Scientists are tracking dozens of sites in Nova Scotia and southern New Brunswick to see if Atlantic Canada’s last warm winter has slowed the spread of slimy marine invertebrates.
Six invasive sea squirt or tunicate species have established themselves in Nova Scotia in the past decade. Two more would have arrived.
The creatures cling to anything they come in contact with and have become a major problem in the shellfish aquaculture industry. Consisting of 95 percent water, they are heavy, weighing down the ropes and increasing shear stress during storms and the risk of loss of equipment and product.
“I have no doubt that most of these species are here because of the warming climate,” said Claudio DiBacco, a federal research scientist who specializes in aquatic invasive species at the Bedford Institute of Oceanography in Dartmouth, NS.
Tunicates are a pocket full of organs and water that are adept at filtering food, reproducing and “biofouling” — accumulating on everything from boats to underwater pipes. When poked or prodded, they shoot water from a siphon – hence the name sea squirt.
The invasive species mainly end up on ship hulls and in the discharge of ballast water.
Tunicates are now a year-round fact of life for Nolan D’Eon, who produces 1.4 million oysters annually on his farm in Argyle, NS
“There are a few tunicates in the cages. They’re very, very small right now. But we never let them grow,” D’Eon said while recently examining an oyster cage pulled aboard a service boat at a location in Eel Lake.
His solution is to turn each cage upside down for 48 hours and let the sun and heat kill the tunicates – but they always come back.
“We have a brood of tunicates at a time we’ve never had before. And they all used to turn green in winter. In the spring they were all dead. Now you lift your cages and they’re all alive They don’t die in the winter, which is a lot more work for us,” D’Eon said.
The creatures are also ugly.
“Scraping tunicates off clams is my life,” joked Peter Darnell, an accomplished mussel and scallop farmer in Mahone Bay, NS. season, so you have two cohorts in one year and there’s just billions of things.”
To track the distribution and survival of invasive tunicates, Fisheries and Oceans Canada monitors between 50 and 70 sites in northern Cape Breton, along the Atlantic coast of Nova Scotia and in southern New Brunswick.
Metal plates hang under public wharves, aquaculture sites and even within the marine protected area in the Musquash Estuary near Saint John.
The busy fishing port of Digby, NS, is known as Site 1.
Fisheries and Oceans Canada biologist DiBacco and engineer Neo Paulin recently pulled up a sign that hung under a floating wharf in the harbor for a year. It emerged, completely covered in half a dozen species of tunicates.
“This is the invasion front for Nova Scotia. This is where we see most of our species appear for the first time, as they typically expand from a southern distribution in the Gulf of Maine,” DiBacco said.
“Now is a good time to come and see these because they wouldn’t have experienced cold water, both because there are warmer winters and this part of the province is warmest.”
Three newcomers are closely monitored: diplosoma listerium, the European sea squirt ascidiella, and didemnum vexillum, better known as a tunicate for pancake batter.
The pancake batter tunicate was one of the first documented in 2013, one year after the warmest year on record in the Gulf of Maine and Atlantic coast.
A cold winter in 2015 knocked down the invasive tunicates, but didn’t stop them.
So-called “thermal refuges” – where the water is just a little warmer – allowed them to survive the next cold winter in 2019.
Using observations of diplosoma overwintering at three measurement sites in southern Nova Scotia, DiBacco and other scientists built a model to predict dispersal based on temperatures.
The model was used to locate monitoring sites and is now being refined to consider tunicate growth and reproduction as factors.
Back in Mahone Bay, Darnell is philosophical about invasive tunicates.
He’s been dealing with it for decades, including ciona intestinalis – a slug-like sea squirt he first saw some 15 years ago.
“Well, it’s kind of a moot point. There’s no more room for anything out there. If something displaces something else, I don’t think it matters much. Anything we put in the water just gets charged with something.”
Ballast water discharge rules
Yet the speed of its spread surprises even him.
He said that DiBacco, the federal government scientist, recently discovered ascidiella, also known as the European sea squirt, in another part of Mahone Bay, away from his leases.
“A year ago last October, he saw this new Ascidiella, another solitary tunicate. About the same size as the others. That was interesting. They’d seen them before in Lunenburg Bay, but I hadn’t seen them before. This year: half at half. Almost as much as ciona. Wow.”
In response to concerns about invasive species including tunicates, Canada is implementing ballast water discharge rules for the shipping industry, urging commercial and recreational boaters to be more vigilant in cleaning hulls and monitoring aquaculture transfers .
In Digby, DiBacco looked down at a plate full of biofouling and saw the beauty.
“If I point to the gold star… that’s this one with the star shape. Each one of those lobes is an individual and it’s that beautiful shape,” DiBacco said.
“If you can look at these, not just under the microscope, the colors, the reds, the browns — just really attractive. I know they’re slimy, but if you can get past that, they’re really quite a diverse and wonderful group individuals.”