Anglers challenge new research that claims salmon farms are not harming wild fish

A new research paper claiming that salmon farming on the west coast of Scotland has no harmful effect on wild salmon populations has been slammed by a leading conservation organisation.

Large floating circular pens in loch
Published: January 5th, 2022 at 1:08 pm

New research claiming that intensive salmon farming off the west coast of Scotland has no harmful effects on wild salmon populations has been dismissed by a leading conservation organisation as “manipulating and cherry-picking statistics”.


Many fish biologists maintain that farmed salmon have a negative impact on wild fish numbers. In particular, they highlight that farmed fish are often infested with parasitical sea lice, which they pass to wild salmon in such numbers that they prove fatal.

But independent researcher Dr Martin Jaffa disagrees. “Those concerned about safeguarding the future of wild salmon should start to address the real issues affecting wild salmon rather than scapegoating the salmon farming industry as the cause of population declines in wild salmon,” he says.

Dr Jaffa, who describes himself as ‘an aquaculture expert who now specialises in the interaction between wild and farmed fish’ published his research in the journal Aquaculture & Fisheries Studies.

Dr Jaffa analysed data of rod-caught fish going back to 1952 (data derived from catch returns self-reported by recreational anglers). He has broken down those catches into ‘grilse’ – salmon that have spent just one winter at sea before returning to their river of birth – and salmon that have spent more than one winter at sea before making their return to their river to breed.

More like this

Dr Jaffa has then examined numbers separately for the east coast of Scotland, where the rivers tend to be slower and longer, and the west coast, where in the more mountainous terrain rivers are shorter and steeper before reaching the sea. It is on the west coast where most of the intensive salmon farming is located.

While acknowledging that overall salmon numbers returning to Scottish rivers have fallen, Jaffa claims that reported catches of ‘grilse’ have not declined, even on the west coast leading him to declare that salmon farming cannot be responsible for falling numbers.

Dr Jaffa concludes that fluctuations in wild salmon stocks are caused by ‘cyclical patterns’ resulting from changing sea temperatures and variations in marine growth rates – not the presence of farms.

underneath shot of worker checking salmon cages
A worker checking cages at a salmon farm in the Sound of Shuna in Argyll on Scotland's west coast. The growth of aquaculture in Scotland has taken place since the first fish farms were introduced in the 1970s./Credit: Colin McPherson/Getty.

Dr Jaffa does acknowledge though that his view is based on limited data and does require further research. As he writes in his paper:

‘In total there are 109 fishery districts in Scotland, of which 59 are located in the NW Highlands. The data presented herein has been selected for 4 fishery districts that most clearly illustrate the importance of disaggregation of the Scottish salmon landings data into morph types [grilse and salmon].’

Haaf Netters Fish For Their Daily Catch In Solway Estuary

Criticism of the new research

Andrew Graham-Stewart, Director of Salmon & Trout Conservation (Scotland) goes further regarding the data upon which Dr Jaffa has based his analysis.

“Dr Jaffa’s latest output focusses on just four west Highland rivers – which are far from representative,” says Graham-Stewart. “The reality on the ground is that the great majority of smaller rivers in the west Highlands and Islands have lost almost all of their wild salmon since the exponential growth of salmon farming occurred from the 1990s.

“Across the northern hemisphere the growth of intensive salmon farming has invariably coincided with the collapse of wild salmon runs.” he continues. “Salmon farms are efficient breeding reservoirs for parasitic sea lice and as such they are a major, indeed lethal, problem for wild salmon – and sea trout – as they migrate to sea. There is international consensus on this with numerous peer-reviewed papers clearly demonstrating an impact.”

Andrew Graham-Stewart also cautions against the use of recreational anglers catch reports to distinguish between grilse which have spent one winter at sea, and salmon which have spent more. Salmon are complex animals and size alone is not reliable – a scale sample and microscopic examination is one reliable method.

“Most anglers are unable to differentiate between large grilse and small salmon and accordingly official declared catches for grilse and salmon are based on nominal, often estimated, weights, which can vary from year to year and from river to river, rather than any reliable biological difference.” says Graham-Stewart.

Scottish salmon: a tale of decline

Wild salmon and sea trout numbers in Scottish rivers have fallen by 70% over the last two decades. Global populations of Wild Atlantic Salmon have fallen from between 8 and 10 million in the 1970s to 3 million today.

Most scientists agree that there are many, complex reasons for the fall in salmon numbers, of which intensive, open-net fish farming is one. Other factors include climate change, loss of habitat in rivers to support life cycles and instream barriers such as weirs preventing migration.

The problems with sea lice and farmed salmon

Sea lice (Lepeophtheirus salmonis) are a copepod crustacean that have lived in our seas for millions of years. The larvae are parasitical, attaching themselves to a host salmon and feeding off its skin and blood. While not thought to be harmful when ingested by humans, sea lice can cause huge damage to the fish, leaving open wounds and legions on its body that make the fish unmarketable. In the wild, a louse – which is less than a millimetre long – can struggle to find a host. However, with the high concentrations of fish kept in open net pens – where the fish are separated from the open sea by just a net ‘cage’ – they have rich pickings to live on.

Most scientists agree that lice from farmed salmon in open net pens do pass to wild fish. Research has shown that an infestation of 10 to 12 lice is likely to have fatal consequences. A farm where the fish have 5 mature female lice can release as many as 3 billion sea lice larvae per month.

The Scottish Government summarised the available science: ‘The body of scientific information indicates that there is a risk that sea lice from aquaculture facilities negatively affect populations of salmon and sea trout on the west coast of Scotland.’

The salmon farming industry has been rocked in recent years by images of grotesquely deformed fish in crowded pens that have been half-eaten alive by sea lice appearing in the national media and posted regularly on social media by campaigners.

Salmon farming is big business in Scotland. A Marine Scotland-commissioned report reveals that it is worth at least £885-million and supports 11,700 jobs, many in rural, coastal communities where work opportunities can be scarce. The report also found that Scottish salmon represents the UK’s largest food export by value.


Salmon & Trout Conservation is campaigning for the end of open-net farming which it claims is unsustainable, and the further development of closed containment systems, which completely separates the farmed fish from the open water. Theoretically at least, this could be done inland.


Andrew Griffiths is an environment and angling writer and podcaster. He writes for publications in the UK and USA, including BBC Countryfile Magazine, BBC Wildlife Magazine, and Gray’s Sporting Journal in the States.


Sponsored content