From Bill Cody:

Fish hug the top of the water because atmospheric oxygen is slowly dissolving into the water as a thin film on top. When the thin surface film becomes saturated with oxygen the rate of diffusion of oxygen into the water under the surface layer is slowed down a lot, thus only the thin film on the very top contains or fairly quickly gets a saturation of DO. Mixing or surface agitation exchanges or disrupts the DO saturated thin film so more atmospheric oxygen can dissovle into the new undersaturated thin surface film. I learned all this when researching my prepared response in the debate about which method transfers more DO into pondwater - plants (phytoplankton) OR mechanical diffused aeration. More on this DO topic may appear in a future issue of PBoss Mag (editor is looking into it for us).

Holding a redear sunfish is like running with scissors.