Blue crab helping fight invasive green crab in Atlantic

Invasive green crab in Atlantic Canada may have met their match in a naturally occurring blue crab that is increasing in waters off Nova Scotia.

The alien green crab, which likely originated in the Mediterranean, were first introduced to New Jersey in 1817. By the 1950s, the crab had found their way north to waters off southern Nova Scotia.

In recent years, they have rapidly expanded their range around Cape Breton, north to Prince Edward Island the Magdalen Islands of Quebec and into even colder waters around Newfoundland.

Chris McCarthy, an ecologist with Parks Canada, said it's thought this second wave of green crab are not from the Mediterranean, but from Northern Europe — and this second wave of green crab thrive in cold water.

The crab have been eating their way up the coast, feeding on clams, oysters, mussels, small fish — but perhaps most alarming is their tendency to destroy ecologically vital eel grass in the process. 

McCarthy said eel grass serves as a nursery for more than half of commercial marine species, as well as for a number of species at risk. Without the eel grass, these lush estuaries become underwater deserts and that can have a huge impact on species that rely on it. 

In order to stop the marauding crustacean eating machines, Parks Canada began a pilot project in the Little Port Joli and St. Catherine's River waterways near the Kejimkujik National Park Seaside to combat the green crab surge.

McCarthy said the pilot project to fish for green crab has exceeded expectations. Ecologists, volunteers and local fishermen have caught more than 1.5 million green crab over the last four years, putting a dent in the damage they cause. 

Here's where the blue crab come in

The green crab, though beaten back, still pose a threat.

That's where seeing a surge in blue crab populations gives ecologists hope.

Blue crab are four times larger than their cousin the green crab. They are naturally occurring in the Atlantic, however they're not common in the colder, northern waters off Nova Scotia. 

In 2013, while checking their green crab traps, McCarthy said they came across 14 blue crab.

In 2014, they've caught about 200 blue crab and he suspects there could be several thousand of the crab in Basin Lake which feeds the estuaries at Keji's seaside adjunct.

Since blue crab compete for the same food sources as green crab — without destroying eel grass in the process — seeing their numbers increase is good news in the fight against invasive green crab, said McCarthy.

He said there have been reports of blue crab eating green crab, another plus for what McCarthy and his team affectionately refer to as "The Blue Cavalry."

He said this year, researchers have seen very little of the eel grass being ripped up, a good indication that green crab numbers are declining. 

McCarthy suspects blue crab made their way to Nova Scotia as larva on warm ocean currents from the south.

He said ecologists are closely monitoring the situation, but healthy eel grass beds, and an increase in shorebirds in the area give them hope.

Parks Canada ecologists will head out Wednesday to see how the blue crab are faring in their effort. 

McCarthy said the effort would not have been possible without the help of fishermen and volunteers in the area that provided vital local knowledge to ecologists. He said the project has had a number of partners, including the Department of Fisheries and Oceans.

He said if people would like to volunteer to fish for the crab — and have a fun time while helping do something good for the environment — they can inquire through Kejimkujik's website.