Other mass deaths are likely caused by human factors. In Vietnam, 150 tons of Red Talapia died last month in their aquiculture farm pens. Turns out the farmers have been allowing the fish pens to become very overcrowded. This, plus an unusual amount of ebb tides, caused the oxygen level in the water to drop. The fish died due to lack of oxygen.
In New Zealand, 10 tons of dead fish, including Red Snapper, were found floating, en masse, just off the Northland Coast. The cause of death was traced to commercial fisherman dumping their loads and broken fish nets.