Scotland's warming seas could see tuna replace species such as cod, a government study has warned.

Large numbers of tuna have been increasingly reported off the Outer Hebrides in recent years.

Now, Department for Environment, Food and Rural Affairs scientists forecast that by 2050 climate change will cause big moves in commercial species’ distributions in the North Sea.

Fish from warmer climes are already populating Britain’s seas, the Marine Climate Change Impacts Partnership found.

Increasing numbers of Atlantic bluefin tuna have been reported in home waters by commercial and recreational fishermen.

At present, there is no quota for this species for British vessels. Mackerel has become dominant off the west of Scotland in the last 30 years, and northern hake has recolonised the northern North Sea after being largely absent for more than 50 years.

Scientists also noted the appearance of warm-water species in UK waters, such as the European anchovy, and local declines of some cold-water species, including the viviparous eelpout.

“Experiments suggest that Atlantic cod larvae may experience higher mortality rates due to ocean acidification compared with European seabass and herring larvae. By 2050, climate-driven changes in suitable available habitat could become a major constraint on some commercial species’ distributions in the North Sea,” the report said.

“Climate-driven declines in primary production and copepods in the North Sea have led to declines in fish stock recruitment for some commercial species, including cod, herring, whiting and sprat.

“A global analysis of fisheries productivity highlighted that the North Sea and Celtic-Biscay Shelf are among the most negatively impacted regions as a result of ocean warming and historical overexploitation.

“Increasing numbers of Atlantic bluefin tuna have been reported in UK waters by commercial and recreational fishers, which may be partly related to warming temperatures. At present, there is no quota for this species for UK vessel.”

Welsh fisheries will be particularly at risk from climate change, the report predicted, as cockles and whelks do not cope well with ocean acidification, which is caused primarily by uptake of carbon dioxide from the atmosphere.

Fish are likely to become smaller as they have to travel further for their prey in warming waters, expending more energy, the report predicted. Warming and associated oxygen solubility, the scientists from the institute said, also appears to be affecting the age at maturation, growth rates, and the maximum size fish can attain.

This all spells trouble for the British fishing industry, which does not have the correct quotas for the newly arriving fish, and will have to cope with catching smaller fish.

The study’s authors said: “Declines in shellfish resulting from ocean acidification may result in significant economic losses.”

By 2050, under a high-emissions scenario, the net value of the UK fishing industry is expected to decrease by 10 per cent.

In some cases, whole fisheries may have to cease operation, as populations of incoming warm-water species with limited quota allocation could “choke” existing mixed fisheries.

However, the arrival of warm-water species presents an “opportunity” for tourism by anglers, the report said, adding that “warm-water fish have been detected in the Channel Islands, including species such as the Atlantic bonito, which is popular for sea angling”.

In November, a Scottish Government minister backed moves to establish a UK tuna angling fishery off the Outer Hebrides. Catching a small number of giant tuna off the isles has been licensed by Marine Scotland to gather more data to support the case for anglers to catch a big game fish described as the “Ferrari of the seas”.

The European Maritime Fisheries Fund (EMFF) has donated towards Bluefin Tuna tagging in the Hebrides. Similar tagging schemes are also being carried out in Wales and Ireland.

The Scottish Government is so supportive of the potential that Rural Economy Secretary Fergus Ewing is backing it.

He said: “We want to support the economic wellbeing, diversity and positive development of our coastal communities and I am delighted this project has been awarded an EMFF grant through the Western Isles Fisheries Local Action Group.

“The project will initially assess the overall viability of a catch and release recreational fishery for Bluefin tuna in the Western Isles and a parallel socioeconomic study is currently being considered, focused on considering wider benefits that may accrue to the Western Isles from a limited recreational fishery.”