The sardines and some mackerel washed ashore in Hakodate on Japan’s northernmost main island of Hokkaido on Thursday morning, creating a sliver blanket along a stretch of beach about a kilometer (0.6 mile) long. Takashi Fujioka, a Hakodate Fisheries Research Institute said the fish may have been chased by larger fish, become exhausted due to a lack of oxygen while moving in a densely packed school, and were washed up by the waves.

Climate change has allowed spotted seals to find new hunting grounds, but the marine mammals ultimately depend on sea ice to survive.