A fish for the future

Within the next two or three months, Mowi Scotland will transfer the first fish into freshwater tanks at a new recirculating aquaculture system (RAS) broodstock facility at Ardessie on the shore of Little Loch Broom.

The move will mark a major step towards improving fish health and staying ahead of the challenges that warming seas can bring to Scotland’s salmon producers.

Since concerns about potential disease transfer prompted the European Free Trade Association to ban the export of eggs from Norway in 2019, Mowi Scotland has relied on its sister company in Ireland to supply eggs from Mowi-strain broodstock, which it has found to be tougher than other strains bred for different conditions in countries such as Iceland.

“What we see is that by growing over in Ireland for many generations, the Mowi strain is much more robust to the kind of challenges that we have there and here in Scotland,” says Mowi Scotland’s broodstock development manager, John Richmond.

But the Irish facility doesn’t have the capacity to provide all the ova that the Scottish operation needs, and the eggs it supplies are only available within the window associated with the standard spawning season, which is primarily between January and April. Mowi’s large Scottish RAS hatcheries at Lochailort and Inchmore want ova supplied over a longer period to make best use of their capacity.

The Ardessie facility solves the problems. It will have a capacity to produce 50 million Mowi-strain eggs and will use photoperiod and water temperature to manipulate fish maturity so that eggs can be made available whenever they are required. This will ensure that Mowi can supply all of its marine farms with Mowi-strain smolts.

The broodstock facility has cost an unspecified number of millions, including £5 million from the UK government’s £100m Seafood Fund Infrastructure Scheme.

“We've tried to keep it very cost effective,” says Richmond. “We bought all the equipment ourselves, we acted as principal contractor on the construction site and brought in all the subcontractors to try and keep the cost as low as possible.”

Strong business case

Even so, it’s a significant outlay, but Richmond says it should be money well spent.

“It's a strong business case. If we get these good Mowi genetics into the sea, the difference in yield from those fish compared to some of the third-party stocks is quite significant and that's where the payback comes. Better survival enables us to keep those fish in the sea to reach a good harvest weight.

“There are other efficiencies, too. We can obviously produce our eggs a good bit cheaper than we would buy them from for a third party.”

The broodstock that will be kept at Ardessie are hatched from “elite eggs” from fish in Mowi’s breeding nucleus in Ireland. DNA profiling is used to identify which fish have traits best suited to production, based on the company’s breeding index.

Resistance to CMS

“We know what areas (of the genome) correspond to good disease resistance, good growth, good pigment quality. And the fish get a score on a breeding index ranking,” says Richmond, who adds that one of the most important current selection criteria is resistance to CMS (cardiomyopathy syndrome).

“That's one of our biggest issues. It's also one that there's no ready vaccine for, so although PD (pancreas disease), for example, is still an issue, we have a vaccine for PD, hence we're pushing on the breeding selections with CMS.”

As part of its efforts to select a more CMS-resistant fish, Mowi is also doing disease challenge work with Aberdeen University. “They’re looking specifically at the strains of CMS that we are seeing on our farms, and then the genetic profiles that correspond to a good level of robustness against those strains,” adds Richmond.

Mowi is also selecting for sea lice resistance, flesh pigment quality, and growth, among other things, although the focus on growth is not simply because the company wants bigger fish.

“We find that growth has a very strong correlation in gill health as well, and complex gill issues are another one of the big challenges in the production environment at the moment,” Richmond explains.

Another winter at sea

Fish hatched from elite eggs are sex sorted then grown in separate pens at Mowi Scotland’s marine sites until it’s time for the farms to be fallowed. Mowi will grade through the potential broodstock and bring the best of them up to a farm in Little Loch Broom, close to the RAS facility. The seawater site is remote and in its own disease management area, which lessens the risk of disease transmission from other sites.

“They'll spend another sea winter in the loch and then in the spring of the following year we'll bring them onshore to the new facility and then the spawning will be throughout the autumn months thereafter.”

The farm contains around 40,000 fish, of which between 5,000 and 6,000 will be selected for broodstock.

