Seals and porpoises near fish farms aren't a threat to migrating Atlantic salmon, report says

A study led by Fisheries and Oceans Canada has found seals and porpoises pose little risk to migrating Atlantic salmon in an area of southern New Brunswick full of fish farms.

Researchers wanted to know if predators attracted to the sea cages full of farmed salmon are also preying on wild salmon migrating to the open ocean. It is believed 99 per cent of salmon leaving Bay of Fundy rivers do not survive in the ocean.

"In the first couple of weeks, in the early marine environment we have shown that seal predation or other warm blooded predators are not a main driver towards the low adult returns and that our effort should be put towards other stages of the life cycle," said Greg English, an aquatic science biologist with Fisheries and Oceans based at St. Andrews, N.B. 

In 2019 and 2021, a total of 310 hatchery-raised Atlantic salmon were surgically implanted with acoustic tags that can also detect temperature changes when the fish is eaten by a warm blooded, or endothermic, predator.

The salmon were released in the Magaguadavic River, which flows into Passamaquoddy Bay, where there are dozens of cages containing farmed salmon.

Few tagged salmon eaten by warm-blooded predators

A network of acoustic receivers and instruments were placed throughout Passamaquoddy Bay and the wider Bay of Fundy in southwestern New Brunswick.

In 2019, 120 instruments were put in the water. That was increased to 197 In 2021.

"While previous research has shown that aquaculture sites can potentially attract marine mammals and other predators, predation by endothermic predators on Atlantic salmon post-smolts was not centered around aquaculture sites in this study," researchers report in a study published in the Canadian Journal of Fisheries and Aquatic Sciences.

"This suggests that predation risk of Atlantic salmon post-smolts by endothermic predators may be low around salmon aquaculture sites."

The study found a total of 11 tagged salmon over two years showed clear signs of being eaten by warm blooded predators, in most cases by seals.

The majority of migration routes passed within one to three kilometres of at least three fish farms. Receivers at fish farms detected 69 per cent of the tagged fish in 2019 and 54 per cent in 2021.

More pressing threats

Jason Daniels, a biologist with the Atlantic Salmon Federation, was a co-author of the study.

He said fish farms pose other risks to the wild population like sea lice, disease and interbreeding with escapees.

"Those are the real concerns I think, when it comes to aquaculture and wild interactions," Daniels said.

'Really encouraging'

The research is part of a broader federal government program examining the impact of fish farms on the ecosystems around them.

For that, Daniels applauds Fisheries and Oceans Canada.

"They're looking at all of these single driving factors. They're working on disease and life studies and obviously marine mammal predation. They're kind of breaking it apart and they're looking at what's attributable from each factor," he said.

"Seeing them take the lead on some of these projects is really, really, encouraging."

Fisheries and Oceans has also tagged some of the most plentiful fish species around aquaculture sites in Passamaquoddy Bay: mackerel, Atlantic herring and sculpin. The data is still being analyzed.

"We're looking forward to analyzing these further and seeing what interactions there may be," said English.