HALIFAX, Nova Scotia – The number of young cod dying in waters off the coast of Nova Scotia has doubled in the last 10 years, mystifying scientists who can’t figure out what’s happening to the fragile stock.

Marine biologists suspect cooler water temperatures, natural predators and ultraviolet radiation might be affecting the vulnerable species, but they can’t pinpoint definitive causes.

“It is a mystery,” Mike Sinclair, a scientist with the Department of Fisheries and Oceans, said at a news conference on Wednesday.

“This is a unique phenomenon that I don’t think has been observed anywhere else in the North Atlantic.”

Scientists estimate the death rate of young cod in waters east of Halifax to the Grand Banks and north to the coast of Labrador has reached 50 percent over the last 10 years.

That’s up from an annual average of about 15 to 20 percent.

The findings have baffled experts who expected the stocks to rebound after a moratorium was imposed on the species in the early ’90s. At the time, scientists suspected cod was being fished to near extinction and needed a reprieve to recover.

But, 10 years after the suspension, the stocks are still collapsing and scientists say they could be facing a new, more elusive problem.

“They’re dying when they’re quite young – as juvenile and young adult fish – and at a much faster rate than what we expected,” said Sinclair, head of the Bedford Institute of Oceanography.

“Major ecological changes are occurring in this ocean area northeast of Halifax.”

Other countries that fish cod and have similar marine conditions, such as Norway and Iceland, haven’t experienced the same kind of rapid increase in fish mortality, making it more difficult to identify the causes, said Sinclair.

He said the changing ecology has also made the cod “a lot slinkier, skinnier fish and in worse condition.”

Scientists have theorized before that the depletion of the Earth’s protective ozone layer could be weakening cod larvae and other microscopic organisms in the Gulf of St. Lawrence.

But it has never been proved that the ultraviolet rays that seep through the ozone layer are compromising the species.

In the early days of the moratorium, fishermen said seals were destroying the once-bountiful cod stocks off Newfoundland. Sinclair says they likely play a role in the high death rate, but it might not be as large as people thought.

Biologists also suspected colder water temperatures might have raised the mortality rate, but temperatures have warmed in recent years, creating better conditions for fish to spawn.

There has been no indication that diseases, such as those that have decimated farmed salmon, have affected the stock.

Sinclair said the moratorium has allowed scientists to get more accurate readings on the stocks, since they are not being fished in the region.

Other groundfish in the area that stretches from the northern tip of Nova Scotia to the Bay of Fundy are also having trouble recovering.

The former backbone of the industry used to account for about 40 percent of the fishery and more than $200 million in the latter ’80s. Groundfish, including haddock and pollock, now comprise just more than 10 percent of the fishery and bring in about $80 million.

“It’s the single-most area that troubles us,” Leslie Burke of the Department of Fisheries and Oceans said in releasing an overview for the Scotia-Fundy fisheries.

“We’re not seeing the signal of a recovery that we would have expected.”

With the downturn in the groundfishery, many fishermen along the province’s south coast have turned to shellfish catches, particularly lobster, which was valued at $286 million last year.

Burke said the region’s entire fishery brought in $592 million last year, making it the top export for the province and amounting to about $2 billion in exports in 1999.