New evidence shows that Atlantic cod off Nova Scotia are recovering from their dramatic collapse two decades ago — and that the ecosystem is recovering with them.

Cod and similar species on the Scotian Shelf have been eight to 18 per cent more massive for their age between 2006 and 2010 compared with 1992 to 2005. (Associated Press)

That suggests major changes to marine ecosystems can be reversed with time, says a Canadian study published Wednesday in the scientific journal Nature.

It also "bodes well" for other cod populations further north along the East Coast that have yet to recover, says the study, led by researcher Kenneth Frank at the Bedford Institute of Oceanography in Dartmouth, N.S.

The waters of the Scotian Shelf east of Nova Scotia, which once teemed with millions of predatory cod, have remained dominated by much smaller, plankton-eating fish such as capelin and herring for more than a decade, despite a cod fishing moratorium since 1993. That led to fears that the ecosystem had been irreversibly altered.

Frank's research eases those fears, partly by uncovering why the cod recovery has taken so long.

"This unfolding drama held many surprises," his paper says.

The study shows that the collapse of predators such as cod resulted in a population explosion of plankton-eating "forage fish." The peak mass of such fish in the ocean reached up to nine times what it was before the cod collapse, spiking in 1994 and 1999. At such levels, they competed with and sometimes ate cod eggs and baby cod, hampering the cod population recovery.

But the overpopulated plankton-eaters eventually began running out of food, and their population started to decline steadily in the early 2000s.

Switch to 'recovering' state around 2005

Around 2005, their ecosystem went into a "recovering" state, where cod populations began rising again.

The researchers suggest that a crash in populations of forage fish provided a "window of opportunity" for predators like cod to recover.

Since 2006, the mass of Atlantic cod on the Scotian Shelf has approached levels in the early 1990s and the mass of haddock, another predator, has grown to an "unprecedented high," the paper says.

In addition, cod and other large fish that live near the bottom of the sea measured between eight per cent and 18 per cent more massive for their age in the 2006 to 2010 period, compared with 1992 to 2005.

"Although the current trajectory is positive, several factors could alter ongoing ecosystem recovery," the researchers warn.

They note that the current dominance of haddock makes it uncertain whether the ecosystem will completely return to the way it was before the cod collapse. They add that in other areas, jellyfish blooms and invasive species have slowed the recovery of collapsed fish populations, while the effect of climate change could add other uncertainties.

Ken Frank with the Bedford Institute of Oceanography cautions that while fish stocks appear to be recovering, the trend needs to be monitored before the fishery can resume. (CBC)

Although there's a promising amount of haddock, pollock and cod being caught for the first time in 20 years, Frank cautions that the size of the fish is another story. Haddock for instance, are half the size they were historically.

"I'm concerned that the growth rates of the individual species aren't as good as they should be," Frank told a news conference.

He also cautions that the situation will have to be watched closely to see if the trends being documented continue, before resuming the fishery.

"We have a solid team of departmental scientists who will look at the most recent information and then with the fishery managers … possibly set a plan in place to determine whether or not it's too early to begin to resume the fishery or possibly institute a test fishery," suggested Frank.

"There's lots of options, but at this stage we want to go slowly and look carefully at the results we're monitoring."

The researchers say their study suggests other cod populations in the northwest Atlantic may also recover, although they believe that could happen more slowly due to colder waters, continued fishing and the lower number of marine species in some of those areas.

In 2010, the Northwest Atlantic Fisheries Organization's scientific council reported that the cod population in the Grand Banks, an underwater plateau southeast of Newfoundland, had grown 69 per cent since 2007. However, that was still just 10 per cent of what the stocks were in the 1960s.