Several of the west- flowing rivers of North Craven support what many fly fishermen believe to be the gamest of all British game fish. This is not, as one would expect, the mighty salmon, but its so-called "poor relation", the sea trout.

For reasons not fully understood, but possibly due to the decline in the salmon population whose fry compete with the young trout for food in their breeding rivers, the sea trout is - or was - growing in both size and numbers, particularly in the Ribble and its tributaries, but also in the Wenning and even the tiny Greta as it runs through Ingleton.

Because this fish is a massive fighter if hooked and makes a wonderful meal when cooked - a sort of delicate cross between brown trout and salmon because in coastal waters it feeds largely on shrimp - it is much sought-after by anglers.

And that means I have a secret pool I visit regularly at this time of the year, particularly when heavy rains have swollen the river, because it is rarely fished: it entails a long walk over fields and some difficult stiles and gates to negotiate in heavy waders and other bulky kit.

In the past, it has been a good provider of fish, although I only take two per session for conservation reasons.

This year, it has proved a blank. And when I arrived there, early one morning last week, I think I saw why: my secret pool has been taken over by an alien invader - and that has put me into possible conflict with an organisation I have long admired, the Royal Society for the Protection of Birds.

The invader is a beautiful duck called a goosander, a bird slightly bigger than a mallard which crossed first from Scandinavia into Scotland and is now breeding in large numbers in Northern England.

It has a similar green head to the mallard and a mainly white body, but has one significant and almost frightening feature: a prominent, serrated blood-red beak which makes it a member of the "saw bill" family for reasons I shall explain.

The goosander is a fish eater of enormous skill - and it is that sharp beak which will take a largish mature trout or small sea trout - and will decimate fish stocks of salmon smolts, as the young fish are known before they leave their home rivers to go to sea. In half a century as a fly fisherman, I have never seen anything quite so awesome as a family of goosanders fishing. The adults have very large broods - often five, seven or even more chicks which mature quickly and learn to hunt when just weeks old.

The whole family make an inverted bow shape and drive shoals of fish ahead of them into the shallows.

There, the slaughter begins - and after a few days of this, even a large pool can be stripped of fish.

Now this put me into a state of unwelcome conflict, for which is the more important to anyone who loves the country-side: fish or fowl?

And if it were only for the goosanders, I might reluctantly turn a blind eye to their depredations; but our Yorkshire trout rivers have been under attack from another invader for the past 20 years or so, the native but equally greedy cormorant.

This was once a cliff-dwelling creature that made its living at sea. But as our seas have been plundered by over-fishing - even for the ubiquitous sand eel which Dutch trawlers take to make into poultry food and, ironically, farmed salmon food - the cormorant has been forced inland in ever growing numbers. And it just loves a diet of salmon, trout and sea trout.

Cormorant numbers have grown so large that one of Scotland's prime trout fishing lakes, Loch Leven, has been closed down as an hotel and boat hire resort because the then Labour-dominated Scottish Parliament refused permission for these birds to be shot. Scores of local people lost their job as a result.

And it so happens that fly fishing is not only important business for hotels, pubs and B&Bs in the Dales, but there are more fish farms breeding trout for angling clubs in Yorkshire than in any other part of Britain. And the open pools of fish farms are a gourmet feast for cormorants and, increasingly, goosanders.

The RSPB admits that there is a conflict over these birds with anglers, but opposes their shooting except in the most extreme circumstances, although the environment department Defra does issue the (very) occasional permit for such culls.

But there is an important question to be addressed here: what is more important, socially, environmentally and economically, than a salmon - one of Britain's most threatened wildlife species which provides leisure for thousands and vital income for many hundreds more - or two birds, one of which, the cormorant, is not even pleasant to look at and a voracious predator which is not even a British native?

Time for the RSPB to put its thinking cap on, perhaps.