HALIFAX - Several East Coast fish stocks that collapsed in the early 1990s are showing the first signs of recovery, according to new research that suggests whole ecosystems can rebound.

Scientists at the Bedford Institute of Oceanography, a federal government research centre in Halifax, released a study Wednesday that says cod, haddock and other once dominant groundfish stocks are making a comeback on the eastern Scotian Shelf.

Ken Frank, an oceanographer and lead author of the paper, said it is a heartening sign after decades of seeing little to no recovery of the stocks that were once a central part of the region's fishing industry.

"It was felt that those stocks and the food chain that supported it might never recover," Frank said after the publication of his research in the journal Nature.

"So we're happy to report that we're beginning to see the initial signs of the recovery of this ecosystem."

The scientists link the stock recoveries to the decline of so-called forage fish, like herring and capelin, that were feasting on groundfish eggs and became dominant in the ecosystem.

In a 2005 paper, Frank said the virtual disappearance of cod and other large species, such as haddock, flounder and hake, due to overfishing led to what he calls a cascade effect. That means large predators declined dramatically, but the fish they preyed on — herring, capelin, shrimp and snow crab — thrived and eventually experienced a population explosion.

Cod, which once sat at the top of the food chain, were replaced by smaller fish that dominated the marine world and stymied their recovery.

There was also a trickle down effect to the lowest members of the marine food chain — zooplankton and algae — which were depleted at a faster rate because more fish fed on them.

Frank said he started seeing a shift in the trend in 2005, when the forage fish biomass began to decline or correct itself.

It's not clear why the smaller fish are dropping in numbers, but Frank said their population burst appears to have "run out of gas."

Cod stocks are at about 34 per cent of their biomass before the collapse, while haddock is close to 100 per cent and pollock is almost 75 per cent of its pre-collapse biomass.

"Atlantic cod and redfish have reached levels not seen since the early 1990s and haddock to an unprecedented high," the paper states.

Still, the researchers are concerned that the actual sizes of the fish have increased slightly but remain smaller than their historic norms.

A six-year-old cod that once weighed two kilograms now weighs 1.5 kilograms, while a six-year-old haddock is a full kilogram lighter than it was historically.

Frank said the habit of catching the larger, faster growing fish year after year may have actually altered the genetic composition of the species, leaving only the smaller, slow-growing fish.

"It's possible that we're kind of locked in to these smaller sized fish," he said.

The scientists caution that while the discovery is encouraging, fisheries managers shouldn't rush to reopen fisheries that have been under moratorium since the mid-1990s.

Frank said they've seen modest improvements in groundfish stocks in other areas, such as the southern Grand Banks, and are linking them to similar declines in forage fish.

"The answer to the critical question of whether or not such profound changes in the dynamics of large marine ecosystems are reversible appears to be yes," the paper states.

By Alison Auld, The Canadian Press