Fish Depth Based on Water Temperature

By: Chris Cook (Admin. for FlyFishingWorld Yahoo Group)

The subject of depth related water temperature is a tricky one. We are looking at two different topics here. Namely how the temperature is affected by depth of water and weather conditions, plus how the fish react to those water temperatures.

Summer Conditions

During the summer months, the water warms in relation to the air temperature, which is dependent upon how much sunshine we get. Last year, we had very few hot, sunny days, so the surface temperature never rose much over 65 or 70 degrees Fahrenheit. Water takes a long time to warm and it does so from the surface downwards. Therefore, in the summertime, the surface water will always be warmer than the deep water.

Fish depend on dissolved oxygen to live and these oxygen levels become depleted as the water temperatures rise. So in the hot summer months, trout will tend to swim where the water is cooler and the dissolved oxygen levels are greater. However, this determines their "cruise depth" and does not mean that they will always feed deep. Nor do they automatically head straight for the bottom of the lake - these are two popular misconceptions. I wish at times that this were true, as it would make catching them much easier. Nowadays, we have a vast array of sinking lines and tactics designed to catch fish at depth. Unfortunately, I believe the fish tend to drop a little deeper in the water to escape the glare of the sun and then go off the feed for hours on end - believe me, this is far worse!

My experiences based on summer flyfishing over the years, is that trout only stay deep if we have a long period of extreme hot weather and constant sunshine. If there are aeration systems present in deep water, then that is where they will head. Sometimes, it is the daphnia that drives them to feed deep, but this is normally the exception rather than the rule.On flat calm, sunny days, the surface fishing is often spectacular, with lots of rising trout feeding on insects trapped in the tight surface film. Once the fish have adapted to the bright sunshine, they will stay "up" for hours, so long as there is plenty of food on top.

Winter Conditions

We have now established that air temperature affects water temperature from the surface downward. This is also true of cold weather. The surface cools the quickest, leaving the warmer water below - quite the opposite from the summer scenario.  Those fish that are fortunate to survive the rigours of winter, will feed mainly on small fish and whatever they can "root out" on the bottom. As there is not much food to eat, they spend less time feeding, so they expend less energy than during the warmer months when food is plentiful. In very cold weather, the fish will almost certainly live in the deep water, where the water is relatively warm. However, they will probably not be feeding down here.

Once spring arrives, they start to feed and the likely food sources are caddis, hoglice, shrimp, snails and bloodworm. All these food forms live on or near the bottom. However, the sun's rays tend to stimulate these invertebrates into life, yet not in 20 feet plus of water, more like four to eight feet. These are the sort of depths to concentrate on for early season fishing, not in the deep water. So if you want to catch feeding fish in early spring or winter, look for them on or near the bottom. But concentrate on depths of around eight feet, rather than 28 feet!