A blueprint for aquaculture in Scotland


by Sam Houston

Funding Manager

Sam Houston of the Scottish Aquaculture Innovation Centre describes recent trials of offshore salmon farming and explains why this approach has potential to increase both sustainability and productivity in the sector.

This article first appeared in the Journal of the Institute of Food Science and Technology.

With waves up to three times higher than traditional farming sites, stronger currents and deeper waters, fish farming offshore is no mean feat. On these ‘high energy’ sites, jobs as seemingly straightforward as feeding fish can become a significant undertaking because of the extremity of the elements.

Great difficulty, though, can also mean great opportunity. Being located offshore means more space and the remoteness of these sites makes them suitable for sustainable expansion with the potential to improve the health and welfare of the farmed fish and reduce impacts on the local ecosystem.

A high-energy site is being tested off the island of Westray, Orkney, a first in Scotland. If the site continues to be successful, it could be a blueprint for the future of Scottish aquaculture.

Breaking ground at sea

Established by Cooke Aquaculture and supported by the Scottish Aquaculture Innovation Centre (SAIC) and Marine Scotland, the Skelwick Skerry site has four out of a planned eight cages installed. The ambitious project received a European Maritime and Fisheries Fund (EMFF) award from Marine Scotland to financially support the project.

As the first of its kind to open in Scotland, Skelwick Skerry represents a potential turning point in how fish are farmed in Scotland. Its achievements to date indicate that this could be an important moment for the future of the industry.

The site offers greater opportunity to sustainably grow fish farming because farming offshore vastly increases the number of potential sites. The deeper waters and stronger currents disperse disolved and particulate wastes more quickly.

The first cohort of salmon was transferred to Skelwick Skerry in November 2018, weighing an average of 2.5kg. After rapidly growing over the following months, the first harvest was carried out in May 2019, at an average weight of 5.5kg.

Fish performance at the site has been positive and, although it is difficult to draw conclusions from a single input of salmon, the health of the fish has been good and mortalities very low. Significantly, gill health of the first salmon cohort has been very good.

This is the first time healthy fish have been harvested from a high-energy site in Scotland and plans are already underway to install an additional four cages on the farm and create a sister high-energy site, thus quadrupling current capacity and directly employing up to 16 staff on Westray.

Of course, high-energy sites are not without risk. Given Skelwick Skerry lies 2.5km from the nearest shore in waters 50m deep, access is not always easy, with the tempestuous waters at times rendering it impossible. The significant wave height of the site is 6.6m – much higher than on farms closer to land.

Battling the elements

According to management, the key design point at high-energy sites is to keep things as simple as possible so that feeding time is maximised and maintenance time minimised.

The site consists of a mooring grid and four flotation cages, each with a circumference of 130m. The cages are fabricated from high density polyethylene (HDPE), which is flexible. Deep, wide nets - over 28,000m3 each - provide plenty of space for the salmon to move about and escape the action of the waves.

All activities at Skelwick Skerry are under the whim of the weather and particularly the wind, which generates the waves. Often in the north-east Atlantic south-westerly winds blow, generating very large waves but with long wave-lengths, gentle giants as opposed to shorter, more violent waves.

However, the site can also experience strong north or easterly winds. Often these winds whip up shorter but higher frequency waves. These waves pass more quickly through the site and can cause workboats to pitch violently, running the risk of of damaging the cages if they are moored to them at the time.

The cage, net and mooring system design become of paramount importance under the environemental loads. The cage design preferred by the site operators has the sinker tube supported directly by the net. This helps the net and cage components move in unison with one another, minimising any abrasion caused between the two.

To combat the dangers associated with this method of fish farming, technologies more suited to this new environment are now being examined. This, it is hoped, will simplify processes and reduce the number of days on which salmon cannot be fed due to weather conditions.

