Japan mass fish death: did lack of oxygen kill at least 1,000 tonnes of sardines and mackerels at Hokkaido beach?

Last week, a large number of sardines and chub mackerels were found dead on a beach in northern Japan. Initially, it was suspected that a potent pollutant might have caused the mass death. However, marine biologists have now concluded that the fish died due to a lack of oxygen in the water. There are two theories explaining this occurrence. The first theory suggests that the fish were being pursued by predators and were driven into the shallow bay near the village of Toi. The large concentration of fish in the bay quickly depleted the available oxygen in the water. The second theory proposes that the fish encountered a sudden pocket of significantly colder water on their migration route, which weakened them.

The fish began washing ashore on December 7. Television footage showed the 1. 5km shoreline covered in silver fish. Local officials estimated that around 1,000 tonnes of fish had washed ashore, but the actual figure might be higher. Kevin Short, a naturalist and professor of cultural anthropology at Tokyo University of Information Studies, explained that large schools of sardine and mackerel migrate south off the coast of Hokkaido at this time of the year. These fish are often preyed upon by larger predators, such as dolphins and tuna. It appears that a large number of these fish were driven into the shallow bay where they quickly consumed all the available oxygen.

There have been similar incidents in the past, although such a large number of fish deaths is unusual. Previous cases have been recorded in parts of Tokyo Bay and the Inland Sea, where 'red tide' algae blooms are believed to have contributed to the mass fish death. In these cases, the microorganisms had drifted to the seabed and consumed much of the oxygen in the water, making it difficult for the fish to survive. Daisuke Imura, an official with the Ministry of the Environment, stated that similar incidents have occurred in Hokkaido in recent years, but never on such a large scale. He also mentioned that climate change might be impacting fish stocks and that rising temperatures are having an obvious effect onshore. Local officials in Toi have advised residents not to consume the fish washed up on the beach as the cause of death is still unclear.