Actually that is exactly what I would like to know. I am staying on a large creek/small river off the Potomac. When I was young, there was a great deal of aquatic life- crabs, fish, shrimp, etc. Through pollution, poor farming practices, and extreme over-fishing/crabbing,this was all changed. It became much shallower, and the crabs all but disappeared; we certainly never saw any. Through new regulations, and with the help of my watchdog relatives, this has slowly improved. Although it is still shallow, and pollution still comes in, it is not as much. The crabs began to come back, and we began to see them every now and again off the dock.
This year is different, though, and very disturbing. The first time I went down to the dock, I noticed it. At low tide it is extremely obvious. Dead crabs line the bottom and the shore. As the water washes the mud on the bottom, more are uncovered. Dead fish are also everywhere. Gone are the huge schools of minnows, ranging in size from 1/2 inch to 3 or 4 inches. Gone are the huge schools of shrimp. If one sits there long enough, you do see a few live animals. A few crabs, some minnows, exactly two shrimp. But almost more disturbing was what these creatures were doing. All of them were jumping to the surface, over and over. This is to be expected of crabs; they breathe air. But fish? they should not be behaving as dolphins. And they were dying, as I sat there. One particularly large minnow's passing was "memorable." He skipped across the top of the water, before stopping and dying, turning over on his back. Almost immediately, minnows of the same species began eating him. What can best be described as a bait ball, where predator is prey, was formed. Soon, not just the dead minnow, but others still alive, were being eaten. Very strange. Other minnows were "swimming" by spinning, over and over, from being on their stomach, to side, to back, etc.
From the fact that they were all jumping out of the water so, I would conclude that the oxygen content of the water was low, perhaps from fertilizer run off. However, I don't know what is causing the fish to suffer from vertigo, perhaps some disease? As to why they have resorted to eating each other alive, well.....
I have no way of telling on my own what is causing the problems. I do know it is a real problem, though, and one that is impossible to escape. I say this as I sit on the porch, the air being filled with the scent of rotting animals.
So, to all you readers, wherever you are, please think before you use any chemicals on your yard, or in fact use any chemicals at all. There are so many natural, environmentally friendly products available, many of which work just as well. And even if they didn't, once your world, your personal environment, is dead, you are not far behind. So why would it matter?