Unusually warm water over an 11- to 13-day period led to low oxygen levels that caused a mass die-off of farmed Atlantic salmon on Newfoundland’s south coast, says the province’s chief aquaculture veterinarian.
Some fish plant workers and the province’s fish harvesters’ union both raised the possibility that sea lice had caused thousands of salmon at a Northern Harvest Sea Farms facility to die early last month, but veterinarian Daryl Whelan rejected that possibility.
“We’ve done the diagnostics and really, what’s occurred is not an infectious process that led them to the mortality,” Whelan said.
“What’s led them to the mortality is the really low oxygen availability to them.”
Whelan said that though sea lice exists in open-pen salmon aquaculture, the lice weren’t the cause of the die-off, which the Fish, Food & Allied Workers union has said killed hundreds of thousands of fish in the affected sites in the Coast of Bays/Fortune Bay area.
He says water temperatures ranged from 18 C to 21 C throughout the entire water column, meaning there was nowhere for the fish to go to cool off. Higher water temperatures mean less available oxygen in the water for the salmon.
“They will try and go to the cooler part of a net or a cage system,” Whelan said, in effect bunching up at the bottom of the sea cage. If too many go down for too long, he said, it exacerbates the problem.
‘We have to accept it as a new normal’
Northern Harvest Sea Farms is looking at redesigning its salmon cages, said Jason Card, a spokesperson for the company.
“We have to act as though this temperature spike is not an isolated incident. We have to accept it as a new normal so that we are ready to deal with it,” Card said.
“So deeper cages, aeration systems, these are different ways that we can keep the water oxygenated, keep the fish cool and keep a good product going.”
The warm-water event was confined to the areas Northern Harvest is operating in, and no other aquaculture companies have reported any mortalities related to warmer-than-average water, Whelan said.
Temperatures on the south coast usually fluctuate, and wind and currents generally prevent the water from staying too warm for an extended period, he said.
“Each of our bays are entirely different and right now all those bays are separated in that region, there’s company by company,” Whelan said.
“The other companies would be in different regions so there would be a different temperature profile.”
FFAW initially raised possibility of sea lice
The massive salmon die-off was first reported to government Sept. 2, Card said.
There’s been debate about what caused the die-off, with some fish plant workers in Harbour Breton pointing the finger at sea lice. Union representative and longtime plant worker Eric Day says as many as 385 sea lice were counted on two salmon.
Northern Harvest Sea Farms, now busy emptying sea pens of dead salmon, has rejected the claim that sea lice contributed to the die-off and also said an extended period of warmer water temperatures was to blame.
Four purse seiners and divers from Newfoundland and Labrador and New Brunswick are still working to clean out the salmon pens.
“We are doing everything we can to clean up the sites as fast as possible,” said Card, who noted that the sites can be as much as 30 kilometres apart on the water and that a round trip to the rendering plant to dispose of the dead fish is 30 hours.
The company has not released any numbers to indicate how many salmon died, but provincial fisheries minister Gerry Byrne said in late September that the potential number of affected fish could have been up to 2 million, but that not all of the salmon in the affected pens died.