Atlantic salmon: life, behaviour and the long reads

The handling question for Atlantic salmon has an answer most species do not: a returning salmon has stopped feeding and cannot rebuild what a long fight costs it.

That single fact reorganises everything. A fish that will feed again can recover from a hard fight over days. A salmon on its spawning migration is running on reserves it cannot replace, so the cost of the fight is permanent and is subtracted from what it has left for spawning. Playing a fish quickly is therefore not merely kinder — it is the difference between a fish that spawns and a fish that arrives unable to.

The mechanics are ordinary and the discipline is not. Keep the fish in the water. Support it horizontally with a hand under the belly and the other around the tail wrist. Never lift a salmon by the tail alone or hold it vertically. Unhook with the fish submerged wherever possible, and revive it facing into the current until it swims off under its own power.

Water temperature changes the calculation sharply. In warm low water the same fight costs more and recovery takes longer, and the honest conclusion in those conditions is sometimes not to fish for them at all.

Where a fishery distinguishes what may be retained, that judgement is made on a fish already in the net — which is another reason the fish stays in the water while it is being made.

Atlantic salmon size is not a smooth distribution. It is bimodal, and the reason is the single most interesting thing about the species' life history.

A smolt that goes to sea returns either after ONE winter — a grilse, typically one to three kilos — or after TWO OR MORE, a multi-sea-winter fish of four to ten kilos and occasionally far more. The difference is not growth rate in the river. It is how long the fish stays at sea feeding before the drive to return overtakes it, and the split is partly genetic and partly environmental.

That is why a spring salmon and a summer grilse from the same river look like different fish and are the same species at different points in one decision. It is also why rivers are described as grilse rivers or spring-fish rivers: the proportion differs by system and by season within a system.

THE EXCEPTIONAL FIGURES are old and are getting older. Historic rod-caught fish of over twenty-five kilos exist in the record and modern fish approaching them do not, which reflects a real decline in the abundance of multi-sea-winter fish rather than a change in what the species is capable of.

And size at return says nothing about size in the river. Parr spend one to several years in fresh water reaching perhaps fifteen centimetres before smolting; almost all the growth happens at sea, in months, on a marine diet the fish will never eat again once it turns for home.

There is no single Atlantic salmon calendar, and any page that offers one is describing a particular river rather than the species.

What is consistent is the SEQUENCE. Adults return from the sea, hold in fresh water without feeding, move upstream as flow allows, spawn in gravel in the colder part of the year, and either die or drop back as kelts. Juveniles spend one to several years in the river before smolting and going to sea in spring.

What varies enormously is WHEN fish enter. Some rivers receive fish from very early in the year — the spring fish that gave the classic salmon season its shape — while others are dominated by summer grilse or by late autumn runs. A single river may have several distinct run components, each genetically distinguishable, entering at different times and destined for different tributaries.

FLOW IS THE PROXIMATE TRIGGER for movement. Salmon hold in pools and run on rising or falling water, which is why rain in the catchment changes a river's fishing far more than anything happening at the bank.

SPAWNING is temperature-driven and happens in the colder months of the local year, on clean gravel in oxygenated water, with the female cutting a redd.

AND THE FISH DOES NOT FEED. A returning salmon has stopped feeding and will not resume. Everything about how it is fished for follows from that, and it is also why a long fight costs a returning fish something it cannot replace.

ESTABLISHED. Atlantic salmon are anadromous: they hatch in fresh water, spend one to several years as parr, undergo smoltification — a physiological transformation preparing them for salt water, involving changes in gill function, body chemistry and behaviour — migrate to sea feeding grounds, and return to fresh water to spawn. Unlike Pacific salmon, some survive spawning and return again as kelts.

HOMING IS REAL AND PARTLY EXPLAINED. Returning fish locate their natal river with high fidelity. Olfactory imprinting on the chemical signature of the home water is well supported for the final freshwater stage. Navigation across the open ocean before that point is far less settled, with magnetic and celestial mechanisms proposed and none conclusively established.

THE FEEDING PARADOX. Adult salmon do not feed meaningfully once they enter fresh water, yet they take angler's flies and lures. No explanation is settled: aggression, territorial response, residual feeding reflex and curiosity have all been proposed. FishingHQ states this as unresolved because it is, and because the fishery's central practical fact rests on it.

WHAT THIS MEANS PRACTICALLY. A fish that is not feeding cannot be tempted by matching its diet, which is why salmon fishing is organised around water height, temperature and lie rather than around imitation.

The most consequential fact about Atlantic salmon reproduction is not how they spawn but where, and specifically how precisely they choose it.

SALMON RETURN TO THE RIVER THEY WERE BORN IN, and frequently to the tributary and the reach. The navigation appears to combine geomagnetic orientation across the ocean with olfactory imprinting in fresh water — the fish learns the chemical signature of its natal stream as a juvenile and recognises it on return, years later, after a migration of thousands of kilometres.

