Seals aren’t mackerel’s biggest predator: study
Mackerel commercial fishery in Atlantic Canada and Quebec has been under moratorium since 2022
CORNER BROOK — While fingers are often pointed at seals for consuming more than their fair share of commercial fish species, a study on mackerel has shown other predators have likely been putting a bigger dent in the fish stock.
The joint study was conducted by the Department of Fisheries and Oceans (DFO) Canada, the Northeast Fisheries Science Center based in Woods Hole, Mass., and Cégep de Rimouski in Quebec.
It indicated that natural predation of Atlantic mackerel seems to be on the rise, but the array of predators is varied and the forage fish is considered a minor prey source for grey seals.
Mackerel are also feasted on by northern gannets, spiny dogfish, bluefin tuna, whitesided dolphins, harbour porpoises and pilot whales.
“A lot of harvesters were asking us about seals and were worried, so we started to wonder maybe there is more going on,” said Elisabeth van Beveren, a DFO biologist who co-authored the study.
“What’s clear was other predators likely consume much more mackerel than seals do. … If we want to look at the future of natural mortality, we have to take into account the ensemble of predators and not just seals.”
TWICE AS MUCH AS FISHING
The study stated that, while the reliability of consumption estimates varied between predators, there was indication that, overall, removals might have increased over time.
Between 2012 and 2021, minimum mackerel removals through natural mortality were estimated to be between 21 kilotonnes and 29 kilotonnes, whereas median values were between 28 kilotonnes and 44 kilotonnes.
According to the report, the amount of mackerel consumed by all predators was likely at least two times greater than reported Canadian commercial fishery landings — which were less than 11 kilotonnes — during the 10-year period studied.
“The relative importance of each predator varied over time and we could not identify a single dominant predator species,” stated the study.
Increases in the abundance of grey seals and northern gannets, though, are believed to be exerting more pressure on mackerel stocks.
“(Grey seals’) impact is likely still smaller or at most comparable to northern gannets,” stated the report.
MORE WORK NEEDED
The Atlantic mackerel commercial fishery in Atlantic Canada and Quebec was placed under a moratorium in 2022. It will remain closed in 2024, although DFO has announced a 470-tonne personal-use bait fishery will open this spring.
Van Beveren said DFO is continuing to work with its American partners on the best approach to more explicitly incorporate their study’s findings into the ongoing assessment of the mackerel stock, although she said that may not be done in time for the next assessment.
“People think it’s easy to do that, but it’s one of the hardest things in an assessment — to include that kind of information because there is a lot of uncertainty associated with it,” she said. “We need to know the abundance of all predators, their energetic requirements, their diet on an annual basis in different months and regions. That is very difficult to do.”