:
Brown trout require a Dissolved Oxygen (DO2) of 9mg/l or higher for a healthy population. 6mg/l is the short term lower boundary and anything below this creates stress on the fish. Below 3mg/l is lethal for brown trout – they asphyxiate through lack of oxygen.
Temperature:
In terms of temperature, fish generally don’t modify their internal temperature very well, so they are affected heavily by water temp. The best temp for maximum growth rate of a brown trout is 13.5 degrees, but most of the time temps are outside this optimum. Anything above 20 degrees can be lethal for trout caught even using best practice catch and release techniques, so that’s the upper alarm. That’s according to the wild trout trust. Actually lethal temperatures without placing further stress on brown trout is 26.7 degrees, that is for 1000 minutes or 17 hours, according to a paper on the EA gov site. The lower limit is just less than 4 degrees for brown trout, below which growth seems to stop, reflecting high stress levels.
Other Factors:
It’s also to be noted that at higher temperatures other factors, pollution, plant growth etc is much faster, causing oxygen depletion, so at higher temperatures it’s even more important to measure oxygen levels, and minimise any pollution.