IT is a condition that can kill more than one in five salmon farmed in Scottish waters and has been linked to rising sea temperatures and increasingly hot summers.

Now scientists are to join businesses to find ways to combat gill disease as the threat of global warming increases. Scottish Sea Farms (SSF) is to lead a second applied research project focused on increasing understanding of gill health in farmed salmon, part of its “prevention over cure” approach to fish welfare.

The £601,000 project, of which SSF will fund the lion’s share, will be the salmon farmer’s second such collaboration with academics at Aberdeen University, feed specialists BioMar and the Scottish Aquaculture Innovation Centre (SAIC).

Combined, the two projects bring the company’s investment in applied research into gill health to almost £750,000 since 2018.

Dr Ralph Bickerdike, head of fish health at Scottish Sea Farms, said: “The gills are hugely important to the overall health and wellbeing of Atlantic salmon, yet the factors affecting these vital organs are as highly complex as they are little understood.

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“This second gill health project seeks to explore further the early insights gleaned in our initial collaboration, helping increase knowledge of the key risks and how to pre-empt and avoid them.”

Testament to how devastating gill-related challenges can be to fish health, Scottish Sea Farms’ Lismore North farm in Loch Linnhe experienced unusually high mortalities of 22 per cent during the last crop as a direct result of the disorders.

Further fuelling the drive to increase knowledge of fish health, the same farm also lost 15% of its stock due to the water-borne mould Saprolegnia, which arises at freshwater stage and is also being researched by Scottish Sea Farms, SAIC and partners.

The two outbreaks resulted in almost one-third of the farm’s fish dying in the water compared with the 90% survival rate recorded for the previous three crops.

More than 200 fish farms now operate in Scotland, producing more than 150,000 tonnes of salmon a year. Farmed salmon are fed on processed feed and treated with medicines to ward off disease and infestations such as sea lice, which can breed among the fish in the pens.

But gill disorders can be caused by the growth of micro-organisms in the water, such as algae and plankton, and are much more difficult to control.

The project team will focus efforts on ways to keep the fish healthy by exploring the effect of local geography and seasonal influences on gill health to identify new preventative measures, while also exploring the accuracy of a range of new veterinary tests which help indicate the health status.

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The insights gleaned will be shared across the industry in an attempt to bring the high mortality rate among farmed salmon down. Every year about 9.5 million fish die in the salmon farms, about 20% of the total, from disease, parasites and even chemicals used to treat them fish.

Caroline Griffin, SAIC aquaculture innovation manager for the gill health project, said: “Gill health has become one of the biggest challenges facing salmon production, not only in Scotland, but in Norway and internationally.

“SAIC is supporting a range of projects across Scotland, drawing on the expertise in our academic institutions and industry to understand the issue and develop innovative solutions.

“The new knowledge gained will be shared across the sector, helping salmon farmers to maintain healthy populations of farmed fish.”

Mr Bickerdike added: “When seawater temperature rises, even by something as seemingly slight as 0.5 degrees, more marine organisms grow. In the summer of 2018 – the joint hottest summer on record for the UK – we experienced a phytoplankton bloom in the Loch Linnhe area that resulted in a challenge to gill health.

“Increasing our understanding of what more we can do to pre-empt and prevent these kinds of challenges is a priority.”

“Across our farming estate, average end of crop survival is now 87.2%; a strong survival rate that has been aided by Scottish Sea Farms investing over £10m in applied research into fish health in the last five years alone. The challenges experienced at Lismore North, whilst isolated, are precisely the reason why we continue to invest and build our knowledge to fully deliver our ‘prevention over cure’ approach to fish welfare and boost survival rates even further.”