John Sheard is looking forward to the start of the fly fishing season. But despite recent improvements, concerns persist over the quality of our waterways as pollution, foreign flora and fauna and building work take their toll.

This is the time of year when wrinkly old fly fishermen like me feel the sap begin to rise. For us, it is a time to unwind and clean our lines, oil the reels and – if arthritic fingers are still up to the job – tie flies with exotic names like Greenwell’s Glory or the more mundane March Brown.

St Valentine’s Day won its place in the calendar from ancient pagan observations of when the birds began their mating rituals but to piscatorial protagonists like me, the year proper starts in March when, after the long winter months, the brown trout season opens.

This year in particular there is an extra interest to grab our attention, a series of findings from an important symposium held in Skipton last autumn which drew together experts from many bodies interested in the state of the Dales rivers: naturalists, the national park, fishermen, Environment Agency officials, the Yorkshire Dales Rivers Trust, farmers, local councils and the highways authority.

The symposium was organised by the Yorkshire Dales Biodiversity Forum and its goal: to discuss many problems affecting water quality, to note some recent improvements, and plan ways of continue those successes into the future.

Although the meeting acknowledged there are many improvements to be made, it is far from being all doom and gloom. As Andy Brown, of the biodiversity forum, told me: “There are some suggestions that things are on the up but there is still some way to go.”

The problems with some of our rivers go back some 300 years to the start of the Industrial Revolution. The Wharfe once had a massive run of salmon all the way up to and beyond Bolton Abbey but industrial pollution created a “choke” in the River Humber which killed off the entry to our rivers to migratory fish from the North Sea.

The last salmon reported in the Wharfe in the Dales was landed soon after the 1915 Battle of Waterloo.

Most of those polluting industries and mine workings have gone now, a disaster for working men and women, but a boon to our rivers: there are salmon back in the Swale and have been reported – along with sea trout – in the Wharfe at Tadcaster and even in the once grossly polluted Aire below Bingley.

That’s the good bit. The bad bit is that there are still obstacles like weirs and other barriers preventing migratory fish – and introduced coarse fish like barbel – from making their way upstream. This is one of the problems that the biodiversity team are tackling. There are many others.

Leakages from silage clamps can be disastrous, high in ammonia as well as other poisons: a single clamp from a 500-acre farm can produce as much pollution as a town the size of Harrogate.

Then there is the invasion of foreign species, both plant an animal: Japanese knotweed which clogs river banks and kills off native vegetation, or giant hogweed which, if touched on a hot summer’s day, can give off a sap that can scar humans, and particularly children, for life.

For years, our river banks were patrolled by American mink, sometimes released from fur farms by animal rights activists. This was to unleash a plague on native species, virtually wiping out the harmless water rat – immortalised as Rattie in the Wind in he Willows – as well as water hens, coots, bank martins and even the iridescent kingfisher.

There is good news there, too, for decades of shooting and trapping have dramatically reduced the mink population and there is even suggestion that otters are moving back into their territories and driving the smaller mink away.

That’s splendid news but, on the other hand, as my colleague Jenny Cornish reported last December, the American signal crayfish – imported as a restaurant delicacy but allowed to escape from fish farms into our rivers – is driving out our native crayfish by bringing with it a deadly pox (just as the American grey squirrel almost wiped out the native reds).

On top of all these is the threat from a boom in house building, which sucks water out of our river and aquifers and flushes back detergents, disinfectants and other nasties: leaking septic tanks are proving to be a serious threat in more remote areas of the Dales away from installed sewerage. So there we have it: good and bad on the river bank.

As I wait for next month’s trout season, there is much to ponder. The fact that almost everyone who loves our rivers, either to fish, to water their stock, or simply to walk along, has got together to nurture them is very good news indeed.