Rising grey seal population eating adult cod at unsustainable levels
Fisheries and Oceans Canada (DFO) has issued a stark warning linking the demise of codfish in the southern Gulf of St. Lawrence to a rapidly increasing seal population.
The warning is contained in the most recent stock assessment of Atlantic cod in the southern Gulf that was released earlier this month.
“At the current abundance of grey seals in this ecosystem, recovery of this cod population does not appear to be possible, and its extinction is highly probable,” the report says.
DFO fish biologist Doug Swain said the cod population is now about five per cent of levels in the 1980s, and the downward spiral is accelerating despite a moratorium on a directed cod fishery in the Gulf since 2009.
The problem is an “extremely high” and “unsustainable” death rate for cod five years or older.
How many cod are dying
Normally, about 18 per cent of adult cod each year would be expected to die from natural causes, like being eaten by a predator.
The DFO assessment estimates between 55 and 57 per cent of adults are dying each year.
Swain believes the fast-rising grey seal population is to blame.
“The evidence all points to grey seals as being the cause of their natural mortality. We’ve looked at many hypotheses and haven’t come up with any other ones that appear to be plausible.”
Why cod are vulnerable to seals
In the southern Gulf of St. Lawrence, cod spend winter in the same place each year in deep water off northern Cape Breton.
While the population has dropped, they still gather in big enough groups to attract seals.
Satellite tagging and stomach analysis have shown they are being targeted by seals, said Swain.
“We’ve observed that the grey seals continue to forage in the vicinity of these cod aggregations, that cod comprise a very high proportion of their diet in these times and places,” he said.
“The adult cod that are the ones that have the natural mortality are estimated to comprise about 40 per cent of their diet by weight.”
Swain said cod have abandoned shallow summer feeding grounds for less abundant deeper water to avoid grey seals.
Fish not eaten by grey seals have not moved.
A downward spiral worsens
Scientists use “spawning stock biomass” as a measure of the health of species.
It’s an estimate of the total weight of the fish capable of reproduction.
There have been sharp drops in the spawning stock since the last DFO assessment in 2015, which used data up to 2014.
Since then, the spawning stock decreased by more than half — by 3,000 tonnes between 2015 and 2016, and a further 12,000 tonnes between 2017 and 2018.
“That is very striking for a stock that is at very low abundance,” said Swain.
Today, the spawning stock is estimated at 13,900 tonnes — the lowest on record.
The assessment predicts a further 4,700-tonne decline by 2023.
When the stock dips below 1,000 tonnes, DFO considers it extinct, which could happen within a few decades.
Swain said the benchmark is somewhat arbitrary, but for a stock that was once 500,000 tonnes, cod are unlikely to persist when the threshold is reached.
At that level, they are vulnerable to harsh environmental conditions.
“The probability is very high the stock will decline to local extinction,” Swain said in an interview from DFO’s offices in Moncton, N.B.
Other threats loom
DFO allows commercial fishermen in the Gulf to take 300 tonnes of cod a year as unintended, or “by-catch,” from other groundfish fisheries.
Ground fishing for other species in the Gulf is now so low, it has negligible impact on the population trajectory.
Preliminary by-catch landings for 2017 and 2018 were 60 tonnes.
But that could change.
Fishing is expected to increase for booming populations of halibut and redfish in the deeper gulf waters where cod still persist, making it more likely cod will be taken as by-catch.
Climate change is expected to have negative effects, causing cod to lose energy faster in warmer water, especially in winter when they are not feeding.
Is it too late? Probably
The assessment says it would take a large reduction in grey seals to stop the decline, but even then there could be unintended results like a rebound of cod predators also eaten by grey seals.
“You know things can change,” said Swain.
“But right now in terms of the ecosystem, it’s most likely that things will change for the worse because you know even if grey seals were reduced … we’re entering a period of dramatic climate change.”