"Forty years ago an estimated 1.4 million salmon returned to UK rivers each year. We are now at barely a third of that -- a new low and evidence of the wider, growing biodiversity crisis," said Alan Lovell, chair of the UK's Environment Agency.

The assessment found 90 percent of principal salmon rivers in England were now classed as "at risk" or "probably at risk", meaning numbers were below minimum levels to support sustainable populations.

Lovell said the agency was working to tackle water pollution, hold polluters to account and improve habitats as a means of boosting numbers.

"The dramatic decline of salmon stocks is an international trend, with similar findings reported in Ireland, Iceland, Sweden and Canada," the agency added in a joint statement with Natural England, a governmental advisory body.

"However the UK shows the most significant decline," they said.

The latest Atlantic Salmon Stock Assessment puts the provisional "declared rod catch" figure for 2023 at 4,911, a 23 percent drop on 2022.

It is illegal to sell any rod or net caught wild salmon in England with most anglers returning any salmon they catch.

The study did not cover Scotland but a report published by the devolved Scottish government said the decline there was less stark.

"Although many areas are in overall good conservation status, there are recent declines in the numbers of salmon spawning in rivers across Scotland which highlights a need for precautionary management approaches," Scotland's Marine Directorate said in August.

It said that of the 79 areas without a significant decline, 59 still had fewer salmon in 2023 than in 2011.

The remaining 32 stocks showed no consistent pattern, it added.