Feeding strategies

Currently, feeding is carried out using a hopper and blower system on the deck of the site’s dedicated workboat, the Annalie. The hopper can carry up to five tonnes of feed. The crew will travel to the site every day and feed each of the cages in turn. To deliver the feed, the vessel must moor up against the cages, plug into the underwater camera systems and blow feed over the water surface. Therefore the violent waves described above can render feeding too risky, primarily because the movement of the vessel could damage the cage, leading to fish escaping.

The Annalie can carry a few extra tonnes of feed on her deck but the crew must load them into the hoppers at sea, which is dangerous in rough conditions. When the salmon require more than five tonnes during a day, each one tonne bag must be loaded by the workboat’s crane. For this reason, two alternatives to the current routine are being considered by the management at Cooke to reduce the number of missed feeding days.

The first solution would be to install a high-capacity feeding barge. Barges are widely used by the aquaculture sector to streamline the feeding process. A feeding barge is an unpowered vessel moored adjacent to an aquaculture site. Onboard the barge large quantities of feed can be stored. A system of hoppers (or silos), dosers, blowers and selectors can help to further automate the feeding process. Depending on the system this can be controlled locally or remotely.

The advantage of a high capacity barge is that it is able to store larger volumes of feed than the Annalie’s hoppers. There are, though, two key drawbacks to operating this kind of system at Skelwick Skerry. The first is that it would require a power source currently not in place and the second is the large capital outlay required.  Despite this, having a high capacity barge would result in fewer trips having to be made by staff and would simplify the feeding process. It should also result in less missed feeding days with positive implications for fish performance.  

The second alternative is a single-point mooring situated close to the site. Feeding pipes to each cage would be installed to meet at the single point mooring and the Annalie’s current blower system would be modified to deliver feed via these pipes. This circumvents the need for the Annalie to moor up to the cage side, eliminating the risk of cage damage in high energy conditions.

The advantages of the single-point system are that the day-to-day feeding operation would be similar to current routines and there would be no need to install a power source. The drawback here, is that it does not increase the capacity to hold larger feed volumes. Both alternatives seek to feed the cages from a safer distance. Feeding fish from a distance should result in fewer missed feeding days and is less likely to result in damaged equipment.  

Salmon is Scotland’s number one food export, so it is not too dramatic to argue that the impact of new and improved fish farming methods could be massive on the economy.

Supporting the economy

The Scottish aquaculture sector has set itself ambitious growth targets to double output, value and employment by 2030. High-energy sites represent one way to sustainably increase production and, due to the deeper waters, having more fish per site does not mean more damage to the ecosystem through waste building up on the seabed.

However, in order to maximise the potential of these sites, challenges have to be overcome. Weather conditions, more robust equipment and ways of farming fish more suited to offshore sites must all be considered. If successful, this could be of great benefit to the local and national economy.

To reach its growth targets, industry will be keen for more sites like Skelwick to come to fruition in the medium term. However, increased specifications of equipment and techology means a requirement for increased investment.

Aquaculture is already a key employer in Scotland’s rural and island areas. Skelwick Skerry, for example, has created eight jobs and supports even more roles in the supply chain, from fish processing to sales. Scottish salmon producers registered a combined turnover of £1.027bn in 2018. When economic multipliers are considered, this could mean that salmon contributed about £2bn[1] to Scotland’s economy. High-energy sites may only be responsible for a small percentage of that at the moment, but this share is likely to grow.

A model for growth

Skelwick Skerry is an excellent example of public-private collaboration and the site should be seen as a blueprint to salmon producers with similar ambitions. As more tailored technology is developed to increase environmentally-friendly growth, SAIC is keen to support the innovation required to expand into new locations by linking research with the needs of the industry.

Salmon is Scotland’s number one food export, so it is not too dramatic to argue that the impact of new and improved fish farming methods could be massive on the economy. With Skelwick Skerry as an early success story and with the ongoing support of SAIC and industry partners, the future looks bright for aquaculture in Scotland.  


1. http://scottishsalmon.co.uk/wp-content/uploads/2019/04/Salmon-Impact.pdf