REYKJAVIK, Iceland -- Researchers in Iceland are blaming low oxygen levels in a shallow fjord for the death of tens of thousands of tons of herring.

Thousands of dead herring have been found in Kolgrafafjordur fjord for the second time this winter, raising concerns about Iceland's fishery.

Morgunbladid newspaper estimated the value of the 10,000 tons of fish found dead this week at 1.25 billion kronur (US$9.8 million). An even larger number of fish died in December.

Johann Sigurjonsson, director of Iceland's Marine Research Institute, said Wednesday that roughly one season's worth of herring has been lost.

He says herring tend to "overwinter" in large populations and may have depleted the oxygen in the shallow fjord. He says the danger should ease in spring when the herring spread out into a wider area.