VIDEO: Mowi launches $30-million vessel to treat farmed salmon for sea lice

It’s a huge ship that’s meant to deal with small but potentially deadly pests.

The Aqua Tromoy is equipped with state-of-the-art gear for removing sea lice from farmed salmon. Mowi, the seafood multinational, gave the Mirror an exclusive tour of the 77-metre ship on Thursday, showcasing a system designed to filter out the parasites and other microbes from its open-net pens.

The launch of the ship comes amid growing concerns about how sea lice may spread from fish farms to juvenile wild salmon as they migrate to sea.

“The idea with this vessel is it’s 100 per cent lice capture,” said Courtlan Thomas, Mowi Canada’s lead well-boat captain, as he described the vessel’s specialized water treatment system. “Anything that’s going over the side is being UV treated and strained.”

The fish are sucked from their ocean-based pens into enormous tanks or wells below the deck. Those tanks carry up to 3,000 cubic metres of water, making the diesel-powered Aqua Tromoy the largest ship of its kind in Canada, according to Mowi – 650 cubic metres was previously considered big.

“Atlantic salmon are lifted aboard from farm pens and immersed in the fresh water, which is harmless to the fish, but removes sea lice and other salt water microbes and parasites that do not tolerate low-salinity water well,” the company said in a statement. “Fine filters then remove parasites from the water before it is re-introduced to the ocean.”

The ship also boasts a sea lice treatment system involving hydrogen peroxide, which knocks the sea lice off the fish. The chemical is discharged into the sea while the ship is in motion, in order to dilute it across a wide area, according to Chris Read, a spokesperson for Mowi.

“Propellers are then churning the water as they’re driving the vessel forward, further mixing, so you’re basically getting dilution at a very rapid rate,” Read said.

The company is trying to reduce dependence on chemical treatments because they’re expensive and have a bad public image, Thomas says. That’s part of the thinking behind the multi-level ship, which is worth more than $30 million.

High above the weather deck is the wheelhouse, where officers can control and monitor the ship’s various high-tech systems – including the creation of fresh water.

It uses a reverse osmosis system that can generate 6,000 cubic metres of clean water from the sea within 24 hours, according to Mowi. After treating the salmon, the water is then recycled using a system that involves ultraviolet radiation and huge barrel filters.

Other features on the European-built ship, which will be operating as needed along the B.C. coast, include a grading system for sorting fish by size, and for removing bycatch – particularly the herring that swim into open-net pens and then grow too large to escape – before returning them to the sea.

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But the ship is perhaps most notable for its complex systems designed to remove sea lice and other pests from farmed salmon. Read said the company wants to avoid sea lice infestations on its farms as much as possible.

“We monitor on a regular basis to stop them being a significant enough buildup to cause an issue where you can have lots of lice larvae in the water,” he said.

Asked how the company disposes of the sea lice once they’ve been filtered out, Read said: “The lice are captured and are stored at the farm, not on the boat. The lice are then disposed of along with any fish mortalities at the site. The process used for disposal of lice and fish mortalities is that they are sent for composting.”

Other methods of controlling sea lice include a drug called Slice, which aquaculture companies put into fish food, and Mowi describes the new ship as part of a larger integrated pest management strategy.

But the parasites are increasingly becoming resistant to all forms of treatment, said Karen Wristen, executive director of the Living Oceans Society, a non-profit ocean conservation group.

“Any treatment you give to sea lice will result in resistance,” she said. “That’s been the experience in Norway. When they started using hydrogen peroxide, the lice evolved thicker shells…. They’re incredibly adaptable little creatures, those sea lice.”

Wristen welcomed the news that Mowi’s new ship features a freshwater bath system for detaching parasites from salmon, saying it’s less harmful than chemical methods.

“The alternative, that we dump more and more toxic chemicals into the ocean, that’s not what we want to see happen,” said Wristen, who co-authored a 2018 study called Lousy Choices detailing sea lice drug resistance in the Clayoquot Sound. “I hope this mechanical treatment can be made to work.”

An unprecedented outbreak of sea lice took place last year in Clayoquot Sound during the out-migration period, between March and June, as wild juvenile salmon were making their way to the ocean.

At least one farm was temporarily shut down by the aquaculture company Cermaq as the abundance of free-moving sea lice exceeded the regulatory limit by several times, and the farmed fish were reportedly euthanized and turned into fertilizer.

Wristen noted that she’s concerned about whether the addition of a ship like the Aqua Tromoy will be enough to keep lice levels under control.

She said that on smaller well-boats, filters would become clogged due to high sea lice levels.

Sea lice occur naturally in Pacific waters, but wild salmon wouldn’t normally encounter the pests in large numbers until the fish had matured enough to cope with their effects.

But with the advent of fish farming, the parasites affix themselves to juvenile wild fish as they swim by the pens, leading to potentially deadly open skin wounds, Wristen said.

“It certainly can kill them… the fish are susceptible to infection,” she said, adding that wild juvenile pink salmon have been found with as many as 50 sea lice.

The ones that survive are also less successful feeders, meaning they grow more slowly and are more likely to be eaten by predators, said Wristen.

She called for the removal of fish farms from the ocean, saying that land-based aquaculture is the way of the future, especially considering the high cost of ships like the Aqua Tromoy.

“It’s not all that much more expensive when you talk about the additional costs of drugs and these phenomenally expensive mechanical treatment units,” she said.

This article was updated on April 24 with more details about the sea lice disposal process.

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