Thousands of fish that washed ashore at the Santa Ana River Lakes last week were killed by decreasing oxygen levels sparked by El Nino weather conditions, biologists theorized Monday.
The fish died when organic materials at the lake bottom surfaced when water temperatures shifted. On their way up, materials likely suffocated the shad and catfish swimming in Warner and Little Warner lakes.
"It basically brings up all of the yucky stuff from the bottom and eats up oxygen," said Ron Wildermuth, public information officer for the Orange County Water District. "This is new for us. It has never happened before. That's why we think it's at least partly due to El Nino rains."
The dead fish were removed over the weekend and the lake was pumped with oxygen to normalize conditions. The recreation area, near the Riverside Freeway and Tustin Avenue, will temporarily be closed to visitors even though there is no health threat to humans, Wildermuth said.