Why a Canadian herring population is dying off

Canadian scientists have delivered a calamitous fish stock assessment in the southern Gulf of St Lawrence, predicting the spring spawning herring population is on a trajectory toward extinction in 10 years.

The grim projection was shared earlier this month by the Department of Fisheries and Oceans, just days before the spring herring fishery is set to open in Nova Scotia, New Brunswick, P.E.I. and Quebec.

Predation is killing six of 10 older fish each year and a warming ocean is knocking down a critical food source for young.

Spring spawners, as the population is called, have been in trouble for many years, but data gathered in 2018 and 2019 indicates very high levels of mortality, said Francois Turcotte, a marine biologist with DFO based in Moncton, N.B.

“So many fish are being removed, and not enough are coming in, that the biomass can only decrease,” he said in an interview last week.

Seals, tuna and warm water

Scientists believe the high level of natural mortality is the result of predation by grey seals and bluefin tuna, and are discounting other potential causes like disease or unreported fishing.

Warming ocean temperatures in the gulf are also contributing to the downward spiral. Herring larvae feed on a cold-water species of energy-rich microorganisms known as zooplankton. That zooplankton is declining.

Turcotte said fewer young are surviving to spawn, meaning as adults die off they are not being replaced.

The size of a fish population is measured by spawning stock biomass, which is an estimate of the weight of all the fish old enough to spawn. The spring spawner biomass is estimated at 33,000 tonnes, down from 200,000 tonnes in the 1980s and 1990s.

The DFO stock assessment predicts that at current levels, in 10 years the biomass will fall to 100 to 1,000 tonnes, a threshold where a population is so low it can be wiped out by random events, like extreme weather.

It’s not a guaranteed outcome, however. With fewer herring available, predators may switch to other prey, relieving some pressure.

“The decline is continuous but it’s hard to get to the level of certainty for such long predictions because there’s things we don’t know, like how predators react,” Turcotte said.

Cutting fishing may not work

A spring spawner fishery still exists, but Turcotte’s model shows it is so small — 1,250 tonnes in 2019 — that scrapping it may not reverse the trend.

“It’s concerning because there’s no lever we can pull to make things better,” he said. “The model does show that if you reduce fishing you do reduce the chances of declines, but only by seven or eight percent. There’s no guarantee here that even if you use the only tool you have you’re going to improve the status of the stock, that’s the concern.”

Confusingly, there are two herring populations in the southern gulf. The other one is a fall spawning stock. It is in better shape, with a biomass estimated at 174,000 tonnes, down from 600,000 tonnes in 2011.


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