Pollution, destruction of spawning grounds and over fishing are reducing catch. The increase is due to development. During the Spring the algae grow in abundance. In the summer the algae die and settle to the bottom cooler non-circulating water. The bacteria feed on the dead algae and use up the available oxygen.
Bottom dwellers are most susceptible to effects of hypoxia.
Oxygen begins to affect life, that is fish, clams, crabs etc. Deadly after a few days.