THE CONSEQUENCE IS GENETIC AND IT IS THE WHOLE POINT. If fish return to their birthplace and spawn there, then each river's salmon breed with each other and not with the next river's. Over thousands of generations that produces populations that are genetically distinct river by river, and locally adapted — to that river's temperature regime, its flow, its timing, the size at which its fish must be to run it. A Spey salmon and a Wye salmon are not interchangeable stock.

WHICH MEANS A LOST POPULATION IS NOT REPLACED FROM NEXT DOOR. Straying between rivers happens at a low rate and is the mechanism by which new rivers are eventually colonised, but it is far too slow to restore a river within a human timescale, and the fish that stray are not adapted to the river they arrive in. A river whose salmon are extirpated has lost something specific and irreplaceable rather than a share of a common pool. That is the reasoning behind river-by-river management and behind the concern about interbreeding with escaped farmed fish, whose genetics are not adapted to any river.

THE SPAWNING ITSELF IS SUBSTRATE ENGINEERING. The female cuts a redd — turning on her side and flexing to displace gravel — deposits eggs in the depression, and covers them. The buried eggs develop over months in the gravel, oxygenated by water flowing through the interstitial spaces between stones. That last detail is why siltation is so damaging: fine sediment fills the gaps, the flow through the gravel stops, and the eggs suffocate in a redd that looks intact.

FISH ON REDDS ARE DOING THE THING EVERYTHING ELSE DEPENDS ON. What is legally permitted in respect of them varies by river and by season and is current local information this page does not state.

An Atlantic salmon needs four quite separate habitats in sequence, and the population is limited by whichever of them is in the worst condition — which is almost always one of the freshwater ones.

THE SPAWNING HABITAT IS CLEAN, LOOSE GRAVEL IN FLOWING WATER. Eggs are buried in a redd cut into gravel of a specific size range, in water shallow and fast enough to force oxygenated flow down through the gravel bed to reach them. That subsurface flow is the whole requirement, and it is what fine sediment destroys: silt from agricultural run-off, forestry or bank erosion fills the interstitial spaces, the eggs suffocate, and the river looks entirely normal from the bank.

THE JUVENILE HABITAT IS FAST, BROKEN, STONY WATER. Fry and parr hold in riffles and runs among cobbles and boulders, each fish occupying a station and defending it. Habitat quality here is a matter of structural complexity and shade: cobble to hide among, broken flow to hold in, tree cover to keep water cool and deliver terrestrial invertebrates. A dredged, straightened, uniform channel supports a small fraction of the parr a natural one does even where the water quality is fine.

THE MIGRATORY CORRIDOR IS A HABITAT IN ITS OWN RIGHT, and it is the one people forget. Smolts must reach the sea and adults must reach the spawning gravel, which requires the river to be passable in both directions at the right times. Weirs, dams, culverts, poorly designed passes and low flows all break the corridor, and a river with perfect gravel above an impassable structure holds no salmon.

THE MARINE HABITAT IS THE LEAST UNDERSTOOD AND THE MOST CONSEQUENTIAL. Post-smolts move to distant North Atlantic feeding grounds, and marine survival — which has declined substantially — now dominates the returns for many rivers. That is a habitat problem occurring somewhere no river manager can reach.

THE TEMPERATURE ENVELOPE BINDS ALL THE FRESHWATER STAGES. Salmon require cold, well-oxygenated water, and warming pushes the southern edge of the range northward and makes low-summer-flow rivers periodically lethal.

WHICH IS WHY THE HABITAT ACCOUNT MATTERS MORE THAN THE DISTRIBUTION MAP. A river within the range is not a salmon river unless it has clean gravel, structurally complex juvenile water, passage from the sea, and enough cold water in summer. Losing any one of the four is enough.

Salmon tackle decisions are made almost entirely on water conditions, and a reader who understands that will make better choices than one who knows what fish are in the river.

ROD LENGTH IS ABOUT CONTROLLING LINE ACROSS A RIVER. A double-handed rod exists to cast a long line without back room and, more importantly, to hold and mend that line across conflicting currents so the fly swims at a controlled speed. On a big river in high water that is the whole problem; on a small stream in low water a long rod is a liability and a single-handed outfit does the job better. The fish are the same fish.

LINE DENSITY AND FLY SIZE TRACK WATER HEIGHT AND TEMPERATURE TOGETHER. In cold, high water a salmon will move a short distance for a large fly presented deep, so the tackle is heavier: sinking lines or tips, bigger flies, more weight. In warm, low, clear water the fish will come further for a small fly near the surface, so the tackle lightens all round. That progression is the single most useful principle in the discipline and it is a physical one about how far a fish will move and how fast the fly is travelling.

