Scientists are monitoring dozens of sites in Nova Scotia and southern New Brunswick to see if the latest warm winter in Atlantic Canada accelerated the spread of slimy marine invertebrates.
Six invasive sea squirt — or tunicate — species have become established in Nova Scotia in the last decade. Two more are believed to have arrived.
The creatures cling to anything they come into contact with and have become a major problem in the shellfish aquaculture sector. They are 95 per cent water and heavy, weighing down ropes and increasing shear stress during storms and the risk of lost gear and product.
“There’s no doubt in my mind that most of these species are here because of the warming climate,” said Claudio DiBacco, a federal research scientist specializing in aquatic invasive species at the Bedford Institute of Oceanography in Dartmouth, N.S.
Tunicates are a sack 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 invasives arrive primarily on vessel hulls and in ballast water discharge.
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, N.S.
“There’s some tunicates on the cages. They’re very, very small as of right now. But we don’t ever let them grow,” D’Eon said as he recently examined an oyster cage pulled on board a service boat at a site in Eel Lake.
His solution is to flip each cage upside down for 48 hours and let sun and heat kill the tunicates — but they always come back.
“We have a spawn of tunicates at a time that we’ve never had before. And they used to all turn green in the winter. In the spring, they were all dead. Now you lift up your cages and they’re all alive. They don’t die over the winter, which is a lot more work for us,” D’Eon said.
The creatures are also unsightly.
“Scraping tunicates off mussels is my life,” joked Peter Darnell, a veteran mussel and scallop farmer in Mahone Bay, N.S. “There’s just so many of them. They’re prolific. They’re unbelievable. They’ll spawn twice in a season, so you have two cohorts in one year and there’s just billions of the things.”
To track the spread and survival of invasive tunicates, Fisheries and Oceans Canada monitors between 50 and 70 locations from northern Cape Breton, along the Atlantic coast of Nova Scotia and southern New Brunswick.
Metal plates are suspended below public wharves, aquaculture sites and even inside the marine protected area in the Musquash Estuary near Saint John.
The busy fishing port of Digby, N.S., is known as Site 1.
Fisheries and Oceans Canada biologist DiBacco and technician Neo Paulin recently pulled a plate hanging for a year underneath a floating wharf in the harbour. It emerged completely covered with a half dozen species of tunicates.
“This is the invasion front for Nova Scotia. This is where we see most of our species show up for the first time as they’re typically expanding from a southern distribution in the Gulf of Maine,” DiBacco said.
“This is a good time to come see these because they would not have experienced any cold water, both because there are warmer winters and this part of the province is the warmest.”
Three new arrivals are being closely watched: diplosoma listerium, the European sea squirt ascidiella and didemnum vexillum, better known as the pancake batter tunicate.
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 the invasive tunicates down, but did not stop them.
‘Thermal refugees’
So-called “thermal refuges” — where the water is just a little bit warmer — enabled them to survive the next cold winter in 2019.
Using observations of overwintering by diplosoma at three monitoring sites in southern Nova Scotia, DiBacco and other scientists built a model to predict the spread 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 them for decades, including ciona intestinalis — a slug-like sea squirt that he saw for the first time about 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 guess it doesn’t make much difference. Everything that we put in the water just gets loaded with something.”
Ballast water discharge rules
Still, the speed of the spread surprises even him.
He said DiBacco, the federal government scientist, recently detected 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 one called ascidiella, which is another solitary tunicate. About the same size as the other ones. That was interesting. They’d seen them before in Lunenburg Bay, but I hadn’t seen them before. This year: half and half. Almost as many of them 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 about cleaning hulls, and monitoring aquaculture transfers.
In Digby, DiBacco looked down at a plate crawling with biofouling and saw the beauty.
“If I point to the golden star … that’s this one with the star shaped. Each one of those lobes is an individual and it’s that beautiful shape,” DiBacco said.
“If you get to look at these, not just under the microscope, the colours, the reds, the browns — just really attractive. I know that they’re slimy, but if you can get past that, it’s really quite a diverse and beautiful group of individuals.”
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