PORTLAND – The collapse of cod stocks off Nova Scotia changed the marine ecosystem so dramatically that it may be impossible for cod to recover, according to a study by Canadian scientists that could have ramifications for cod stocks at Georges Bank.

Once the top predator, cod is now a bit player in waters off Nova Scotia. Its population on the Scotian Shelf has plunged 96 percent since the 1850s, according to archaeological evidence and old fishing records. In its absence, the entire marine ecosystem has been transformed, said Ken Frank, who co-authored the report published recently in Science magazine.

“It was always thought that the effects of overfishing were reversible,” Frank said. “It’s pretty shocking when you stop fishing, the fish don’t return.”

The research of Frank, Brian Petrie, Jae Choi and William Leggett could have ramifications for Georges Bank cod, which also has been decimated by overfishing.

Frank, who works for Canada’s Department of Fisheries and Oceans at the Bedford Institute of Oceanography, says the virtual disappearance of cod and other large species such as haddock, flounder and hake led to what scientists call a “cascade effect.”

As the population of large predators declined, the fish they prey on – herring, shrimp, crab and lobster – have undergone a population explosion. That means cod, which used to sit on the top of the food chain, has now been replaced by smaller fish, Frank says.

Cod’s departure is felt at the bottom of the food chain, where zooplankton and algae are now being depleted at a faster rate because more and more fish are feeding on them, he says. Eventually, the lack of food could bring about a crash in the numbers of fish, he says.

Frank says it’s unclear whether cod can make a recovery in this new environment.

A big obstacle, he says, is that cod spend the early parts of their lives at the bottom of the food chain. Cod eggs, larvae and juvenile cod are food for other fish. Because there are fewer adult cod, there are now a larger population of fish to eat the young cod.

“The king of the jungle are typically large as adults and are safe,” said Robert Steneck, a University of Maine scientist who has studied the cascading effect in the Gulf of Maine. “With overfishing, we are left with babies, and they are not safe.”

Because of cod’s decline, lobster populations have exploded to record levels, despite increasing fishing pressure, Steneck says. And lobsters today account for more than 70 percent of the total value of Maine’s marine resources.

While that may seem like great news, Steneck says, fishing communities have become dangerously dependent on lobsters. “If anything happens to this one species, we have a disaster on our coast,” he said.