Californian fishermen and harbor chiefs were puzzled after millions of dead anchovies were found floating in a marina just south of Los Angeles on Tuesday.
Experts said the fish, found at a harbor in Redondo Beach, may have from lack of oxygen due to algae, while locals wondered if high winds overnight might have something to do with it.
"It looks like what happens to goldfish when you dont change the water in the tank, mouth open and belly up," said local official Bill Workman, cited by the Los Angeles Times newspaper.
"There are no visible signs of any toxins that might have caused (the deaths) and our early assessment is that this was oxygen depletion," he said, adding: "There are no oil slicks or leaking of substances into the water."
Locals at Redondo Beach, where the stench from the dead fish hung over King Harbor, suggested that high winds could have driven the fish into the harbor, where their sheer numbers would have reduced the oxygen available.
Experts were cautious. "What we're trying to tease apart is whether it's a consequence of algal buildup, a fish buildup or something toxic in the water," said David Caron of the University of Southern California.
Explore further: Mexico sets 25 pct pollution cut by 2030 for climate talks