I have witnessed a similar event up in Alaska. We flew up to a river system on Kodiak Island that was experiencing drought conditions for the last 2 months, very low water running over dark rocks. The early morning salmon would make it the mile and a half to the upper lakes. Later in the day as the sun rose and the water temperatures did the same the fish all ran out of oxygen and would loss conciousness. A line of the bigest seagulls I had seen around waited anxiously to peck out the eyeballs then start on the rest of the fish if they could catch them. Fish piled up at the mouth like trash until they fisheries people closed the weir to prevent the fish from coming upsteam. Easy pickins for the birds and bears, though they were not keeping up with the piles.
Once the rains came and water levels rose the fisheries people opened up the weir for the fish to run upsteam again. Sad sight to see for sure, mother nature can be cruel.