As climate change’s warming events continue to become more frequent, anglers’ attitudes also need to change in relation to trout and the effects of warmer temperatures. Trout species have optimum feeding temperatures, and some are able to withstand warmer water while others can be intolerant of it. Brown trout, for example, can tolerate slightly warmer temperatures than rainbows, which can tolerate warmer temperatures than sea-run cutthroat.

The temperature differentiation these species can withstand is due to the amount of dissolved oxygen in the water that each can tolerate; essentially, cold water has more dissolved oxygen than warm water. While trout may comfortably feed in warm temperatures, playing them in the warmer water increases mortality as fish revival becomes problematic. This article outlines the ideal temperatures that will prevent catch-and-release mortality and gives some hints on how to target fish based on water temperature.

This Dolly Varden proves that summer fishing at 8 PM pays off!

Using Your Fishing Thermometer

Few things in fishing beat wet wading up your favourite stream or river in the hot summer sun, and several techniques can be used to prevent fish mortality. Attempting to revive a trout exhausted by heat or other factors taints the victory.

A few steps are required for an accurate water temperature reading. The first step, which is possibly the most ignored, is once you remove your thermometer from your pack, let it rest in the shade for 60 seconds. Next, choose a spot on the river that is a moving piece of water with a depth that is average for the river and hold the thermometer here for another 60 seconds. If you doubt the accuracy of your reading, take multiple measurements.

Large river bed exposed in summer

Optimum Temperatures for Trout

The reward of fishing the evening of a hot summer day

It is a common perception that the early bird catches the worm when trout fishing, and this is most certainly true in the summer heat as dawn is the coolest part of the day. Lake surface temperatures and some rivers can fluctuate up to 10°C, so arriving early in the morning provides a chance to beat the day’s heat. Productivity also increases in the evening as trout start to feel the oppressive heat lift and the water cool. Conversely though, winter is an afternoon fishery. I often remain fishless until noon, as the best fishing occurs around 2 p.m. when the day is the warmest. In the warmer months, trout will congregate around stream and tributary mouths, deep dropoffs, lake outflows and inflows, deep pools, underwater springs, behind boulders, and practically anywhere that provides water aeration or the introduction of cold water. Aside from seeking out these more obvious structural habitats, either move up in elevation to alpine lakes or lower to the ocean for some sea-run cutthroat action.

The effects of water temperature on trout pertain to both the dissolved oxygen levels in a river and the rates of metabolism at which fish digest food.The warmer the water becomes, the faster a trout’s metabolism works.

But when the water temperature becomes too warm, at 23°C and over, fish can quickly become exhausted by the lack of oxygen, especially when being fought—just imagine running at high altitude in the heat. As water temperatures rise, fish congregate en masse in cold water refuges and therefore become susceptible to predators and anglers. One should never fish when a river is 23°C or above, so make sure to bring your thermometer in the warmer months. In warm water environments, trout may be fine while swimming behind a riffle or in a pool in a river and beneath the warmer upper layer of a lake, but once hooked they’re taken out of this oxygen-rich area, reducing the chance of a successful revival. While hot temperatures can cause severe lethargy, there is no minimum temperature to fish for trout, as they are comfortably caught at water temperatures of 0°C, although they’re somewhat sluggish.

Since the Goldilocks zone for trout fishing hovers around 11°C and 18°C, it can be posited that this water temperature meets their metabolism and oxygen requirements. If the water is warmer, fish can digest faster (which means they’ll eat more) but if the water is colder, it takes longer to digest food so they can become sluggish and refuse (larger) presentations. If there is a week of 7°C to 9°C air temperature forecasted during winter, it signals a shift in feeding response, as fish start to digest meals faster. This is the time to be out on the river! The optimum water temperature for brown and rainbow trout ranges from 11°C and 18°C, so any movement of water temperature towards this optimum range will increase their feeding and your chances at catching. Even a shift from 3°C to 7°C will create a positive change in behaviour, even though it does not fall within the optimum range.

And, as expected, any drop in water temperature from 15°C to 10°C will shock the fish. Trout need time to adjust to changes in temperature, but only when the water temperature shifts away from the optimum range, not towards. So a positive shift in temperature of 0°C to 3°C won’t create a period for adjustment, but rather increase their feeding.

Tips to Give Fish a Better Survival Chance

Leave fish in the net even when removing the hook

• Beef up your leader to bring the fish in faster to release it. I suggest a quite reasonable 8-lb test minimum, as your presentations are most likely large dry flies, streamers, or lures.
• Fish in the cooler parts of the day, either morning or evening.
• Check regulations to make sure there are not local closures on your intended river.
• Use a silicon landing net to prevent trout from hitting their heads on rocks and to prevent the fish’s slime layer from deteriorating in your hands.
• Make sure to keep the fish in the water as much as possible, especially when taking photos—a warm fish will tire much quicker than one from spring or winter.
• And last, but not least, bring a thermometer to ensure the water temperature is below 23° C.

This article appeared in Island Fisherman magazine. Never miss another issue—subscribe today!