Could be several reasons; A are there known springs in that area? Springs/ "weeps" underwater can be all but impossible to identify/ locate, generally though they push heavily oxyogenated water at a constant 56 degree temperature up into a lake or body of water ALL yr long regardless of the surface temp or weather conditions; you might as well say during the hottest summer they are "air conditioned" and during the coldest winter they are "heated" compared to the rest of the same body of water- KEEP IN MIND in shallow water or if the discharge is great enough any ice over a spring CAN be much THINNER (or not frozen at all) due to warmer water rising towards the surface! THEY CAN BE DANGEROUS TO APPROACH: PLEASE USE EXTREME CAUTION!!! Fish don`t have to worry about that, and tend to FLOCK to springs, especially in colder temps; shad especially so, and if cover is present (or is introduced HINT HINT!!), so much the better. B seasonal prey availability could also play a role; aquatic redworms become very abundant in the fall as the food chain breaks down; along with other aqutic morsels that briefly "bloom" on softer bottoms. C Is there any outstanding bottom features that could account for the abundance of the fish; old creek channels, deep basin edges, transitional zones, ect. Gotta be a good reason so many are there...