Hundreds of striped bass wash up dead in northern Cape Breton

University biologists say striped bass that recently washed ashore in northern Cape Breton probably died from a sudden temperature change in the ocean.

A video posted to the Port Morien Wildlife Association's Facebook page on Monday shows what looks like hundreds of dead fish in North Bay, near Dingwall, N.S.

Trevor Avery, a biology professor and lead researcher with the Striped Bass Research Team at Acadia University in Wolfville, N.S., said he doesn't know for sure what happened.

However, he said, it's not uncommon for fish in relatively warm water near the surface to get forced into colder water down below when the temperature changes all of a sudden.

"About 20 years ago there was a big bay full of cod up in Newfoundland that flash froze because of ice crystals raining down on them from one of these turnover events," Avery said. "Same sort of thing. You've got a bunch of ice crystals in the water and then they sort of sink down."

Avery said striped bass have a kind of antifreeze in their cells, but that doesn't help when the temperature change is like a shock.

"If the fish is in a certain temperature of water, say five degrees, something like that, and then it goes to zero degrees with this sort of slurry, it's like taking a fish and then sticking it in an ice bath," he said.

"They might not be able to recover quickly enough from that."

Fish kills rare but not uncommon

Cape Breton University biologist Bruce Hatcher said natural weather events that kill fish are not rare, but not common, either.

"We had one of those in the Bras d'Or Lake a few years ago with mackerel," he said. "It's known as a superchill."

A similar event happened with striped bass near Pictou, N.S., in 2013. Researchers later said it was caused by a sudden temperature change when Nova Scotia Power briefly stopped putting warm water from the Trenton electricity plant into the harbour, but the power company disputed that assessment.

Avery said he is trying to get samples of the dead fish from Dingwall, but called it "highly unlikely" there would be any proof of the cause of death.

He said freezing samples to be shipped to a lab for examination would make that difficult.

However, Avery said it would be important to determine which population of striped bass was affected by the kill.

The Committee on the Status of Endangered Wildlife in Canada had deemed the St. Lawrence River population to be extinct, however in recent years, the fish have made a comeback in the river.

Avery said about a million striped bass are in the Gulf of St. Lawrence, but there is another group that lives mostly off the Atlantic coast of Newfoundland, Nova Scotia and New Brunswick, and another population that lives further south, off the U.S. coast.

It would also be good to know where the populations regularly overwinter, Avery said.

Due to its salt content, ocean water can go below zero, he said, so many striped bass move into freshwater for the winter, where the temperature is not as cold.

"The overwintering grounds of striped bass is ... not completely unknown, but we don't know all of the places," Avery said.

Avery said the Department of Fisheries and Oceans would likely also be looking into the fish kill.

In an email, DFO confirmed it was aware of the "mortality event," and said it had determined there were no projects or industrial activities underway that would have caused it.

"Fishery Officers are gathering samples of the striped bass for further physiological analysis by DFO Science," wrote spokeswoman Lauren Sankey.

"It should be noted that mortality events can be the result of natural die-offs. Cape Breton is at the northern range of this particular species and therefore, local populations can be more susceptible to conditions such as rapid temperature changes."

Sankey also noted that Environment and Climate Change Canada would follow up to assess water quality in the area.