Nutritious wild-caught fish is being squandered if it continues to be used as feed for farmed salmon, a new study maintains.

Scientists analysing the Scottish salmon farming industry calculate that an extra six million tonnes of seafood would be available annually if wild caught fish is diverted away from aquaculture feed.

Farming fish is often billed as a way to relieve pressure on wild stocks, but caged-reared species such as Atlantic salmon rely for feed on fish oil and meal made from millions of tonnes of wild-caught fish.

The new study published in the research journal PLOS Sustainability and Transformation says that limiting salmon farming to using feed made from fish by-products could result in 3.7 million tonnes of fish being left in the sea.

Global annual seafood production could increase by 6.1 (six point one) million tonnes by avoiding use of nutritious wild-caught fish, the team of scientists from Cambridge, Lancaster and Liverpool universities and environmental NGO Feedback Global, state.

The team collected data on fish nutrient content, fishmeal and fish oil composition, and salmon production, and examined the transfer of micronutrients from feed to fish in Scotland's farmed salmon industry.

The scientists say that results showed that over half of the essential dietary minerals and fatty acids available in wild fish are lost when these fish are fed to farmed salmon.

The team developed alternative production scenarios where salmon were only produced using fish by-products, and then added more wild-caught fish, mussels or carp for human consumption.

All alternative production scenarios produced more seafood that was more nutritious than salmon, and left 66-82% of feed fish in the sea.

The researchers then collected global salmon, fishmeal and oil production data to apply their alternative scenarios at a global scale.

One scenario shows that farming more carp and less salmon, using only feed from fish by-products, could leave 3.7 million tonnes of wild fish in the sea while producing 39% more seafood overall, according to their calculations.

“Fish and seafood provide a vital and valuable micronutrient-rich food source to people worldwide, and we must make sure we are using this resource efficiently,” the study leader, Dr David Willer of Cambridge University, said.

“ Eating more wild fish and using alternative feeds in salmon farms can achieve this,” he said.

The authors acknowledge that not enough is known about the source and species composition of fishmeal, but there are positive signs that use of plant-based feeds is growing.

Dr James Robinson of Lancaster University said more data on the volumes and species used for fishmeal and fish oil was required, as “this can show where salmon farming places additional pressure on fish stocks”.

Dr Karen Luyckx of the Feedback ngo said that “until the salmon industry kicks its wild-caught fish oil and fishmeal habit, chefs and retailers should help citizens switch away from unsustainable salmon by offering ultra-nutritious mussels and small oily fish instead.”

The authors call for a reduction in marine aquaculture feeds, as this will offer opportunities to produce more nutritious seafood while reducing pressure on marine ecosystems.

Maximising sustainable nutrient production from coupled fisheries-aquaculture systems by David Willer, James Robinson, Grace Patterson and Karen Luyckx is published in PLOS Sustainability and Transformation.