Chilling effect: a mass kill has destroyed hundreds of tonnes of endangered Canadian cod. Credit: FISHERIES AND OCEANS CANADA

More than 700 tonnes of Atlantic cod have frozen to death in chilly waters off eastern Newfoundland.

The dead fish first began to surface in the waters of Smith Sound, Trinity Bay, on 3 April, but scientists are struggling to explain what induced the cod to swim into the lethal waters.

Such mass culls are very rare, although one in the northeastern United States killed an estimated 10 million to 100 million 'warm water' tilefish fish in 1882 (R. Marsh et al. Fish Oceanogr. 8, 39–49; 1999).

At this time of year, cod are usually found in warmer waters near the bay. Local theories on why they entered the sound include a bid to escape from seals and a fateful pursuit of herring. But none of these ideas passes muster, says George Lilly, a biologist with Canada's Department of Fisheries and Oceans (DFO), who is investigating the deaths.

A team of biologists and oceanographers from local universities and the DFO has been monitoring water temperature, salinity and oxygen saturation in the sound since last week. They have also collected dead cod for analysis and confirmed that they were perfectly healthy when frozen. They have used acoustic and video monitoring equipment to track down survivors.

In early April the temperature of the water column in Smith Sound fell to −1.7 °C, says Eugene Colbourne, an oceanographer with the DFO. Historical temperature profiles from the region indicate that such temperatures are very unusual for the sound.

Cod produce antifreeze proteins in their blood to safeguard them from very cold water, but the proteins take two months to build up to maximum levels and the dead fish had very little of them in their blood, say investigators.

Some environmentalists are describing the mass kill as an environmental disaster, as it destroyed one of the last remnants of the region's few cod stocks.

Canada's fisheries minister, Robert Thibault, is expected to make an announcement shortly on Newfoundland's fisheries, and may ban cod fishing. Cod used to make up 70–80% of the total stock in the province, says Jeffrey Hutchings, an evolutionary biologist at Dalhousie University in Halifax, Nova Scotia, but 99.9% of the stock has been lost. North Atlantic cod is classed as being of 'special concern' by the Committee on the Status of Endangered Wildlife in Canada.