HALIFAX — Scientists remain baffled by what has caused tens of thousands of dead herring to wash ashore along Nova Scotia's southwestern coast, despite a battery of tests.
"What it is, is a bit of a mystery,'' said David Whorley, director for Fisheries and Oceans Canada in southwest Nova Scotia.
The first sightings were reported late last month along the eastern edge of St. Marys Bay, and more dead fish were later spotted on beaches in the Annapolis Basin and farther west, near Pubnico, N.S. There have been no major sightings in recent weeks.
"We've made some progress and have ruled out a few things, but we haven't found the actual cause yet,'' Whorley said in an interview Wednesday.
Dead herring are shown at Gilberts Cove, N.S., in this recent handout photo. (Photo: Joan Comeau/The Canadian Press handout)
The tests have produced negative results, including the fact that no infections or infectious agents have been detected in the small, silvery fish.
As well, Whorley said examinations of the herring have failed to find any obvious signs of damage, and molecular testing of tissue samples showed they were clear of some types of viruses and harmful bacteria.
More testing is on the agenda, he said.
The Canadian Food Inspection Agency, for example, is looking for toxins caused by algae, and for the possible presence of domoic acid — a toxin sometimes found in shellfish.
"The fact that (the testing) is coming up negative is good news,'' Whorley said. "Some of these are nasties."
Pollution, parasites possible causes
Among other things, the herring don't appear to have infectious salmon anemia, a viral disease that can wreak havoc in open-pen fish farms. Testing has also ruled out infectious pancreatic necrosis and hemorrhagic septicemia.
Earlier speculation had suggested the herring could have been driven ashore by whales or other predators, but three surveillance flights have failed to spot any large mammals along that section of the coastline, Whorley said.
"We continue to monitor areas in southwest Nova Scotia for any other incidents."
Pollution and parasites have also been mentioned as possible causes.
Dead herring are shown at Gilberts Cove, N.S.
(Photo: Joan Comeau/The Canadian Press handout)
People should report any sightings of dead herring, but they should refrain from collecting samples, as a precaution, he said.
Local fishermen have said they can't recall seeing herring wash up on the beaches in such large numbers.
Herring are known as a forage fish, which means their large schools play an important roll in feeding whales, seabirds, seals and larger fish, such as cod.