One interesting aspect of the most notoriously reported fish kill is that all the fish were bottom feeders.

All common fish have a "swim bladder" that retains a pocket of air that's used for "balance" and "bouyancy control." Most fish, however, use that bladder only for those purposes.

Many bottom feeding fish have an additional "air bladder" or have "overdeveloped" swim bladders adapted to supplying oxygen to the circulatory system in addition to the balance and bouyancy functions.

Cat fish generally have the most obvious "air bladders," and are able to surface and suck air to fill the bladder, after which they can spend prolonged periods in oxygen deprived water, where there is insufficient concentration of oxygen for extraction by the gills. (Their behavior in this respect is somewhat like porpoises?)

Many other bottom dwelling fresh water fish have similar but less developed "scuba" capabilities, carrying "breathing air" of a sort to support some time in oxygen poor water while feeding.

Fish generally are cold blooded, so they assume the temperature of the surrounding water. For the most part they can sense water temperature and move to an appropriate area where water temperature and oxygen content are sufficient. If the entire body of water is cooled, however, it's common for fish - especially bottom fish - to become quite "lethargic," and response to slow cooling may be almost entirely missing.

With an inability to sense gradual changes in water temperature, a fish resting in cold water with low oxygen availabiity may not sense the need to move before the oxygen available from swim and/or air bladders is depleted sufficiently to deprive the fish of the ability to rise to the surface (loss of swim bladder bouyancy), or to swim far enough to find a more oxygen rich area (CO₂ accumulation in the air bladder). Since cooling the water reduces its ability to contain oxygen, while at the same time making some species of fish progressively less likely to move about, the result can be -

Dead fish.

Cat fish, with their fairly well developed ability to suck air at the surface, are seldom found in oxygen deprivation kills. They also tend to "loiter" much nearer the surface than some other bottom fish - and in much shallower water than many self-proclaimed expert (cat)fishermen will believe. On the other hand I have seen catfish killed on a stringer that kept them in a poorly oxygenated area in waters where they were quite vigorous when free to move about. (A really sad way to lose your supper.)

The common carp that are reasonably abundant in my area also tend to stay quite close to the surface or in fairly shallow water, and are less commonly found among the dead from this effect. The carp have a tendency to enjoy browsing on "green stuff" near the surface as well, and so far as I've observed seldom venture into deeper parts of the waters they live in, unless it's for short excursions while feeding.

True "bottom feeders" like the drum reported in the most recent kills probably explore bottoms at more variable and deeper depths, making them more susceptible to being caught in oxygen deprived water; but they weren't common enough in my area when I was fishing regularly for me to have observed their habits in as much detail as for the more common (and more edible) kinds of fish.

With information available from the news reports, this is only a "plausibility" argument, and not necessarily something to be taken as an explanation for the specific events reported in the news.

John