It looks like the waves of animal deaths aren't over yet: Millions of dead anchovies, sardines, and mackerel washed ashore in Los Angeles on Tuesday. Scientists said the cause of death was “critically low” levels of oxygen in the water, but they are still working to figure out what caused those conditions. Massive amounts of stinking fish have washed ashore in King Harbor, Rotunda Beach—in 2003 and 2005—after algae blooms depleted the harbor water of oxygen, which caused the fish to suffocate and die. This problem is not specific to Rotunda Beach: Marine scientists say nearly half of all U.S. bays, estuaries and other waterways have low-oxygen dead zones. Tuesday’s die-off followed unusually heavy rain in Southern California, dumping all kinds of human waste into the water.