Even so, such natural boosts are not enough for the Environment Agency. Backed by alarmed fishery owners, next week it introduces strict new by- laws to force fishermen to return prized spring fish and use barbless hooks, and to ban worm and shrimp baits.
For now, however, as the autumn spawning migration gets under way, the salmon have to rely on the elements and, in spite of this October's heavy rain, fewer fish than ever will make the journey. Catches on most rivers have declined dramatically over the past two decades. On the Wye, for example, the annual spring rod catch has fallen from 4,300 in 1967 to 730 in 1997. This year John Hopkinson, chairman of the Wye Salmon Fishery Owners Association, reckons that the final tally on what was once the premier salmon river in England and Wales is between 600 and 700.
Worse, even these stark figures disguise another problem. Salmon begin life as "parr" in gravel beds in upland streams. When about a year old, the fish - now called "smolts" - migrate to the sea to feed in rich areas off Greenland. When ready to breed, at about four years old, they return to the river where they hatched.
The first, and biggest, enter freshwater in early spring and it is these fish that are prized by fly fishermen. Later in the summer the "grilse" return. These have just spent one winter at sea in shallower coastal waters, and are considerably smaller. Accordingly they are less prized by anglers, but, more important, have lower reproductive success.
Unfortunately, the proportion of grilse that fishermen have caught has been rising over recent years, reflecting their increase in the rivers. And scientific measures reinforce the worries of fishery owners. The Environment Agency has produced figures for the numbers of eggs laid in each major salmon river and these are well below what would be hoped for in healthy water.
For years falling stocks were blamed on poaching, but scientists dismiss this as relatively insignificant. The most likely cause of the current crisis is probably climatic. Ironically, in this age of fears of global warming, scientists believe the problems stem partly from a cooling of Britain's coastal waters as melting polar icecaps shift the Gulf Stream westwards. This results in poorer feeding for all marine life, but hits the predatory salmon particularly hard.
"The overall pattern is one of salmon numbers going down across the North Atlantic," says Guy Mawle, fisheries officer at the Environment Agency. "And we're particularly concerned about the larger, sea-winter salmon, whose numbers have crashed," he adds.
Naturally, fishery owners are worried. Along the Welsh border, they have formed the Wye Foundation, which tries to improve breeding habitats by removing debris blocking smaller streams and cleaning up the gravel beds where the fish spawn. It also runs a restocking programme, based on live fish donated by anglers.
These are moved to a hatchery where their eggs are removed and fertilised, and most of the resulting progeny are released. These go to suitable areas where natural stocks are low or non-existent. A minority are kept back for a year, however, and released as smolts ready to migrate straight to salt water. But while such schemes may help stocks, everyone recognises that this alone will not solve the current crisis.
Another partial answer is to reduce the number of fish killed legally on the river by introducing a policy whereby anglers return fish unharmed to complete their breeding cycle. This runs counter to angling tradition, however: "Game fishing has always seen itself as akin to game shooting," explains Hopkinson. "To some people the idea of releasing a salmon that you've pitted your wits against for hours, or even days, is like shooting at pheasants with blanks. Nevertheless, it is critical that anglers should support catch-and-release; the only alternative is to close the fisheries completely."
Next week the Environment Agency will introduce by-laws forcing fishermen to release salmon caught before 16 June. After that it is up to the individual as to whether they catch-and-release, but there may be local extensions and even total bans on killing salmon.
Nevertheless, in spite of the general gloom on traditional rivers, there are rays of hope from unexpected quarters. Improved water quality on many industrial rivers has seen salmon returning to haunts abandoned 150 years ago.
The Tyne, for example, is now among the best in England, while this summer a coarse fisherman on the Don in South Yorkshire was amazed to hook a large salmon.
Even on the Wye there is hope: "We've had two cracking good floods this year and it was a wonderfully wet summer," says Hopkinson.
Meanwhile, anyone wanting to see salmon leaping should try the weirs on the Taff in central Cardiff, or the new barrage on the Tees Estuary - or contact their local Wildlife Trust.Reuse content