More females are required than males, but Mowi wants to make sure the milt it uses has the best provenance.

From 6,000 males to 20

“We'll look to select maybe 5,000 or 6,000 males for DNA sampling, and then we'll refine that down to 300 or 400, and then ultimately we might only use 20 or 30,” says Richmond.

“The better we get at it, the fewer males we'll try and use. It’s different for the females because they've only got a certain number of eggs in them, although we’d expect at least 15,000 eggs from a good two-sea-winter fish of maybe 15 kilos, and they’d be good size eggs, too.

“We’re looking to get quite big fish as they generally produce bigger eggs which means a bigger alevin and a bigger first feeding fry, and that gives freshwater production an advantage and it produces a more robust first feeder as well.”

When the fish are brought into Ardessie they will be distributed among 10 concrete tanks, each measuring approximately 9 metres diameter and 2.8 metres deep.

The tanks each have their own freshwater RAS and temperature control. Each tank will also be encircled by a blackout curtain extending from the roof of the hall to below the rim to the tank. This, along with control of water temperature, enables Mowi to use artificial light to control when each tank of fish is ready for stripping.

Wild salmon do not eat once when they return to their natal river to spawn, and the fish at Ardessie won’t be fed, either.

“They’re big fish by that point,” say Richmond. “They have a huge reserve and they're basically putting all their energy into the egg and milt production. It mimics what they do in the wild.”

Wild salmon die after they’ve spawned, and the same fate awaits the Ardessie fish, albeit a little earlier.

Euthanised before stripping

“When we get the fish to maturation, we’ll euthanise them in a tote bin containing a lethal dose of anaesthetic, then open up the body cavity of the fish to take the eggs and the milt out,” says Richmond.

“Another method is to use compressed air to inflate the body cavity and the eggs come out of the vent, but that can force some eggs out that might not have been ready. When we open up the body cavity, the eggs that were going to release will just flow out, and any that are not quite ready will remain bound up in the membrane.

“It also allows you then to inspect the carcass of the fish. We also screen all the fish for any issue. The day before we strip them, the guys go into the tank and squeeze out a little bit of ovarian fluid which we'll send off to the labs for testing so that we know that fish is disease free. We do the same with the males.”

The euthanised males have their gonads removed. This method gives Mowi more milt from each fish than catheterising fish to extract their sperm.

Once the eggs have been stripped, they are washed in a saline solution before being mixed with milt, disinfected, and placed in Ardessie’s incubation facility. This is the second stage at which Mowi can control the timing of its ova deliveries. There are three separate incubation units in the facility, each with its own RAS, and Mowi can control the development of the ova by running water at a different temperature in each unit.

“Generally we're sending eggs out after around about 380 degree days - the temperature multiplied by the number of days,” says Richmond, although he adds that the formula doesn’t hold true at lower temperatures. “You can't just keep an egg at one degree for 380 degrees, so we use a calculator that assesses the percent development.”

The longest period that Mowi can keep a viable fertilised egg is 170 days at 2°C.

This, combined with the extended stripping window, means Ardessie can meet the demands of Mowi’s hatcheries which produce four batches of smolts per year.

As well as receiving eggs from Ireland, Mowi Scotland has established a broodstock facility at its Inverpolly hatchery, although it is also seasonal. Although Ardessie has not yet stocked its tanks with broodfish, it has been incubating eggs from Inverpolly.

As a result, Richmond says between 80% and 90% of eggs going to hatcheries this year will be Mowi stock.

“There were some in the early season that we couldn't supply because we were just spawning the fish under a natural photoperiod. Next year we'd hope to have 100%, all of the eggs going in as Mowi stock, but then we’ll have a lag in terms of when those will come through as harvested stock. They've got a year or so in fresh water, and another year and a half in seawater before being harvested.”

Technical tour: the tanks, lamps, chillers and filters that give greater Mowi control of egg production

The tanks in which Mowi’s broodstock will be triggered to produce eggs and milt hold 150 cubic metres of water and are made of prefabricated concrete sections supplied by Nottinghamshire firm A-Consult. They sit on in-situ concrete bases. Tensioned cables running through the concrete panels hold them together.

