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.