SWING SPEED IS THE THING BEING CONTROLLED. Everything above — rod length, line density, fly size, casting angle, mending — is aimed at making the fly travel across the current at a speed the fish will intercept. Tackle that cannot control that produces a fly moving too fast, which is the commonest reason a well-fished pool produces nothing.

HOOK CHOICE IS INCREASINGLY A CONSERVATION MATTER. Many salmon rivers now operate mandatory catch-and-release, and hooking damage governs survival. Single, barbless hooks reduce injury and unhooking time relative to trebles, and their use is required on a great many rivers. What is required on any particular water is a matter for the competent authority and it changes; several jurisdictions also restrict methods entirely in parts of the season.

AND WARM LOW WATER IS WHEN TACKLE CHOICE BECOMES A WELFARE DECISION, exactly as with barbel: a long fight in warm water on light tackle kills salmon that would have survived a short one. Where rivers impose temperature-based restrictions, that is the reasoning.

Salmon lure fishing rests on a premise that has to be stated before anything else makes sense: a salmon in fresh water has stopped feeding, and every lure presented to it is provoking a response rather than offering a meal.

THE FISH DOES NOT NEED TO EAT AND DOES NOT INTEND TO. Returning adults cease feeding on entering fresh water and run on stored reserves, which is why a river salmon can ignore an entire day's presentations and then take savagely. What is being triggered is variously argued to be aggression, territoriality, irritation or a residual feeding reflex from the fish's marine life, and the honest position is that nobody has settled it. FishingHQ records it as unresolved rather than picking an explanation.

WHAT FOLLOWS PRACTICALLY IS THAT SPEED AND DEPTH MATTER MORE THAN PATTERN. A lure passing at the right depth and the right speed in front of a fish gets a response; the same lure passing too fast, too high or too far away does not. That is why salmon anglers talk about covering water systematically rather than about selecting a bait.

WATER HEIGHT AND TEMPERATURE SET THE SIZE AND DEPTH TOGETHER, and this is the single most transferable principle in the discipline. Cold high water: bigger lure, deeper, slower, because the fish will move only a short distance. Warm low water: smaller, higher, faster, because it will come further. That progression works for spoons, spinners, plugs and flies alike.

SPINNERS AND SPOONS WORK BY FLASH AND VIBRATION, which is why they earn their place in coloured water where a fly is hard to find. Plugs and heavier lures cover deep holding water in high flows.

AND HOOK CHOICE IS INCREASINGLY THE BINDING CONSTRAINT RATHER THAN A REFINEMENT. Many salmon rivers now operate mandatory catch-and-release with single, barbless hook requirements, and treble-hooked lures are prohibited on a great many waters. Where they are permitted, the record's tackle section explains why a shorter fight matters in warm low water. What applies on any river is a matter for the competent authority and it changes between seasons.

Eggs hatch in gravel in spring; alevins absorb the yolk sac and emerge as fry, which become territorial parr. After one to several years in fresh water, parr smolt — a physiological reorganisation that allows survival in salt water — and migrate to sea. Marine feeding is rapid, and fish return after one winter as grilse or after several as larger multi-sea-winter fish. Unlike Pacific salmon, Atlantic salmon do not necessarily die after spawning, and a proportion of kelts survive to return again, though most do not.

Spawning takes place in late autumn and winter in clean gravel, with the female cutting a redd and burying the fertilised eggs. Salmon show strong natal homing, returning to the river and often the tributary of their birth, which means populations are genetically distinct river by river — and which means a lost population is not readily replaced from a neighbouring one.

At sea, salmon feed heavily on crustaceans and small fish, which is where essentially all their growth occurs. On entering fresh water to spawn they effectively stop feeding and metabolise stored reserves. Why they nonetheless take a fly or a lure is not settled — aggression, territorial response, and residual feeding reflex are all proposed, and the honest position is that the question is open. FishingHQ records it as unresolved rather than picking the most satisfying explanation.

Vision is central to a taking response in fresh water, which is why fly size, depth and speed dominate salmon angling far more than colour or pattern do. The homing mechanism itself is thought to combine geomagnetic orientation at sea with olfactory recognition of natal water in the final stages — the fish smells its way home once it is close.

Runs are timed by river and vary enormously: some systems receive fish from early spring, others principally in summer or autumn. Movement within the river is driven overwhelmingly by water height and temperature, which is why salmon angling is so conditions-dependent. Fish that have been in the river for weeks are progressively less inclined to take than fresh-run fish.

The most prestigious game fish in Europe and the subject of an angling tradition, an economy and a body of technique of its own. Salmon angling is now conducted under heavy conservation constraint in much of its range, with catch-and-release the norm and access, seasons and methods closely regulated in ways that vary substantially by country and by river.