They were showing how a huge school of sardines can actually be so dense the sardines in the center of the school can die from lack of oxygen. I wonder if this happens to shad on our lakes? Just thinking, maybe this is why you see a bunch of dying shad sometimes. Maybe not.
Sometimes around here....
A large reservoir will be drained through say a 6 or 8 inch pipe for many days or weeks into a bayou or creek. Many fish will come up to the running water. If it is either turned off, or detiorates significantly in quality (close to the bottom) all those fish can die pretty quick.
Now I'm not sure if the shad can school so thick in open water to die of oxygen starvation, but they are particularly susceptible to low DO and will be among the first fish to show stress at such times.