Hundreds of herring fish, about 20 tons, washed up dead on a beach at Kvennes in Nordreisa, northern Norway.
Residents reported spotting the dead fish early Monday.
The mysterious death of the fish seems to have baffled scientists and residents who said there could be two to three possible causes of the death.
The fish could have been killed either because they were trapped in an oxygen-depleted area by the tides or may have moved away from large predator fish, landing dead ashore. Any disease or stormy weather could also be the reason for fish death, according to marine scientists.
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While researchers are trying to find out the cause, cleaning up of the beach is now a headache for the local authorities.
Mass death of ocean fish points to environmental instability. Contribution of human actions also can't be ruled out.
In March last year too, over a million fish (sardines) washed up dead in a Californian harbor because of oxygen deprivation.
In May 2010, about 500 metric tons (500,000 kilos) of milk fish were seen floating dead in Taal Lake, south of Manila in the Philippines. A sudden drop in temperature because of climate change was the most probable reason.