Halibut flourishing, cod struggling, says DFO stock assessment

Wild food fish populations off Atlantic Canada continue to confound scientists, with some species flourishing and others floundering.

The latest examples are halibut and cod.

The big flatfish is flourishing off Nova Scotia and Newfoundland, according to the latest stock assessment released by the Department of Fisheries and Oceans.

However, another new DFO report shows cod stocks off southern Nova Scotia remain in dire shape.

Researchers believe they are seeing the impact of warming ocean temperatures — especially off southern Nova Scotia and the rich fishing grounds of Georges Bank.

Warming oceans

"Most likely it's something in the environment. We do know there has been increases in temperatures," said Kirsten Clark, a research scientist at the St. Andrews Biological Station in New Brunswick.

"That can have a direct impact. For example, the young fish are not surviving, the eggs are not viable for some species.… For other species, it can be great," said Clark.

Warmer temperatures can also trigger an earlier arrival of phytoplankton blooms, causing fish or their prey that rely on those blooms for food to miss the event.

The winners 

The department said the halibut stocks off both Nova Scotia and Newfoundland and Labrador have rebounded from depleted levels in the early 1990s to the point where scientists recommended the department increase the total allowable catch by 15 per cent in 2018 — the maximum amount allowed — to 4,164 tonnes.

"The updated halibut survey index shows that abundance of exploitable halibut continues to increase, with 2017 being the highest in the 20-year time series," DFO said.

The healthiest groundfish population, however, remains Georges Bank haddock, which has seen a huge growth in numbers over the past decade.

It is estimated in 2013, one billion haddock were hatched on Georges Bank, an event that will keep fish plants busy for many years. 

"Undoubtedly, there were cyclical changes in the past, but in terms of the time spans we are looking at, we haven't seen these levels of haddock in the past or cod at the levels we are seeing," said Clark. 

Silver hake is also doing well, she said.

The losers 

For Atlantic cod, once the bulwark of Atlantic Canada's ground fishery, the turnaround has not materialized off Nova Scotia. And the stock took a big step backward off northern Newfoundland and Labrador in 2017.

In March, DFO reported a dramatic 30 per cent drop in the spawning biomass of northern cod off Newfoundland and Labrador.

In its assessment off Nova Scotia, DFO said cod has not recovered from its 1990s plunge.

For decades, fishing grounds from Halifax to Georges Bank and into the Bay of Fundy produced annual landings of 20,000 tonnes. Now cod is caught as a bycatch in other fisheries. 

In 2016, landings were 717 tonnes.

"Given the very low biomass, low productivity due to low recruitment, truncated age structure and high total mortality, the current outlook for this stock is extremely poor," said the stock assessment.

DFO said the reason for the elevated rate of mortality in this and other cod stocks is not fully understood but may include predation by seals. The Canadian grey seal population continues to increase at approximately four per cent per year.

Yellow flounder is another species that is struggling.

Investigation ongoing 

Clark said the link between environmental changes and their impact on various species is not understood.

That information is needed to better predict fish populations and the basis for setting fish quotas.

"We are keeping track of these environmental variables. We are keeping track of the predator species. We are looking at the prey species. We are trying to figure out what those links are that are driving those different populations," Clark said.