TAKEOFF
BASS SCIENCE
UNDERSTANDING STRATIFICATION
I
TEMPERATURE GRADIENTS AND TURNOVER CAN IMPACT BASS FISHING SUCCESS
f you’ve been around fishing long enough, you’ve undoubt-
edly heard someone whining at a weigh-in line, explaining
away a tough bite by saying the lake is “turning over.” This
is a reference to the limnologic process of temperature strat-
ification, or when the water in a lake or reservoir forms dis-
tinct layers of different temperatures. Often misunderstood
by anglers, stratification and the resulting “turnover” definitely
play a role in how fish move about a lake. Understanding the
process can help you catch more fish.
What is Stratification?
Unlike many substances, water is unique in that it is less
dense as a solid than as a liquid (which is why ice floats).
Specifically, water is at its densest at 39 degrees. Both
above and below this temperature, water is less dense. So,
as the temperature of the surface layer (epilimnion) rises,
that water becomes less dense than the water in the cooler
By TJ Maglio
• Because there’s relatively little mixing below the thermo-
cline, in highly productive lakes the water below the thermo-
cline can suffer from oxygen depletion, making it inhospitable
for fish.
• Some open-water baitfish (alewives, ciscoes, blueback her-
ring, etc.) will also use the thermocline as a type of “structure”
and hang out right on top of it. As a result, the thermocline can
be used to predict the depth of game fish such as bass.
Turnover?
Once surface water temperatures start to lower in the fall,
there is a point where the water above and below the ther-
mocline reach temperature equilibrium. At this point, the
thermocline disappears, and wind and wave action mix the
water, making the temperature relatively constant from top
to bottom. This also causes waters from the bottom to reach
the surface, and vice versa.
Epilimnion
Thermocline
Hypolimnion
layer below (hypolimnion). At a certain point (usually in
spring or summer) this difference in density becomes a bar-
rier to mixing and forms what scientists call a thermocline.
The thermocline is a thin band of the water column
between the upper and lower layers where, if you dropped
a thermometer down through it, you’d see the temperature
rapidly drop.
Do All Lakes Stratify?
One of the biggest misconceptions about stratification is
that all lakes stratify. In reality, stratification is a fluid process,
more likely to occur in deep, stable systems where a large
temperature gradient can form between the surface and the
bottom. Stratification is also non-uniform. Parts of a lake
might stratify while others won’t. Many reservoirs also don’t
stratify due to constant current from power generation.
Impacts of Stratification on a Fishery
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• On multi-species fisheries, stratification usually causes
warm-water species to stay in the warmer water above while
cold-water species thrive down below.
Turnover
Turnover Fishing Tactics
Although anglers are quick to blame turnover for tough
fishing in the fall, it’s unlikely that the event has any real sig-
nificant impact on the feeding activity or aggressiveness of
the fish.
What turnover does impact is their location, which is
probably the real culprit in most cases when the bite gets
tough. Water turning over and mixing opens up the entire
lake to baitfish and bass, which can cause a steady summer
and early-fall pattern to die in just a matter of days as the fish
go through a transition.
If you suspect the lake you’re fishing is turning over, the
keys to success are mobility and versatility. Don’t be afraid
to abandon the patterns that had been working and try
something new. Cover lots of water and keep your head on
a swivel while looking for baitfish visually and with your elec-
tronics. Odds are, the bass will be following the baitfish, and
you might find them in unexpected places.
Top choices include topwater walking baits, lipless
cranks and swimbaits. Once you find a few fish, slow down
and fish the area thoroughly.
FLWFISHING.COM I WINTER 2019