The fish may have been chased by bigger fish and died of suffocation, one researcher told AP

Takashi Fujioka, a researcher at Hakodate Fisheries Research Institute, said it was the first time he had observed such a phenomenon, per AP.

Fujioka speculated that the fish might have been chased by bigger fish, run out of oxygen while swimming together, and then been carried away by the waves.

https://www.businessinsider.com/tons-fish-washed-up-japan-beach-officials-dont-know-why-2023-12