We have average of 8' thermocline on the shallow lake I fish most . Looks like your question was answered. In summer low oxygen means few if any fish below the thermocline .I see guys set trotline then act shocked when the fish all dead . They anchor them down and fish can't move into good water. I have no trouble seeing it on my Humminbirds . It appears as a cloudy area from bottom up to the thermocline on my screen .