View Full Version : Doc...Thermocline question

08-29-2005, 02:45 PM

I fish Pine Mountain Lake just out of Groveland on HWY 120 fairly regularly. Elevation is around 2800 feet. The lake is roughly 200 acres with a max depth of 90 feet. I would guess average depth to be in the 35-40 foot range. Surface temp has been in the 77-80 degree range for the last month or so.

the question? is there any rule for establishing where and how thick the thermocline is? does it take a minimum size body of water for a thermocline to form? I have been cathing a lot of fish in the 20-25 foot range and just wondering if I'm in the thermocline, or are the fish leaving to hunt where the food is.

Thanks again,


08-29-2005, 03:41 PM
Hey WishnIwasfishn....had any luck lately?? Been to Cherry at all??
I hear it's been hot in Groveland....hoping to get up there in a few weeks.

08-29-2005, 06:06 PM
There is no rule of thumb to determine where or if a thermocline will form in a lake. The variables are the depth, average air temperature, amount of sunshine, and the amount of wind activity to stir up the water. Eagle Lake, which is about 90 ft at the deepest point not have a sharp thermocline this summer. With a surface temperature of 73 degrees, the temperature dropped to only 63 degrees at the deepest point. That lake gets lots of sun and the afternoon winds mix things up quickly. Yet 25-35 ft seemed to be the best fishing zone 2 weeks ago. Rainbow trout will come up into warmer surface water if that is where the food is. Their feeding slows down when the water temperature exceeds about 68 degrees.

08-30-2005, 12:28 PM

I was at Eagle this past Saturday and the fish were hanging from 22-30 feet of water. I had baits down to those depths for a VERY long time with no hits.

An old timer that I talked to said that the fish werent hitting at that depth due to the thermocline. Instead, he said that the fish were hitting better at around 18 feet.

Surface temp was 67 degrees.


08-30-2005, 04:34 PM
The depth at which the fish will hit readily is the depth where they are feeding. That is determined by where the bait is. At Eagle, the chubs are perfectly happy at temperatures up to 75 degrees. So they stay in the shallows or just below the surface to minimize exposure to the trout. The chubs go into deeper water to feed if that is where most of the zooplankton and small insects are found. That's where the trout meet up with them. The best depth varies from day to day.

The thermocline issue is overplayed by fishing magazines that want you to believe that there is a simple formula for catching fish. A thermocline is where the temperature changes rapidly with the depth. If enough oxygen is present, most salmonids will hold at temperatures between 50 and 54 degrees. If there happens to be cold water bait fish such as cisco or smelt that prefer the same temperatures, trout will feed actively within the thermocline at their preferred temperature range. But if the zooplankton, insects, and baitfish prefer warmer temperatures, the trout will make forays into the upper part of the water column to feed. This is what happens in most of our lakes (Tahoe, Donner, and Fallen Leaf are exceptions). The trout hold in cooler deeper water but won't bite. The actively feeding trout are at the same depth as the feed. And that depth can change daily with the plankton levels.

08-30-2005, 04:54 PM
so i guess if you see schools of bait fish...troll at those depths? i would think you might want to fish through schools of baitfish if the trout are in those areas, maybe troll a broken back rapala at that depth will attract willing biters.

08-30-2005, 07:08 PM
Troll just under a school of bait fish so that your lure will look like a straggler. Troll in the same direction that you think the school is moving for best results.

I've posted this many times, but I'll say it again: The most common mistake that most trollers make is to troll at the same depth where they are marking fish. Always troll above the fish you are targeting or just below the bait.

08-30-2005, 08:26 PM
Thanks Doc,

It was my first time using a downrigger since watching my Dad use them when I was a kid. I'll make sure to troll above the fish on my next trip.