Surface tension of water clinging to the gill collapses the lamellae and the effective surface area is reduced to a tiny fraction of normal. Catfish survive better than most other fish because they have cartilage supports for the primary filaments and have the secondary lamellae thickened and separated more than usual. Hemoglobin in the blood will react with the oxygen to form oxyhemoglobin, keeping the DO low and high. As a result one ml of blood will carry as much oxygen as 15 - 25 ml of water. Hemoglobin exhibiting the Bohr effect will require a higher oxygen tension to fully load under conditions of low pH. Hemoglobin showing the Root effect can never be fully saturated with oxygen when pH is low.
Gills are normally low in carbon dioxide and high in oxygen.
Hemoglobin loads on the high curve and unloads on the low curve. Suppose conditions deteriorate in the pond. Decomposition uses up much of the oxygen, producing abnormally high carbon dioxide levels. They can't load as much in the blood because they have to load on the low curve. They can't unload as much to the tissues since they will be unloading on the same curve they loaded on.