What A Thermometer Can Tell You

The recent thread on stream thermometers on a fly fishing bulletin board got me to thinking; always a dangerous undertaking. Just what effect does water temperature have on trout, and why should I care?

The first thing to realize is that water temperature is not consistent over depth, and the difference in temperature is directly related to the amount of mixing that the water undergoes. In an undisturbed water column, the warmest water will be found at the surface and the coldest water will be on the bottom. Mixing occurs either by the influence of wind (more important on lakes and ponds), or by turbulence such as found in riffles and rapids.

The largest influence on water temperature is sunlight, the more sun exposure and the longer the sun is on the water, the greater the temperature change; this change affects trout in two primary ways.

The first way is that temperature affects the amount of dissolved oxygen (DO) in the water, and this is the oxygen that trout breathe through their gills. Dissolved oxygen is measured in parts per million (ppm). Trout require a DO level of 3ppm to survive. When water reaches a temperature of 75℉ 3ppm is the maximum DO the water can hold without some form of turbulence to enhance gas exchange (oxygenation). At water temperatures above 75℉ with no oxygenation of the water trout will suffocate. That’s why in summer months as the water warms, trout will more often be found in, or directly below sources of turbulence, particularly later in the day. Water will contain the greatest amount of DO at water temperatures at or near 32℉.

Secondly, a trout’s metabolism is controlled by water temperature. While very cold water has the highest DO content, as much as 30ppm, these cold temperatures slow a trout’s metabolism to a very low level, and the slower the metabolism, the less food a trout requires.

While metabolism rates verses temperature varies among species, as a general rule of thumb the following guidelines apply:

At temperature ranges of 32℉ to 44℉ a trout’s metabolism is slowed to the point that it only takes about 1 bellyful of food per week to sustain them. At these temperatures trout will seldom move from their holding position to take food.

Between 45℉ and 50℉ the metabolism starts to speed up and the level of activity and food intake increases. Trout will begin to move slightly to intercept food.

From 50℉ to 55℉ the metabolism really starts to ramp up and trout will eat as much as a bellyful of food per day.

The ideal temperature range as far as metabolism and food intake is concerned is between 55℉ and 65℉. At this range, the DO level is 12 to 18ppm, food is plentiful, and a trout will consume upwards of 2 bellyfuls of food per day.

When the water temperature climbs to 68℉ and above the trout's metabolism begins to burning oxygen as fast as they can absorb it from the water. As the sun heats the water, they use the DO faster and faster, and without some type of escape valve they will suffocate.

While some trout, particularly brown trout, seem to adapt to warmer water conditions more readily than other species, once the water temperature hits 70℉, we should be looking for other activities to occupy our days, and restrict our fishing to late evening/early morning hours. At this temperature, fish mortality rates will increase dramatically. Above 72℉ we shouldn’t be fishing at all.

So, get a good stream thermometer, use it several times a day to check temperatures, and take readings at various depths to help figure out where the fish are most likely to be working.