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By Axel Wessolowski

Axel Wessolowski investigates how water temperature can affect fish, and therefore, fishing.

The first scientifically proven conclusions into the behaviour of fish at different temperatures were published in the 1940s.
The first scientifically proven conclusions into the behaviour of fish at different temperatures were published in the 1940s.

Even 200 years ago, people were wondering about water temperature and its significance in the life of fishes. On the record, WJM Menzies, Inspector of Salmon Fisheries of Scotland, wrote in the 1930’s about the importance of water temperature: “Temperature may play an important part in encouraging trout to feed or in limiting their opportunities of so doing”

Correlations between water temperature, fish activity and the underlying principles were quickly realised, too, as a paper from the early 1940s indicates. Van Someren, a zoologist from Glasgow University gave attention to environmental factors which lead to rising of trout. Fly fishing as a scientific job, so to speak. Nice!

Anyway, he concluded: “Temperature, or conditions immediately dependent on temperature appear to be the principal factors controlling rising.” But what exactly is meant by that?

Either too cold or too little oxygen
Water temperature influences the well-being of our fishes – salmonids – in two ways. To begin with, let’s consider the fishes’ metabolism (respiration, growth, digestion). During fluctuating temperatures both the metabolic rate and metabolic efficiency may fluctuate. I will explain this, using two extreme examples.

If the water temperature drops near 0° Celsius the body temperature of a trout would drop to that level, too. In other words: the trout is on the verge of becoming frozen and will not feel comfortable. There are only a few fish that can tolerate temperatures at, or below, freezing point.

The fish’s chemical processes slow down, and such a cooled trout will appear very lethargic.

If the temperature rises, the metabolic conditions are inverted. Until a point where the temperature either changes or even destroys specific biological microstructures. Also, the fish’s chemical reactions will speed up, but of what use is that for a cooked fish?

The second factor influenced by water temperature is the oxygen content of the water. Generally, the warmer the water, the less oxygen is soluble in that water.

Poor salmonids. At lower temperatures, more oxygen will be available, but their metabolism will be slow, and, when the temperature increases, they get going, but oxygen levels decrease. Still, these are extreme examples: there is a ‘golden mean’ – perfect conditions – and every serious fly fishermen should know that.

Water temperatures to feel comfortable
Being asked, what would be the ideal temperature for fishing the Beaverkill river (USA), a greying Lee Wulff answers, that the ideal temperature is between 60° and 65° Fahrenheit! About 17° Celsius. Basically, he is right, because there is such an optimal temperature range, which lies between two extremes.

When it comes to the optimal temperature, scientific data displays slight variations. Depending on the latitude and habitat, where the fish used to live, they show different levels of acclimatisation. Hence, unsurprisingly, fish who are used to cooler temperatures, like trout and grayling, experience earlier difficulties or even die sooner when temperatures are rising, than fish who are used to living in a warmer environment.

However, a clear basic trend exists: the critical thermal minimum is 1° Celsius, and the critical thermal maximum is about 25°C. Both these values indicate the narrow temperature range in which our salmonids are able to survive.

The absolute ‘feel-good’ temperature is not exactly the median, and lies about 15° Celsius (59° Fahrenheit). At this value fish show active feeding, best growth-rates and increased swimming activity. In scientific experiments, where fish were allowed to choose their preferred water temperature, they always swam towards this value.

Despite the fact that the air temperature had reached almost 38° Celsius (100.5°F), this nice brass-coloured trout rose to a dry fly, because for the activity of fish it is the water temperature only which is crucial.

Where are the fish in summer?
With knowledge of optimal water temperature brings logical consequences. Norman Maclean (no, not the Norman Maclean of A River Runs Through It) notes in his book on trout and grayling, that, when the water temperature rises above 20° Celsius, one would be better playing a round of golf than go fishing.

Of course, this is not true for every stretch of water, but in case you are going to practice catch-and-release you should keep an eye on the water temperature. Higher water temperatures decreases the survival rate of a hooked-and-released fish.

If you fish for trout in lakes, keep in mind that colder water is more dense than warmer water and this results in a temperature gradient. On hot days or during hot summer periods the water at the surface may be warm, but below that one will find much cooler layers ... and that is where the fish will be.

