OSLO—Tens of thousands of dead herring have carpeted a stretch of coast in northern Norway — and then disappeared again.

The fish appeared on New Year’s Eve, leading to speculation that predators might have driven a huge school ashore or the fish could have been washed onto the beach by a powerful storm that hit Norway on Christmas Day.

Jens Christian Holst of Norway’s Institute of Marine Research says a combination of several factors was the likely explanation.

Locals also started pondering how to clean up the estimated 20 tons of dead fish on the beach in Kvaenes, near Tromsoe, north of the Arctic Circle, before it decayed. But before they could start, the fish disappeared.

Holst said Tuesday the herrings have likely washed back into the North Sea.