Test breakthrough slashes cost of novel salmon lice solution  

A Scotland-based company that has developed a retrofit permeable salmon pen enclosure to keep out lice and jellyfish is now quoting for trials and close collaboration with salmon farming companies after scale-model testing revealed that water flow through the enclosure’s nylon mesh walls was much better than previously estimated.

This means that the enclosures won’t require pumps, slashing the capital cost by more than half and making operation simpler.

The Smoltscreen by Salar Solutions is designed to fit inside a net pen, and comprises two layers of mesh, one of 150 microns and the other 200 microns, which are small enough to block all stages of sea lice, including the eggs, but big enough to let water pass through.

Salar Solutions originally proposed using pumps inside the pen to draw water through the Smoltcreen mesh but results of the model testing by respected Norwegian technological research institute SINTEF Ocean show that they aren’t required.

A massive breakthrough

“This has really been a massive breakthrough in our design model,” said Salar Solutions chief executive and founder Andrew Bett.

The flow-through efficiency was demonstrated in SINTEF Ocean’s towing tank in Trondheim. Water coloured with fluorescein was poured into the tank at the leading edge of a model Smoltscreen made with the same nylon mesh layers as a full-size version. The coloured water was then timed as it tracked across the surface of the model.

The results were extrapolated used Froude scaling to calculate the time it would take for water to pass through a Smoltscreen retrofitted into a cone-shaped 160-metre circumference, 50-metre diameter pen.

Froude scaling is a method used in fluid mechanics and naval architecture to scale physical experiments by keeping the Froude number identical between a small-scale model and a full-scale prototype. The Froude number is a dimensionless ratio comparing inertia forces to gravity forces.

8 million litres per minute

Bett said everyone involved in the testing, including marine engineers and scientists, was “ impressed and excited” by the permeability demonstrated at SINTEF.

“It has proven that the water in a 23,000m³ pen, in a tidal flow of 0.7 metres per second, would be exchanged inside three minutes,” he told Fish Farming Expert.

“This is around 8 million litres per minute water exchange, which indicates to us that we will not need to be pumping 270,000 litres a minute with four pumps, or requiring complex pipework inside the pen.”

The fact that pumps won’t be needed should make it easier for fish farmers to adopt the system, said Bett.

“Most farmers tell us they don’t like things sitting in their pens, which we can fully understand. So, our designs now do not include pumps and pipes.”

Tidal flow

Testing shows that pump-free operation relies on the farm site having somewhere between 0.2 and 1 metre per second tidal flow, explained Bett, who added that the tank tests also showed that there would be no structural challenges in such sites.

“The scale modelling at a significant wave height of 1.5 metres was absolutely fine, with no overtopping.”

One aspect of Smoltscreen operation that remains unchanged is the importance of keeping it clean, a task that Bett said shouldn’t put farmers off the technology.

“We work with highly skilled engineering companies who produce solutions for cleaning pens. We particularly like Ocein’s Stealth 1, Stealth 2, and their Ocein Resident,” said Bett. “They know that we’re very keen to continue to dovetail with them.

“I think it is starting to become accepted that there’s good water exchange with the Smoltscreen as long as the pens are kept clean, but farmers say they don’t always have very good oxygen levels in the fjords for various reasons. When that’s the case, we encourage thinking about nanobubble technology with Moleaer, another company we've anticipated working closely with.”

Ideal for Norway's red zones

Bett believes the Smoltscreen could offer an affordable solution for fish farmers who have lost production capacity under Norway’s “traffic light” system, but says the enclosure doesn’t currently qualify for the country’s environmental flexibility scheme.

Farms in production areas that are given a red traffic light in the biennial designation must reduce their production volume by 6%, and some have lost 18% over the past few years. The flexibility scheme offers them the chance to use that suspended capacity if they adopt floating closed containment. The aim of both the traffic light system and the flexibility scheme is to protect migrating wild salmon smolts from lice believed to emanate from the fish farms.

Lost in translation

Salar Solutions says the Smoltscreen, manufactured in Norway by Plany AS, would achieve that objective, but doesn’t meet the legislated criteria. Bett said this is because of the regulators’ interpretation of the Norwegian word “tett”, directly translated as “tight”.

“When a public consultation was invited in December 2024, we submitted our designs in detail; and when the environmental flexibility scheme draft regulations were issued in the following quarter, we saw that the definition for the required design was two barriers, and one of the barriers had to be what is described as ‘tight’.

“You might describe our mesh as a tight barrier. The definition didn't appear to describe or require an impermeable barrier made of PVC, steel, or other form of composite material.

“We thought they had acknowledged that our permeable systems could comply, but the Directorate of Fisheries at the moment are sticking with their interpretation which is that the word ‘tett’ means impermeable, and that means that we are not at the moment eligible for the environmental flexibility scheme,” said Bett.

Meeting with ministry

“It is a great shame because otherwise farmers that are losing biomass in the red zones could be turning to our Smoltscreen system, which is very low cost and can be retrofitted into existing sites.

“However, we haven’t left it there. Our Norwegian consultant, INAQ, have had meetings with the Ministry of Fisheries, and the Ministry have invited us to make an approach and to appeal - and we will do so.”