The science of salmon fishing

Richard Shelton (with rod) and Bob Sherwood on the River Tay

Salmon fishing often seems like a triumph of hope over science. Why else would anyone try to entice a fish that feeds at sea to bite on a manmade fly in a river? Once in freshwater, salmon live entirely off their energy reserves. They do not need to feed. What is surprising is not how rarely these fish take a fly, but that they do at all.

One man who understands fish behaviour better than most is Richard Shelton, biologist and author of a new book on the life of salmon. In a bid to put more science into my salmon fishing, I have persuaded him into an early season expedition on Scotland’s river Tay.

Despite his professional background, Shelton insists that he is a poor Spey caster. It’s perhaps not surprising: as head of the UK’s Freshwater Fisheries Laboratory and then research director of the Atlantic Salmon Trust, he has spent much of the past two decades tracking, catching, filming and recording salmon at sea. But there has been little time for angling.

It is bitingly cold as we venture into the Tay, famed for the large size of its early-run spring salmon. Shelton and I find a good spot just where the river emerges from Loch Tay. I suggest we share a rod, under the guise of offering a few casting tips. In reality, I just want to plunder his encyclopedic knowledge.

As the bright Willie Gunn fly swings round on the chilly current, Shelton reflects on how little we still know about salmon, in particular their navigation at sea. “They are complicated machines. We only know a fraction of what they are capable of,” he reflects.

We haven’t had a single touch on the line, so I quiz him for more on the science behind catching fish.

Following the barometer is one tip. Salmon are often caught as a spate begins to drop, which is usually accompanied by a climbing barometer, Shelton explains. “The rise in air pressure tells the fish it’s time to move and that’s when they are much more catchable.”

For much of their time in the river salmon are conserving energy, which offers clues to where they hold station. “Salmon are idle,” he says. “They like a quiet life, in slacker current, behind boulders, under the bank. That’s why beginners often catch fish. They are fishing under the bank because they can’t cast any further.”

Now we’re getting to the good stuff. “It’s mainly water height that matters,” he goes on. “A lot of lures go over the heads of the fish when they really should be at their level or just above them. They need to see it close up. Nothing is more important than getting the fly at the same depth as the fish.”

I try to ascertain a scientific rationale for fly colour. “The redness of shrimps in the sea looks black at depth,” Shelton notes, perhaps explaining why black is often a killing colour in a lure.

A quietly spoken Englishman who has spent most of his life in Scotland, Shelton has an ear for a humorous anecdote, often told in the brogue of a Scots ghillie. His book, To Sea and Back, is similarly lively, intertwining the life of the Atlantic salmon with memoir, history and tales of ghillies, gamekeepers and naturalists.

Environment, too, is a key theme. “In the southern part of the salmon’s range, which includes Britain, they are not doing particularly well,” he tells me, explaining that warming temperatures restrict the availability of crustaceans and small fish.

The salmon faces other threats. The most immediate, he says, is sea lice infestation of juvenile salmon and sea trout caused by intensive fish farming on Scotland’s east coast.

“The lice pose a colossal threat to salmon and sea trout. They eat the smolts [young fish] alive. We are doing nothing about it and it is scandalous.”

Shelton believes that intensive fishing is upsetting the balance of the marine ecosystem, exerting unknown effects on salmon. He also accuses the fishing industry of unwittingly supporting the growing population of grey seals, which eat salmon, by discarding so many dead and dying fish.

“If fishing and fish farming were to stop, I have no doubt we would see dramatic improvements in salmon and sea trout stocks despite climatic changes,” he says bluntly.

The Tay recorded a number of big “springers” last year, some over 20lb. These fish travel as far as Greenland, spending three winters at sea, and are not as badly affected by lack of food as summer-running fish that travel less extensively.

“It’s a little bit surprising there are not more of them,” says Shelton. “Something is happening to stop them surviving, as generally good growth goes with good survival.”

The Tay District Salmon Fisheries Board this year introduced a code asking anglers to return salmon in response to a declining run of spring fish. As someone whose instinct is that fishing should be for food, Shelton accepts the importance of catch-and-release fishing where stocks are low.

A Tay springer is the stuff of angling dreams, and today it looks like staying that way. Without apparent irony, Shelton makes it worse by telling me spring fish are the easiest to catch.

Many authors have speculated about what makes a salmon take a fly, ascribing reactions such as curiosity, anger and even playfulness to the fish. Shelton is unequivocal. “It’s an attenuated feeding behaviour. Those reflexes lessen as the fish matures sexually. The early spring running fish still have that instinctive reaction to marine prey.”

The reality is that much about salmon behaviour remains a mystery. And this mystery captivates Shelton. “Science is a form of worship. The sense of wonder you get from studying even the few things that I have never leaves you.”

Sadly, there are no fish yet in the river to wonder at. Again, Shelton corrects me, spurring hope: “In Scotland, there are fish entering the rivers in all months of the year.”

In spite of the ridiculous odds, I remain hopeful in my salmon fishing. And now I have a scientific justification for my optimism.

pursuits@ft.com

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From stream to sea and back

Atlantic salmon hatch from eggs spawned in rivers, becoming “alevins” that grow into “parr”, remaining in freshwater for one to five years. As “smolts”, they migrate in late spring to deep-sea feeding areas such as the Norwegian Sea and off south-west Greenland. It is believed that they navigate with receptors sensitive to the earth’s magnetic field.

Salmon stay in the ocean for up to three or four winters. The majority return to their birth rivers, guided by a chemical memory of the water. Spawning in the UK is usually in November and December. About 90 per cent of Atlantic salmon die after their first spawning but some can survive to spawn two or three times.

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The details

‘To Sea and Back: The Heroic Life of the Atlantic Salmon’, by Richard Shelton is published by Atlantic (£18.99)
The Atlantic Salmon Trust funds and sponsors research; www.atlanticsalmontrust.org