HALIFAX -- Officials have ruled out human activity as the cause of Nova Scotia's dramatic fish kill, but say we may never know exactly why tens of thousands of dead herring washed ashore.
Fisheries officials say the fish kill is subsiding -- they found just 10 to 15 fish Wednesday in an area where thousands washed up at the peak of the die-off in mid-December in St. Marys Bay.
"What we're seeing are the remnants of dead herring that have been existing on those beaches for the last couple weeks," Doug Wentzell, regional director of fisheries management for the Department of Fisheries, told a briefing Thursday.
"This event really appears to be subsiding."
He said tests for toxins, viruses, infections, contaminants and other possible causes have come back negative, and testing is now largely being wrapped up.
Officials say their search for answers was costly, but inconclusive.
"In terms of the likelihood that we will identify the cause of this? I would say it's pretty low," said Alain Vezina, regional director of science for DFO.
"There is no human related cause that we can detect from the analyses that we've done, which were quite exhaustive. This leaves really a natural event that would have led to the mortality, which remains unknown."
The die-off started in the last week of November, in a 100-kilometre swath from St. Marys Bay to Tusket, with most found between the mouth of the Sissiboo River and Plympton, and started winding down just before the holidays.
Crabs, starfish and other sea life also began turning up dead around mid-December, which officials said might have been caused by a sudden temperature change in the water.
Fish kills are not uncommon, especially in fresh water, but the last kills of comparable size involving herring occurred in the Bay of Fundy in 1976 and 1979.
Vezina said the die-off is the first in his six years as science director in the region.
Possible causes that have been discounted include a small December earthquake, and the Cape Sharp Tidal turbine that began producing power in November in the Minas Passage, about 150 kilometres away.
Vezina said scientists have been looking at the impact of climate change on ocean waters, but "to relate it to that particular event would be incredibly difficult."
Officials say testing Wednesday found far more herring swimming in the St. Marys Bay area than normal, but they're not sure why.
Vezina said predators could be herding them in the bay, or warmer-than-usual temperatures could have kept them there.
"The only thing we know is that there does seem to be dense aggregations," said Vezina.
