Why are they where they are??

[Calico Kid]

Looking at the US Army Corp of Engineers report on Green River Lake - Water Quality- it shows that oxygen levels are too low below 17 Feet. Also, there is less than one degree variation in temperature over that 17 ft. So....Why are the where they are at? Why aren't they anywhere in that range. Why aren't they as likely in shaded stumps in 10 ft of water as anywhere else????? (assuming there are baitfish present -of course.)