Species such as cod, the mainstay of traditional British fish and chips, may be a fifth smaller by 2050, according to Canadian marine ecologist Dr William Cheung.
The reason is that climate change is reducing levels of oxygen in the oceans, he claims.
Another effect of warming waters around the UK is likely to be a change of fish menus, Dr Cheung, from the University of British Columbia, believes.
With cod heading north to colder waters, fish more commonly found off Spain and Portugal will routinely be hauled onto the decks of British trawlers.
It could lead to alternative offerings of sea bass or anchovies and chips in fish bars.
As surface ocean temperatures rise, the water holds less oxygen for fish to breathe, said Dr Cheung, speaking at the 50th Anniversary Symposium of the Fisheries Society of the British Isles at the University of Exeter. Fish more easily become “out of breath” as they grow larger, he explained.
Efforts to reduce the impact of climate change and conservation programmes could make a big difference to Britain’s fisheries, said Dr Cheung.
He told his audience: “International actions in achieving the Paris Agreement will benefit Britain’s fisheries by substantially reducing climate impacts on fish stocks.
“At the same time, local actions that improve the health of fish stocks, and protecting their critical habitats, can also reduce help moderate climate impacts on the fisheries.”
The northerly migration of temperate zone fish under the influence of climate change could be good news for Arctic fisheries in northern Canada, Russia and Norway, Dr Cheung predicted.
Atlantic cod could find a new Arctic home along with salmon now found off the west coast of North America.