I can't chime in for everyone who live in the cold regions, and performed DO monitoring, but I do noticed for my colder mountain areas. Trout tend to search for food sources, and will venture into the low DO zones during day time, attempting to stay in the warm zone, then end up belly up due to the lack of O2 with no food in sight. Same for carp. They would do the same thing, searching in familiar grounds that normally have dense vegetation, but during winter time, plants died off, and they end up trapped in the low DO zone with no food insight. They were unable to return to the depth where the high DO was. They became easy scavenged for predators.
