Oxygen levels in the river are now well over 60%
The Mersey - once the most polluted river in Europe - can support fish throughout its estuary for the first time since the Industrial Revolution, scientists have discovered.
It now has sufficient oxygen in it to support life, following a 20-year clean-up costing £1bn.
Scientists have recorded oxygen levels of well over 60%, compared to the late 1970s when tests found none at all in many areas.
Fish need oxygen levels of at least 30% to survive.
The pollution was the legacy of raw sewage and industrial waste including chemicals being discharged into the river.
George Ager, of the Environment Agency, said: "This is fantastic news for the Mersey.
"It was once a prolific fishery famed for its salmon, but after the Industrial Revolution it became infamous for pollution.
"Not so long ago, people said there were no fish in the estuary, and folklore was that you couldn't drown in it because you'd be poisoned first.
"But nearly 20 years of hard work have paid off and now we have an estuary to be proud of."
Tighter regulations on industrial pollution and a huge effort to stop sewage getting into the river have helped clean the river.
In 2002 the agency captured and returned 26 salmon at a fish trap at Woolston Weir, near Warrington.
Other varieties of fish, including brown trout, sea trout, lamprey, dace and cod are also now frequently caught by anglers.
Water sports, including sailing and windsurfing, are also becoming increasingly popular as a result of the clean up, the agency said.