A researcher who spent years removing millions of green crab from a Nova Scotia estuary says evicting some of the cantankerous crustaceans has proven effective in controlling an invasive species that has wreaked havoc on marine ecosystems around the world.

Known as the "cockroach of the sea," the green crab can decimate marine environments as it reproduces quickly, mows down eelgrass with its claws and devours just about any species it comes across that's comparable in size or smaller.

"They can upset entire ecosystems. They cause cascading problems," said Chris McCarthy, a Parks Canada scientist at Kejimkujik National Park.

"Ecosystems are getting hammered because of this new invasive species."

Aggressive green crabs have caused some problems with local species.

Acclimated to cold water

The green crab is originally from Europe and first arrived on Canadian shores in the 1950s, but there was a second invasion from northern waters during the 1980s, said McCarthy.

It's these green crab that are causing the most trouble on the East Coast, as they are acclimated to cold water temperatures.

McCarthy said estuaries at the park's seaside site on Nova Scotia's southern coast near Port Joli have been under attack by green crab for nearly a decade.

Roughly 268 hectares of eelgrass — a marine plant with long ribbonlike leaves that provides food for birds and nurseries for young fish — was almost completely annihilated in two estuaries.

Fisherman-invented trap

McCarthy said a plan was developed in 2010 to try to deal with the uninvited visitors.

He said a trap invented by a local fisherman was used to catch more than two million green crab from one estuary over a few years.

Since then, eelgrass in that estuary has recovered by about 34 per cent and soft-shell clam populations are on the rise, while the second estuary remains mostly bare beneath the surface.

Millions of green crab were caught by just a few people using small, motorless boats. (CBC)

"We'll be able to export this knowledge to other areas," said McCarthy, adding that the millions of green crab were caught by just a few people using small, motorless boats.

"People are seeing the damage that they're doing and they're looking for some way to respond. This is one of very few projects that actually tried to control them. We have a measured level of effort to get control of this estuary, so people can take that and use that to plan their own interventions."

How to use green crabs

McCarthy said the challenge now is finding a use for the crustacean — which can grow to be about eight centimetres.

He said they can be used as bait for lobsters and studies are currently underway to develop green crab for the dinner table.

"If the use can be developed, then we can really knock them down and keep them down," he said, adding that complete eradication is impossible, as one green crab can produce 175,000 eggs per year.

Ecotourism trapping possible

But the long-term effects of removing large quantities of green crab are not yet known.

McCarthy said researchers aren't sure yet if they can stop fishing green crab for a period of time or if control efforts need to continue on an ongoing basis.

McCarthy says he hopes the general public will soon be able to help control the green crab population in the second estuary at Kejimkujik.

He said plans are underway to develop an ecotourism experience in which people would help set traps.