Reading The Water Series - How Water Temperature Effect Trout

finding fish Apr 01, 2019

In our previous post, we discussed how trout are cold-blooded and how they react to changes in water temperature. This is important as it determines how active trout are and which habitats they'll prefer. Now that we have a better understanding of when trout feed with regard to temperature, let's focus on warmer water temperatures. This is more critical to trout than cold water and tends to overlap more with anglers' fishing habits. More fly fishers are likely to be out fishing in the summer than the brutal cold months of winter.

We have established that a trout's metabolism increases with temperature and so far that increase has been good news. Warmer water means fish are more active and aquatic insects are more likely to be active. The problem is that a trout's metabolism will continue to increase with temperature. After water temperature exceeds 65 degrees trout will begin to take measures to save energy because their metabolism will be too high.

 It makes sense to believe that a trout will be extra hungry when water temperatures increase their metabolism into overdrive, but the exact opposite happens. The problem a trout has is there is no way for them to eat enough to make up for the energy their body burns. This requires them to seek out the coolest, quietest spot and hunker down. This means that most fish will not be out feeding, and in turn, fishing will be poor.

 In our previous post, we compared trout to financial investors to describe how they expend their energy for the best "return on investment". A financial comparison to a trout's dilemma with warm water might best be thought of as "rent". Cold water doesn't burn many calories for a trout while warm water can burn an extraordinary amount of energy regardless of how active a fish is.

Imagine a thriving restaurant that's busy all the time and is highly profitable. Suddenly the landlord triples the rent on the building and all of the restaurant's profit is now going to pay the rent. In a play to make more money, the owner blasts TV and the internet with advertisements. His business increases, but the restaurant only has so many seats and now he's spending about as much on advertising as he's gaining in revenue. The restauranteur is now working harder without making any more profit. He's even had to start dipping into savings to cover his expenses. His problem isn't a lack of business; the rent is too high. After a local real estate search, the owner finds a comparable space for the original rent he was paying. After the rent has gone back down, the restaurant returns to profitability.

 This is essentially what happens to trout during a heat wave. Their metabolism gets so high it's very hard to do anything to maintain body weight. Some may take the same action as the restaurant owner and move where conditions are more in their favor. That's not always an option for fish depending on where they live.

Trout has developed a variety of ways to get through these stressful periods. Wild and native trout have adapted over the ages to seek out cooler areas. In some locations, fish will migrate into cooler tributary streams or springs seeping into the stream. Not every river has features like this and even the ones that do can only harbor so many fish. Extended periods of warm water can lead to mortality. This is a problem that may be encountered by traveling anglers on summer vacation who arrive on the river during a heat wave and there are a few ways to get around this problem.

Small headwater trout streams typically remain cold during the hottest days of summer. They will fish better than many larger trout rivers even as the fish are certainly smaller.


The first thing an angler can do in this situation is fish very early and pack it in by lunchtime. Cooler nighttime temperatures can give the fish a respite and get them active around daylight when the water is at its coolest. Another tactic is to find a tributary with cooler water. This often means fishing much smaller water but may mean the difference between fishing overactive fish or sweltering in a warm river where fish are hunkered down on the bottom with no interest in biting. A third, often overlooked solution, is to target different fish species. Here in the Southeast where we live smallmouth bass are very active in conditions that are unfriendly to trout. Guides in the Rocky Mountain West have taken to offering trips for carp when fishing for trout is unproductive because of extreme heat. In some instances, local fish and game agencies will close trout river to fishing because of heat, so it's important to be willing to consider other options.

 Anyone who chooses to fish during a heat wave needs to consider the resource. Always be sure to land fish as quickly as possible and keep the fish in the water after landing them. Keeping fish out of the water is never good for them but particularly bad when they are already stressed. We have personally found it better to find another option rather than fish over stressed fish. It's easier on the resource and usually leads to better fishing somewhere else.

 While fishing in the heat is a problem for some anglers, fishing in the cold weather has its challenges as well. We'll discuss that in an upcoming post.


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