Brown trout depend on cold, well-oxygenated water, with an ideal temperature range of about 13°C to 19.5°C. As temperatures rise above this range, trout experience metabolic stress, reduced growth, depleted fat reserves, and eventually cardiac problems. At around 24°C and above, conditions can become lethal due to low oxygen and physiological shock.

Fishing should stop once water reaches about 18°C because handling fish in warm water can cause severe stress and delayed mortality. A fish tank thermometer costing £5 can be bought on Ebay.