ask a bIoLogIst
q.
UNDERSTANDING THERMOCLINES
a.
by Dr. Brian Weidel
Thermoclines are thin portions of the water
column where the temperature changes
rapidly over just a few feet. They
form when surface waters, heated by solar
radiation, become less dense than the
cooler, deeper waters below them – effec-
tively creating a boundary between the
layers. This process is called stratifica-
tion, and it occurs annually on many
lakes and reservoirs.
Although common, thermoclines can
be variable from year to year, and even
within a season. Some water bodies don’t
ever form a thermocline, some will period-
ically and some do annually. Thermoclines
are also variable across the same lake. Certain
parts of a lake might stratify, while others won’t.
The depth where a thermocline forms is also variable
seasonally and annually. Thermocline depth and formation tim-
ing are primarily influenced by how much solar radiation is hit-
ting the lake and mixing due to wind.
From a fish’s perspective, thermoclines can be looked at
the same way as any other edge or transition zone. Baitfish
how do
game fish
and baitfish
relate to a
thermocline?
Meet Our Expert
Dr. Brian Weidel is a
research fishery biologist
at the U.S. Geological
Survey Lake Ontario
Biological Station.
26
might congregate near or right at the thermocline
to feed on zooplankton, which in turn might
cause game fish to cruise the thermocline
as they would a depth contour, weedline
or reef.
Anglers often assume that there
will never be any fish below thermo-
clines due to lack of oxygen. This can
be true, partially true or false – but it
depends on the lake. When thermo-
clines are stable, bacteria and other
organisms use up oxygen to complete
their life cycles below the thermocline,
which, without any mixing from oxygen-
rich surface waters, can cause the water
below to become anoxic (low in oxygen).
This occurs most commonly in productive
waters and reservoirs, creating oxygen levels too low
for fish.
In contrast, in clear infertile lakes where there are fewer
nutrients to spur anoxic conditions, there is ample oxygen
below the thermocline, providing habitat for many cool-
water species.
TJ Maglio produces the Ask a Biologist department. If you
have a question you’d like to have answered by a fisheries
expert, please email it to TJ at [email protected].
fishing tip: Turning up the sensitivity of
your electronics will often allow you to
“see” the presence of a thermocline on
your graph in the form of a fuzzy band
or line. Identifying this depth can be
helpful when patterning fish in late
summer and early fall, and to recognize
when the thermocline breaks up and
the lake “turns over.”
fLWfIshIng.com I august-september 2016