Our Man In Canada
November 23rd, 1998

Lake temperatures and Fish

Adapted from "The Essential Guide to Fly-Fishing" by the author
By Clive Schaupmeyer

For me, passion in fly-fishing is found in streams. But everyone loves variety, and I fish for trout in lakes a few times each summer. And I enjoy the thrill of pike fishing in local reservoirs. Despite my slanted desires in fly-fishing, the dynamics of lakes, water temperatures and fish are still interesting.

The depth at which fish are found in a lake is affected by temperature and oxygen which vary considerably with the seasons. Their location in summer is complicated by the presence or absence of temperature layers in the water. Lakes may or may not form distinct temperature layers.

Whether or not a lake forms temperature zones is related to lake depth, wind speeds and shoreline cover. There is no magic lake depth at which layering develops for sure in summer. Where I live, on the open plains, the wind can howl, and I'm told that lakes of 25 feet or less will not layer because the wind mixes the water. Although distinct temperature zones do not form in shallows lakes, the water temperature will still vary by depth, and fish will seek the level where the combination of temperature, oxygen and food is optimal. In a shallow lake, less than 25 feet or so, trout are likely to be at or near the bottom in summer. Even warmwater species like bass will drift into deeper water when the temperature rises above their comfort zone elsewhere.

Deep lakes in summer are another matter. Early in spring the water temperature in deep lakes is quite uniform. However, as summer approaches, the surface water warms up and becomes less dense, and at a certain point it will no longer mix with the cold, dense water below. The warm, oxygenated top layer varies from a few feet down to about 50 feet. Rooted plants, algae and insects thrive at various depths in the upper layer.

At the lake's bottom is a cold, stagnant layer that does not have direct contact with the surface, and the oxygen declines there in summer. Little light penetrates the deeper layers and plant growth is minimal. Fish avoid the cold, dark, lower depths of deep lakes during the summer months, which is just as well because attaching a fly line to a downrigger sort of defeats the purpose, doesn't it? You will be hard pressed to get a fly down past 30 feet or so even with the heaviest sinking line. (Sure you could add a bunch of weights but well . . . you might as well get out your spinning outfit.)

Just below the warm surface layer there is a zone of rapid temperature change called the thermocline. Fish, particularly trout, move into the thermocline in summer. Within a short depth range they can find a comfortable temperature, the proper oxygen level and enough light to look for dinner. This doesn't mean fish stay in the thermocline all the time. Most game fish are notorious for heading to shallow water in the evening to chase small food fish. Water temperatures may be more tolerable toward evening and foraging game fish are less likely to be snatched up by an osprey in the low light.

The layers in a deep lake mix together in the late fall and are more uniform in chemistry and temperature until the next spring.

The significance of all of this lake environment stuff can get lost if we don't know what the preferred temperature comfort levels are for fish. Trout are most active and comfortable between 50F and 68F (10C and 20C). In deep lakes they're likely to be down near the thermocline, which may be 10 or 15 feet in early summer and as deep as 20 to 30 feet or more during the dog days. In early spring and late fall they'll seek warmer shallows.

Warmwater species like bass and their sunfish kin (panfish) prefer temperatures between about 65F and 75F (16C and 23C). Until water temperatures in the spring rise to 55F or 60F (12C or 14C), bass are in a funk, and it will take many warm days to get them moving. But as their metabolism increases, they head to the warmest shallows even when water temperatures are below their preferred range. They'll stay in relatively shallow water during the spring spawn (and after that) until it gets too hot. Then, during the day, they'll head down to where it's more comfortable, moving back to shallows to feed in the evening.

The preference for a certain temperature range is over-ridden by things like availability of food. You may not like the cold of winter or the searing heat of summer, but you'll go out to the local pizza shop to get a meal if you have to. Same with fish.

For more information about water temperature and the feeding habits of trout check the FAOL Our Man in Canada Column for March 23, 1998: "Water temperature and trout feeding habits."

Closing thought: "Only after the last tree has been cut down; only after the last river has been poisoned; only after the last fish has been caught; only then will you find that money can not be eaten." Cree prophecy. ~ Clive Schaupmeyer

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Copyright ©1998 Clive Schaupmeyer

Clive Schaupmeyer is an outdoor writer and photographer. He is the author of The Essential Guide to Fly-Fishing, a 288-page book for novice and intermediate fly anglers. His photo of a boy fishing was judged the best outdoor picture of 1996 published by a member of the Outdoor Writers of Canada. He fly-fishes for trout in Alberta's foothill and mountain streams and for pike near his home in Brooks, Alberta.
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