In summer, lakes may reveal a phenomenon that is called ‘summer stagnation’. The top few feet of the water column are stirred by winds and the surface layer (epilimnion) is almost uniformly warmed. Beneath that follows a layer (metalimnion), which is less stirred and therefore slightly warmed. If you are going to fish the layer, which displays the optimal temperature for salmonids, it is helpful to fathom out the water column with a thermometer. In the third layer (hypolimnion) the water temperature is stable.

If, in winter, a lake is covered with ice, there will be no wind-driven stirring of the epilimnion and the lake cools equally (‘winter stagnation’). In between, spring and autumn circulations happen, completely overturning the whole water body. But in rivers, different conditions apply.

In principle, a river’s water temperature is lower closer to its source and higher towards the river mouth. Due to the fact that at the river source the temperatures are stable, the water is cool in summer and warm in winter. In these summer-cool brooks fish will display a constant activity. But in other types of river salmonids rely on alternative areas, in case the temperatures rise.

Water temperature determines body temperature of trout and grayling, so fish will seek areas which show temperatures close to their biological optimum. Biologist call this behaviour thermo-regulation. During warm or hot weather periods fish will prefer intensely shaded sections over sun-lit sections of river.

Interestingly, salmonids’ thermo-regulation is also influenced significantly by oxygen levels in the water. If the oxygen concentration decreases below a certain value, fish will swim to cooler areas. Conversely, one can reason that the fish will seek sections with elevated oxygen levels when the water temperature rises – water falls, rapids – any place with surface turbulence.

In contrast to fast-running mountain streams, in which oxygen concentrations are virtually constant (over a period of 24 hours), middle river sections are subject to daily fluctuations. These occur as a consequence of water plants and their metabolic activities.

During night the plants consume oxygen, but during the day, thanks to photosynthesis, they also produce oxygen. Therefore, oxygen levels may be higher in the afternoon compared to the early morning. Assuming everything else, eg Mayfly hatch and weather, is ideal, the fishermen can hope for good feeding activity into the afternoon and evening. Although the river bank can be attractive early in the morning, the fish might be less active.

An imperative purchase?
By now you probably agree with me that it can be beneficial to know the exact water temperature and that purchasing a thermometer might be sensible. There are different models for you to buy, and it’s up to your taste and wallet, which one you finally add to your fly-fishing paraphenalia. Don’t be tempted to get one of the older mercury thermometers, because an unforeseen breakage will not be environmentally friendly. Latest models normally come with alcohol as the temperature indicating liquid. Less susceptible to breakage are electronic thermometers, but these need batteries.

Relatively new on the fly fishing market are infrared thermometers. Measuring without getting your hands wet! Unfortunately, these gadgets measure the water surface temperature only. If you go fishing in lakes the infrared thermometers are absolutely useless. And who goes fishing and expects not get his hand wet? Macramé is a nice hobby, and dry, too!

For most of the thermometers it is common to have two scales: one in Fahrenheit, the other in Celsius. Whereas it is quite simple to convert Fahrenheit into Celsius ((°F - 32) x 0.55). Especially in all Anglo-Saxon countries where Fahrenheit scales still go strong.

Meanwhile a small thermometer has found its way into my chest-pack. I really do hope that I will never again have to deal with extreme weather situations, but you never know. I also think, that successful practicing of catch-and-release makes purchasing a thermometer almost imperative.

Recently, I was finally able to outsmart some trout and grayling, despite the hot weather. They were lying in a deep scour, which lay directly after a section of small rapids. Although I did catch them with a nymph, and not with a thermometer.


Every lake fishermen should be aware of the fact that there are different temperature layers. In summer, distinct warmer and cooler layers develop. To catch trout in late summer, when the surface layers can be too warm, the transition areas between epi- and metalimnion, are cool, rich in oxygen and will hold the fish. In between summer and winter the whole lake will be stirred by spring and autumn circulations.

In winter, a lake covered with ice will eventually assume a uniform temperature. (Diagram shows epilimnion orange, metalimnion yellow and hypolimnion yellow.)

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