I used to do a lot of scuba diving, in the winter we'd use gravel pits and quarries to train novices, i must have spent hundreds of hours over the years in freshwater training sessions, in the very coldest weather i noticed carp - and tench would often lie up right in the middle of snags, normaly a few feet off bottom, but never on it, or near the surface totally motionless, you could spend an hour or so moving around the lake, come back and find the fish in the same spot apparently having not moved an
inch, the denser the snag the more they seem to prefer it, i think because in that stupor state they feel vulnerable,and seek a place where an attack from a predator would be difficult, i don't think they necesarily use these places to feed just to lie up, unless obviously a food source was close by, one venue used for diving is stoney cove in Leicestershire, actually in Stoney Stanton where Stef Horak lives, its an abandoned stone quarry used for scuba and commercial diver training and has depths to 120 feet during winter the thermocline is almost always steady at about 60 feet you can hover at one point in a water temp of about 8 degrees C. and put your arm into water of 2 degrees C. Weird! The fish life - No carp!, but truly massive roach in abundance would always be between the surface layers and 30 feet, the odd few might venture a few feet deeper but very few, and never below 50 feet in winter.
This place is a massive hole basically the southern side has about a sixty foot sheer drop to the water and so is always shaded,
i would say about 60% of the fish where in the brightly lit areas of the northern side
particularly attracted by cover such as the vickers viscount cockpit at about 20 - 30 feet it was like looking at a coral reef, anywhere there was a place to suddenly dive for cover was surrounded by hundreds of fish, on the few occasions i've fished for carp in winter i've tried to apply what i've seen to my fishing -i.e. bright sunlit areas , not too far from snags or cover of some sort,- then just hope they're having it on the day , i've not done too badly considering the little time i've devoted to winter carp fishing, hope this is of some help to you all.