A polyurea coating on the inside provides a smooth surface for cleaning and for the fish that might otherwise abrade themselves on a rougher surface, and it seals up the concrete to give it greater longevity.

The 10 tanks are laid out next to each other in two lines of five, and there are two plant rooms on the outer edges of the tank hall, each containing five recirculating aquaculture systems, one per tank.

Each tank has a circular steel door in the side for access after the water level is lowered to sort the fish.

“We’ll put a light anaesthetic in the tank when we're sorting the fish just to make them sleepy and easier to handle and once we've finished sorting the fish we'll run it almost on a flow-through basis to remove that (sedative) from the water before we return the tank on to the recirculation system,” explains broodstock development manager John Richmond.

Forever autumn

The tanks will be each have a curtain around them to block out ambient light and enable Mowi to use photoperiod and water temperature to mimic autumn and induce spawning.

Ardessie has LED lights specifically designed for lighting over fish tanks, producing a white light frequency. 

“The main thing is we have the ability to control the dimming and the ramping up of the light so that we keep the fish settled during those transitions,” says Richmond. “You set a programme, and we can obviously have each tank set to its own individual programme.

“We’ll keep them on 24-hour light if we don't want them to spawn and then we reduce down gradually to an eight-hour day length. After about eight weeks of eight-hour day, keeping the temperature down around 8°C, that’s enough to cue them.”

Filter upon filter

In one of the plant rooms to the side, Richmond and broodstock manager Paul Russell explain the elements of a tank’s individual RAS.

The water from the tank first goes through a drum filter, above left, and then a protein skimmer to remove particulate waste.

“Although they're not feeding, the fish will still produce some faecal waste as they're essentially metabolising their body tissues, especially when they first arrive in,” says Richmond.

The water then flows into two moving bed biofilters, and is degassed in two trickle filters, before being pumped back into the tank.

“We've got some ancillary process: temperature control via a heat exchanger, and oxygen injection to replace the oxygen the fish are consuming. We have a little bit of pH buffer in as well,” says Richmond.

The temperature difference between the tanks is no more than 4°C, between a ‘steady state’ 12°C and 8°C.

Always chilling

“We'll probably have to be chilling all of the time, because you are putting a little bit of heat energy into the water through the mechanical process. Simply by blowing air in, pumping the water around, you create a bit of frictional heat,” says Richmond. 

“And in summer months, there'll be a bit of load coming from the top-up water, so we'll always be chilling just to offset that and then more chilling when we need it to cue the spawning.”

The fresh water used for the broodfish at Ardessie is taken from the Allt Airdeasaidh burn which flows adjacent to the hatchery into Little Loch Broom. 

The burn water is first filtered through a 60-micron drum filter to remove the larger particulates and then pumped through pressurised sand filters where it is possible to remove particles down to 15 microns. 

Fish-free water source

“We don’t disinfect the water as it abstracted from a fish-free source above a waterfall which is impassable to fish,” says Richmond. “Also, as the fish are resident in a seawater farm prior to stocking into the Ardessie freshwater farm, we can never ensure the facility is fully biosecure so there is no logic to trying to sterilise the new water as we do in our production hatcheries where we only stock eyed eggs certified disease free and disinfected upon arrival.”

Water used in the incubation units is, however, treated with a high dose of UV during the recirculation process to kill any saprolegnia, a water- and airborne fungus that can kill eggs and young fish.

It’s the last stage of a process that begins with putting water from the egg silos through sand filters to remove eggshell debris and other particulate waste.

The water filters down into two moving bed biolfilters containing bacteria that metabolise the dissolved waste that the eggs are producing. “Just like the fish, they excrete ammonia. So the biofilters will break down the ammonia to nitrate and then nitrate,” explains Richmond.

The water then moves into a trickle-down biofilter to remove CO₂ from the water, before being exposed to UV.