So if the surface temp is about 44* in my lake, how much warmer is it (if any) at 30' where I am catching bass this winter?

 

 

It would be colder.

 

The deeper you go the colder the water.

 

In the warmer days you can get in the water and feel the thermocline.

Im talking winter time after the turnover. Why else would the bass be deep unless its warmer?

So if the surface temp is about 44* in my lake, how much warmer is it (if any) at 30' where I am catching bass this winter?

After turnover the temperature and oxygen content of the water are consistent throughout the water column. 

 

It is most likely colder at 30'. Water is most dense (heaviest) at about 39.2°F (4°C.) So the concept of warmer water at deeper depths is only applicable when the surface temperature is below 39°F. If the surface temperature is 44° then there is heavier (colder) water beneath it. In lakes that are iced over the water just beneath the ice will be warmest.

Why is it common to catch bass in deeper water in the winter?

Why is it common to catch bass in deeper water in the winter?

I don't think that is always true. I've caught a lot of bass in 3-8' of water over this winter, and the water temps here are around 35° at the surface. I also catch fish at 20' and deeper all summer long when water temps are in the high 80's. 

 

An entire book (or several books) could be written on why bass go deeper water in the winter. The short (and way oversimplified) answer is that they are lethargic and forage is scarce. In order to conserve energy bass will orient to vertical structure because it takes less energy for them to move vertically through the water column than it does to move horizontally.That vertical structure can be anything from a drop-off or a bluff to submerged trees in man-made reservoirs. This is however a gross oversimplification of the process. There are too many variables to count, and many are unique to a particular body of water. Where are you fishing?

 

Depending on the water temperature, bass go through a number of physiological changes when wintering.