I was looking down at the River Wye from Boughrood bridge on a sunny autumn morning with Simon Evans, who runs the Wye and Usk Foundation. Simon, it is fair to say, is a fish enthusiast. “I’ll come here at times and see the river almost boiling with fish activity,” he said. “As many as 500 shad can be churning the water for a group spawning. There are barbel as well. It looks like pigs rippling through the undergrowth of a forest!”

He told me that another sight worth coming for is the sea lampreys, which swim inland to build a hatchery by moving large rocks with their eel-like bodies and strong jawless heads. The lampreys have to work together to move some of the heavier rocks: this is one occasion when having a surfeit of lampreys helps. A sea lamprey can, according to Simon, be as long as a human arm, which is a spectacular and rather alarming idea should one of these “vampire fish” break the surface of the water.

But the icon of this river is the salmon – and the trail I set off on with Simon, the Wye Valley Walk, is signposted with leaping salmon waymarks. Just a little upstream from the bridge, we came to the large pool of Chapel Catch, named for the remains of a Methodist church among nearby trees. Salmon gather here before crossing the Llangoed rapids above: they like to feel the fast-flowing water running through their gills and gather strength before they attempt the rapids. When they do, you can just see their bow waves rippling behind them as they crest the shallow rills.

There’s nothing like walking with an expert to enhance the pleasure of a ramble. Simon told me that by the time they get to this bit of the Wye, the salmon will already have come 100 miles from the river’s mouth in the Severn estuary – and the larger ones some 2,000 miles before that, from the ocean banks off Greenland.

The salmon need to travel this far up the river to spawn because the females lay their eggs in gravel on the riverbed, and the Wye is too muddy lower down. Above Hay-on-Wye, it is a fast-moving mountain river coming down fromthe Cambrian Mountains, over bedrock and gravel. Here salmon can be easily spotted in the clear, tumbling water.

Simon had suggested I bring polarising dark glasses to help see the fish through the water. “The only way you can see the riverbed any better is by getting into a wetsuit and swimming up and down the bottom.” It was remarkable how effective the glasses were – and what fun – particularly in small tributaries such as the wonderfully named Sgithwen brook, where the water was flowing less rapidly. We could see clearly how the hen salmon had constructed their nests – called redds – on the bed of the stream by laboriously scraping gravel with their fins.

The hen salmon often prefer small tributaries like the Sgithwen to the main river, as there are fewer predators. The hens in their turn attract cock salmon, who congregate and bide their time before courtship by attacking each other with their sharp “kype”, a ferocious hook on the lower jaw grown for the spawning season. The fighting is not just salmon-on-salmon, though: we came across a 10kg cock salmon lying dead in the stream, its head ripped off by an otter who had daintily eaten just a small amount of the best flesh, then left the rest to rot.

Next to some falls along the Sgithwen with a particularly high leap, Simon told me how once a dozen salmon had congregated in the pool below to gather their strength and an otter had taken every single one, like a fox in a hen coop, then left them uneaten on the bank.

Over the past 25 years, the Wye and Usk Foundation has worked hard to make life easier for the salmon. It has removed 117 weirs and cut fish passes through others to make it easier for the fish to swim up and, later, down the river. At these difficult Sgithwen falls, for instance, they have constructed staggered pools, so the salmon can rest at intermediate stages. That said, a full-grown fit salmon can still leap four metres, an extraordinary distance and higher than any human high jumper.

The river is exceptionally beautiful upstream of Boughrood bridge, and largely empty except for Llangoed Hall, once home to Laura Ashley and now a hotel. I saw dippers and wagtails playing across the water as I listened to Simon enthusing about his salmon projects.

The numbers were revealing and startling. In 1988, as many as 6,300 salmon were taken by rod on the river. By 2002, this had declined to just 357. The Wye has never been artificially stocked and depends wholly on wild salmon finding their way back to their birthplace from the Atlantic. Clearly urgent action was required to make the river more navigable for the fish.

But removing weirs and making fish passes has only been half the battle. Pollution from farming practices and acid rain from south Wales’ industrial past were also problems. The trust had to put large amounts of alkaline lime into the headwaters around Plylimon where the river rises, to try to rebalance pH levels; it also worked with local farmers to prevent damaging phosphate run-off from their land. The extreme winter floods the Wye experienced earlier this year have only made people more aware of the delicate ecosystems on which the drainage depends.

As with any farming ecosystem, a complex balance needs to be negotiated between competing forces and interests. When more free-range chickens were introduced to local farms – a boon for ethical farming – their feed also increased phosphate run-off, with a resulting algae bloom.

For a while after the trust’s formation, the conservation work proved remarkably successful. Salmon returned to the Wye in much greater numbers, but started to decline again a few years ago – because of warmer spring weather, which encourages the salmon to hatch before there are insects to eat, and also to little-understood complications in their north Atlantic habitat near Norway.

We didn’t just talk about salmon – although by the time Simon and I had been walking for a few hours, I was certainly ready to pass my piscine oral examination. He is also a keen fungiphile: as we climbed up to Brechfa Common, we passed an exotic assortment of mushrooms including hedgehog fungi, charcoal burners, coconut milkcaps and blewits, along with the more familiar ceps and chanterelles.

“Look, there’s some honey fungus mushrooms growing on a log!” exclaimed Simon with his customary enthusiasm.

I thought I should call his bluff. “Isn’t honey fungus the poisonous spore that kills off garden plants?”

“Uh, yes it is,” laughed Simon. “And even I have to admit it doesn’t taste very good.”

As we got higher, ash trees gave way to Scots pine, and we reached the beautiful Brechfa Pool, a waterlogged haven for wildlife. It was one of those days of crystal-clear sunlight, and looking south we had a spectacular view down towards the Black Mountains, with Mynydd Troed and the Dragon’s Back cresting across the horizon, while even further away the Brecon Beacons broke through the clouds.

Just a short drop down below was the Griffin Inn in Llyswen, where we ended our walk. Over lunch, Simon told me that much of the conservation effort to protect the salmon and trout on the river had come from fishermen. The trust had been set up by the late Dr Stephen Marsh-Smith, whose 38.5lb (17.4kg) Wye Special salmon, caught nearby, is proudly displayed on the wall of the pub.

I’m not a fisherman and am not temperamentally inclined to killing anything, in the water or on land. But, as Simon said: “When everyone else has forgotten about the rivers, the fishermen will remember them”. Much of the conservation drive has come from them.

Sitting under the mounted Wye Special after our 10km walk, we raised a pint of much-needed Otter bitter “To the river and all who swim in it!”

• The Wye Valley Walk runs for 136 miles (219km) from its source in Plynlimon to Chepstow, just shy of the mouth of the river. The 10km circuit from Boughrood bridge runs up the Wye then along Sgithwen brook to Brechfa Common and Brechfa Pool, before descending back past the Griffin Inn