Department of Fish and Wildlife
Kentucky Afield Outdoors: Find the thermocline for better summer fishing success
FRANKFORT, Ky. – Fishing instruction
for decades preached anglers should fish deep, cool water during the hottest
days of the summer.
This makes sense
as fish are cold-blooded animals and their bodies are the same temperature of
the water that surrounds them.
deep water in summer on a lake such as Taylorsville Lake may mean your
offerings are in a dead zone. Warming air temperatures change the nature of the
water in a lake. Anglers need to consider these changes while summer fishing.
The water column
in lakes begins to stratify in late spring, like a layer cake with a warm top
layer, a mixing layer in the middle and a cold layer on the bottom. Warm water
is less dense and sits on top of the cooler and denser water below.
“The mixing layer
in the middle is the thermocline,” said Dave Dreves, assistant director of fisheries
for the Kentucky Department of Fish and Wildlife Resources. “The thermocline is
a band of water with rapidly changing temperatures.”
progresses, the water below the thermocline grows increasing hostile to fish.
“As organic material from plants or animals sinks to the bottom, decomposition
ties up all of the available dissolved oxygen,” Dreves said.
dissolved oxygen to survive, so the layer of the water below the thermocline
becomes a dead zone as the air temperatures rise. Lures or bait presented there
is the same as casting into a desert.
“The further you
go into summer, the more the thermocline matters,” Dreves said. “It varies from
year to year how long it takes for it to set up.”
Anglers want to
target the area just above the thermocline for the best summer fishing. “You have that happy medium of the coolest
water with good dissolved oxygen,” Dreves said. “You might find baitfish at
that level as well. That zone will be the active part of the food chain.”
The fertility of
the lake determines the location of the thermocline. Water clarity is a good
indicator of fertility; murky lakes contain more nutrients in the water than
lakes where you can see the bottom in 12 feet of water.
On a highly
fertile lake such as Taylorsville, the thermocline starts at about 8 feet deep
in mid-summer and ends at roughly 12 feet. Bass anglers fishing a heavy jig 18
feet deep on a channel drop are wasting their time, the same as anglers dunking
cut bait for channel cats at that depth. There is no dissolved oxygen that
Anglers need to
pound that water from 6 to 10 feet deep or so in the summer heat on
Taylorsville for bass, crappie, catfish and even bluegill.
“On a clear, low
fertility lake like Laurel River Lake or Lake Cumberland, the thermocline may
start at 30 feet deep,” Dreves said. “On these lakes, there can be good
dissolved oxygen below the thermocline. They have a two story fishery where
cool water fish such as walleye or striped bass locate below the thermocline in
On mid-depth, moderately
fertile lakes such as Nolin River Lake, Barren River Lake, Green River Lake or
Rough River Lake, the thermocline usually starts around 11 to 12 feet deep in
summer with little dissolved oxygen below 16 feet.
channel drops or humps at that depth and fish them,” Dreves said. “I always
remind myself to fish the right depth in summer.”
The U.S. Army
Corps of Engineers Louisville District webpage has charts showing the water
temperature and depths of the dissolved oxygen at www.lrl.usace.army.mil.
Click on the “Water Information” tab, then the “Lake Temperatures and Dissolved
Oxygen Levels” tab.
The depth where
the oxygen line takes a left toward the zero side of the graph is the top of
the thermocline. “You can also go out in the middle of the lake and turn up the
sensitivity on your sonar unit until you see a band in the depths,” Dreves
said. “That band is the thermocline. It seems the older units might pick this
band up better than the newer ones.”
Lakes with high
flow through rates such as Kentucky Lake and Lake Barkley usually do not form
thermoclines in summer in the main lake, but thermoclines may appear in their
major creek arms or embayments. The current in rivers prevents thermoclines
Remember the role
of the thermocline while fishing this summer. It will put more fish in hand.