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 50°F and 68°F (10°C
and 20°C). 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 65°F and 75°F (16°C and 23°C). Until water
temperatures in the spring rise to 55°F or 60°F (12°C or 14°C), 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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