Predators take big bite out of declining Atlantic mackerel population

Predators ate at least twice as many Atlantic mackerel as commercial fishery landings in the decade leading up to Canada's region-wide moratorium, according to new research by Canadian and American scientists.

The study also found seals are a major predator, lending credence to what many fishermen have long claimed.

The consumption estimate is the first attempt to quantify removals by predators on the declining population. The finding that natural mortality outweighs fishing underscores the challenge of rebuilding a declining stock through government management measures like reduced quotas.

The modelling study was published this month by the federal Fisheries Department and the Canadian Science Advisory Secretariat.

The top predators were gannets, grey seals, dogfish and bluefin tuna. In the most conservative estimate, predators removed between 21,000 and 29,000 tonnes annually between 2012 and 2021 — at least two times greater than Canadian commercial landings reported as 11,000 tonnes per year.

"The thing is with natural mortalities, it's such an important parameter in an assessment and we had almost no information about it," said Elisabeth Van Beveren, lead author and a research scientist with DFO in Quebec.

"It gives us a better grip on the predators that are important, what we need to pay attention to. And it did give us an idea of the potential scale of that natural mortality."

In the case of seals, DFO believes mackerel makes up a very small part of their diet — from virtually nothing up to four per cent. But with the grey seal herd now estimated at over 300,000, their sheer numbers mean they have an impact.

By 2020-21, estimates of consumption by seals varied between 7,000 and 13,000 tonnes.

"Scaling that up to their huge abundance still means that they are of certain importance. So it did allow us to get a better sense of the importance of seals relative to those other predators," Van Beveren said in an interview.

"If I'm being honest, then yes, seals popped out more than I thought they would."

The estimate that seals are a major predator is not a surprise to Andrew Bourgeois, a fisherman in Chéticamp, N.S.

"Obviously that population has exploded, and I'd say is the major part of the predation," he said.

"We've seen it over the years and we've been saying it over the years that seals are having a major impact on the stock, and slowly DFO is coming around."