our three rafts headed for a tributary with a nice shelving sand-bar to moor up on for the night. We crunched onto the beach and were stunned to find huge salmon lying dead on the water’s edge.
There was a biologist aboard. He looked closely at the fish and saw that their gills were clogged. They were king salmon, the big boys, and there were around forty of them stranded down the river-edge for a few hundred yards, all just above the tributary’s junction with the main river. The biologist said the fish were all within a day of spawning. So a valuable stock or ‘year-class’ of a rare and wondrous fish lay wasted about us never to breed, eradicated in the last moment of its evolutionary purpose. Why? The brown water was still silty from a landslide further upriver. A natural event had wiped out the big fish in this tributary for one breeding season.
What gave the event an added twist was the furious debate taking place in west Canada’s media that summer about threats to king salmon, their precarious status, and the need for firmer protection laws. Nature had thrown a joker onto the gaming table and we were staring at it.
But it was also where genetic straying and fish unfaithful to their natal imperative step in. Suppose, as we know is possible, that one pair of king salmon had gone up a tributary close by. They bred there. That strain of salmon thereby dodges fate and escapes elimination. In due course some of the progeny from that union relocate themselves as adults in breeding livery back in the original natal stream, and re-populate it.
Straying is Nature’s way of spreading risk. The same is true, surely, about the differing ages at which young salmon go to sea. If some ‘smoltify’ and migrate in their second spring, and some in their third, the risk of total elimination is spread. On big rivers in Scotland like the Tay, different grilse runs climb the system from early spring to the autumn. They are all fish which spent only one winter in the sea, the definition of grilse. Bookies call it hedging bets.
Peter Malloch might have had a lot to say on some of the purist preconceptions about river-stocking only with site-specific strains which have crept into modern management. It was Malloch who realised a century ago that salmon sometimes remained at sea a long time, and that not all fish were grilse as had been assumed before. He understood the salmon’s admirable diversity. The migratory fish turn homewards to reproduce, swimming south. It is presumed, but only so far tentatively claimed, that they follow the same passage, but going the opposite way, as that which they used as teenagers – another neat theoretical twist made available by modern science which this aristocrat of salmon analysis long ago would have appreciated.
Left behind in the marine larder are the others, the non-movers, growing and growing. Or not growing: some salmon a long time at sea are not especially big. Maybe they too are hedging their bets, passive actors in an evolutionary insurance policy.
A salmon’s eventual size is determined by its length; it can be fat or it can be thin, but without length it can never match the biggest. Girth is the feeding which beefs up the body length. Some of these wintering salmon spend two years in the vicinity of the Arctic, some three, some four, and some even prolong their Arctic sojourn to five years.
Turning southwards as the new year awakens, they ultimately acknowledge the ritual of reproduction, or so it is assumed. Scale-reading shows that after January sea-feeding picks up again following the short winter check. Thus, the fish achieve peak condition prior to the demanding migration south.
Today’s existence is tougher for salmon, for as the northern hemisphere has warmed, dragging the food supply northwards, the return journey south lengthens. Migrating salmon are starting further from home.
Krill may move, natal streams do not.
How long they take to swim home is unknown. Scale-reading differentiates between sea-time and freshwater-time, but we do not yet know in which sea, at what time. Could we? If scales were better able to track diet perhaps we could pin down more accurately how long the journeys take. Sand eels are on known sandbanks and capelin live in the north – the diet could position the predator.
However, there is another lateral-thinking way, a technique not yet fully developed for practical use. We need tags that could measure the angle of the sun at midday, then, as polarised sun filtered through the seawater, it would be possible to calculate latitude. Temperature and the intensity of the light are recordable now; the next step is a reader for polarised light. The tags used on smolts register temperature and depth, but so far they cannot measure the angle of the light. This tantalising technical advance may be not far off.
What is hard fact is that salmon are fast movers. When they do reach rivers they can travel incredible distances in a day’s journey, proven by the presence on them of sea lice. These are saltwater parasites which can only remain attached in freshwater for up to two days. Sea-liced salmon have been found thirty miles up rivers, and more. Fresh from the marine, Atlantic salmon are true Formula One fish, as some sparkling-eyed anglers have cause to know.
They do not, though, just arrive at the mouth of their natal river and motor into it. They savour the moment of transition, whilst making a vital physical adaptation to their water-balance mechanism. In the sea a salmon has to cope with water that is chemically stronger than its own body tissue, making it lose its body liquids. To compensate, a salmon at sea actually drinks seawater, which then passes through its stomach and intestine. Its kidneys expel small amounts of the salt in concentrated urine, conserving some of the swallowed water to make up for the liquid losses.
In his 2009 treasure-trove about the Atlantic salmon, To Sea and Back, Richard Shelton refers mischievously to the ‘uriniferous’ smell of farmed salmon: he has sound physiological reasons to do so! So many fish excreting powerful body wastes in a basin of seawater, only lightly refreshed by the average tides, might well produce some fairly rank odours and taint the aroma of the imprisoned inhabitants.
Entering the milder environment of freshwater a salmon finds its fluids more concentrated, not less, than those of its surroundings. It starts to absorb water through the permeable linings of its mouth and gills. The excess is filtered as dilute urine through the kidneys. So fresh and salt water are quite different environments. Salmon biology has the sophistication to occupy both.
Sometimes for weeks the fish pregnant with breeding preoccupations ride the tides near the river-mouth, tasting the fresh water debouching from the estuary, sampling it, experiencing the reawakening of chemical memories from long ago when they were fish only inches long. The expectant river-runners can be spied from the cliffs above estuary mouths drifting on the tide, maybe fleeing a hungry seal.
The year 2011 was dry for a while in midsummer in Scotland when I was sailing with the life-boat crew along the coast of east Sutherland. On a bright day in a calm sea we approached the River Helmsdale where the boat was to tie up. There, offshore, were leaping salmon, throwing themselves into the air. Those who see this often say they do it prior to entering the river, usually when a freshet of rain has gushed into the sea, bringing oxygen and water particles to their sensory receptors, stirring fusty recollections. Then, often waiting for the moon, they hitch a lift in like any surf-rider, selecting the highest tides.
Just as salmon unerringly follow the shelf-edge off Norway, so they unerringly seem to know what lies up rivers in the hinterland. If a river has lochs and lakes feeding it, salmon swim it earlier, for they can progress to a safe haven up ahead in the expanse of the loch. Scotland’s west coast has what are called ‘spate’ rivers, because they are shorter than eastern rivers, and salmon generally run into them during or after rain when water levels are higher. These fish know there is often no sanctuary or a loch; their only refuge is a deep pool, and it might already contain other occupants. On the River Helmsdale we know from the presence of sea lice on fish caught by anglers that many salmon run straight through the 24-mile system and forge on through the fish-pass into the chain of lochs at the top where they can safely bide their time until autumn spawning.
The river system made in heaven is perhaps the one with big headwater lochs providing a sanctuary for the salmon and an inducement to run the river to get to it, thereby presenting anglers with chances of an encounter. Even better, possibly, would be the system with a big loch in the middle, with salmon filtering into it from below and out of it from above, the loch also acting as a natural regulator