From fish tanks to farmers’ fields, Scotland’s salmon growers are transforming nutrient-bearing fish waste into fertiliser to enrich agricultural land


Costing an eye-watering £58million, Scottish Sea Farms’ new Barcaldine RAS Hatchery on the shores of Loch Creran near Oban is equipped with some impressive technologies, all with one thing in mind: sustainable food production.

Minimal fresh water usage, reduced fossil fuel consumption, a wood chip biomass system and provision for its own hydro scheme – the environment has been considered at every turn, right down to the responsible re-use of fish waste.

Scottish Sea Farms’ Operations Manager for Barcaldine RAS Hatchery, Noelia Rodriguez, explained: “Our fish arrive with us as tiny eggs and are hatched and raised in fresh water until they are the right size and weight for transfer to sea pens.

“Throughout the growing process, the hatchery’s recirculating aquaculture system – or RAS for short – ensures that the fish have a continuous supply of clean, oxygenated water that’s maintained at a steady temperature.”

During this same ongoing cycle of cleaning and recirculating water, any waste material such as fish faeces or uneaten feed is removed and captured for recycling.

Lead Engineer for Barcaldine RAS Hatchery, Ewen Leslie, explained: “Using technology by Norwegian engineering company Scanship AS, we first aerate the waste to prevent any unwanted bacteria from germinating, then we bind it together into larger particles via the addition of a cationic polymer.

“That done, the waste is filtered to separate the solids from the water. These solids, which are now of a sludge-like consistency, are then collected in a storage tank.”

Ensuring the sludge is both safeand suitable for agricultural land is Invergordon-based waste management company Rock Highland, part of the Avanti Environmental Group.


Scottish Sea Farms’ Lead Engineer for its Barcaldine RAS Hatchery Ewen Leslie

They remove the stored sludge via arctic tanker barrel and take it to their holding tanks strategically located across the Highlands.

Samples of the sludge are tested periodically at the UKAS-accredited Scottish Agricultural College, part of the SRUC, and an organic waste report is produced identifying all the components present in the material. This includes Nitrogen, Potassium and Potash; a combination of which gives the sludge its high nutritional value.

Once the sludge has been certified as being safe for use on agricultural land, the nutrient-rich by-product is then uplifted from the storage tank by tractor and barrel and spread onto the land to enrich the soil and aid crop development.

Rock Highland Divisional Director Neil Barker said: “We first started out by working with one or two of Scotland’s whisky distillers, helping recycle nutrient-bearing effluent originating from barley into fertiliser.

“Over 15 years later, we’re now working with distilleries from the Orkney islands all the way down to the Central Belt, spreading around 175,000 tonnes of distillery effluent per annum back to the land via a network of registered land banks across the whole of Scotland.

“Recent years have seen us diversify and apply the same sustainable service to Scotland’s salmon farmers.

“In addition to Scottish Sea Farms, we’re also working closely with MOWI Scotland and the Scottish Salmon Company, with our proven model now collecting sludge from most of the salmon hatcheries across the Highlands and Islands of Scotland.

“For the country’s farmers, this salmon hatchery byproduct offers a really high value, low cost alternative to synthetic fertilisers. No-one turns it down.”

Back at Barcaldine, Scottish Sea Farms’ freshwater team are hard at work on phase two of their fish waste recycling plans to add further value.

“We’ve already identified reducing technologies that would enable us to remove the remaining water content and convert the sludge into dry pellets,” said Ewen Leslie. “Now we’re scoping out the best uses for it before conducting some preliminary trials.

“The benefit to the environment of moving from wet to dry form longer-term would be that we could reduce the volume of waste material, thereby reducing the number of tankers and road miles required to transport it from hatchery to farmland.

“For land farmers, dry form would provide an even more nutritional and valuable natural fertiliser alternative that’s easy to handle.”

“We’re always thinking, always mindful of the environment, always trying to reduce our impact in order to produce fresh farmed salmon in the most sustainable way.”