The washing ashore of thousands of jellyfish at San Francisco’s Ocean Beach this weekend was likely due to a combination of a large swell and an increase in the species’ population in the area, a National Park Service spokesman said Tuesday.

More than 10,000 circular, translucent moon jellyfish washed ashore on Saturday in an area about three miles long and 20 feet wide between Pacheco Street and Lawton Street, park spokesman George Durgerian said.

Most of the jellyfish had been washed back out to sea by high tide, but several hundred remained as of Tuesday, Durgerian said.

He said strandings frequently occur in the area during the summer and fall, but usually they’re in much smaller numbers than on Saturday.

“That makes this case unusual but not alarming,” he said.

Durgerian said marine biologists with the park service determined there has been an upwelling of nutrient-rich cold water near the ocean surface, which has made it easier for jellyfish to reproduce, causing their population to soar.

“The higher numbers at the beach may simply reflect higher numbers in the water,” he said.

Park officials will allow nature to take its course with the remaining jellyfish carcasses on the beach, Durgerian said.

“Unless there’s a public health hazard like a whale or it gives off a significant odor, we’re not going to bury or remove something from the beach, and there’s no impressive stench from these things,” he said.

Although the dead jellyfish do not pose a threat, Durgerian encouraged patrons of the beach to leave them alone.

“Every national park discourages visitors from picking up wildlife, whether it’s alive or dead,” he said. “We prefer natural processes to take them back into the sea or decompose, and that’s what’s going to happen.”

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