Over the years I have heard and read much about thermocline. Why it develops, its impact on fish, why some lakes have it and others don't, how deep it is, etc. Then I read other stories of fishermen down south fishing for crappies down 40 or more feet. It was always my understanding that a thermocline set up around 20-25 feet or so, maybe a little deeper on some lakes, maybe a little shallower on others. Supposedly there is very little fish activity below the thermocline due to oxygen levels but I'm wondering if anyone has other experiences that contradict these "gospel truths" or "old wives tales". On some lakes that go down to 80 feet I cannot pick up a thermocline on my graph, even with the sensitivity turned all the way up. Does anyone care to comment on this?
Wisconsin Fishing Discussion
Thermocline
I have no doubt that sonar can be used to find the thermocline if conditions are right but when an author presents such statements as a constant fact I have to believe the author is using some salesmanship to fulfill his duties to his sponsor or trying to give some impression of his expertise. I have seen what appears to be thermoclines on my screens and given that my units are all under $200 and the results are inconsistent I dont really focus on this. Best of luck.
Mendota and Others who responded--Thank you for your input. Mendota, I will look up that Queg-thing. It looked like you had it built into a console or something but I'm looking for something portable. I just know that I've read that a regular locator should be able to pinpoint the thermocline when one turns the gain/sensitivity way up and sees a fairly solid and level band on the screen but mine (a Garmin 312DV) doesn't show one on a lake that should have a thermocline. It's over 50 feet deep. I'm trying to eliminate fishless water. Thanks again all.
Jim
The product is/was the QUEG Fish Finder. Mine was built by SANGER GmbH & Co, Germany.
I recall paying maybe $60 for it. It records temperature in 5' increments all the way down. Works as advertised; mine was still functioning when I went to find it last week. although I had not touched it for 5 years. I did replace the battery afterwards (CR2032 button cell) to ensure another 10 years.
I've used it on perhaps a dozen different waters in the US and Canada.
I actually built a digital thermometer with a 50' cord way back when in order to investigate this issue, as well as look for those legendary "spring holes".
Nowadays there are several available commercially, but they are not cheap.
About 15 years ago I got a castable probe, which records the temperature and depth as it sinks, every 3' or 5' as I recall. Let it hit the bottom, then reel it in, and read back the temperature every 3-5' all the way to the bottom. Very slick, although it takes perhaps 30 seconds to get it 60' down.
I'll have to look later and see who made it.
Since I brought this topic up I'm still pretty confused about the whole thing. Is there a somewhat practical way (read "cheap") of finding if a lake has a thermocline? There used to be a cheap thing called a "Depth-O-Therm" as I recall where you attached a line to this depth-temperature glass tube and lowered it into the water. Then you'd pull it up real fast and you'd get a depth and temperature reading. By doing this a number of times at different depths you'd get a lowering of temperature and supposedly this would be the thermocline. I don't know if it was very accurate but shouldn't there be something around now that you lower into the water that gives a temperature reading? Even if it doesn't tell you the depth one can figure it out rather easily by the old "arm-span" method of around 6 feet for every length of line you drop down. The whole thermocline and where it sets up still seems rather hit and miss.
Jim
This graph shows the thermocline beginning around 8m (26'), and also shows only mild mixing on windy days:
Lake Mendota is one of the most studied lakes in the world (perhaps second only to Lake Geneva in Switzerland) due to the UW's Limnology Dept.
No shortage of data available for L. Mendota. The graph here http://limnology.wisc.edu/lake_information/mendota/mendota.html shows the poster Mendota is correct on the thermocline for this lake.
There isn't much argument to make about the depth of the thermocline on Mendota, you can see from the buoy data that it is about 10m (33') down right now:
The depth or even existence of a thermocline depends on how much mixing can occur from the wind, and the size/shape of the water body
I've been on couple-acre borrow pits, 20’+ deep, surrounded by trees, where the thermocline was only 5-6' down. On 10,000 acre Mendota, it sets up at around 30-35'. On 400-acre Devil’s lake near Baraboo, it sets up at about 20-25’.
Whether there is oxygen below the thermocline depends on how much stuff is decaying down there and using up the oxygen. On lakes like Mendota, the high algal productivity means the oxygen is quickly depleted down there, and it becomes a virtual desert at the bottom. On Devil’s, oxygen is not depleted, and species such as trout can be stocked and will thrive down there. There are many grades in between, just depends on the lake.
In order to mark the thermocline, it needs to be distinct and rather abrupt, with no trash in the water column above it, or else you get a screen full of garbage when you turn the sensitivity up. You will have the best luck looking early in the morning, before boat traffic, and after an extended period (days) with light winds. Any mixing caused by the wind will cause the boundary to blur, and if you get an algae bloom, it will cause lots of trashy echoes above it that will obscure it entirely.
I find I only rarely see the thermocline on Mendota for exactly these reasons, but on the much more infertile lakes up North, I usually can get at least some indication.
