Why fish migrate
Explainer · Last reviewed 2026-08-14
Migration is an energetic bet: a fish moves because the pay-off from feeding, breeding or surviving somewhere else exceeds the very large cost of getting there. Every migration pattern is a different version of that calculation.
MIGRATION IS EXPENSIVE AND IT IS COMMON, WHICH MEANS IT PAYS. A fish that moves hundreds or thousands of kilometres is spending energy it could have used to grow, and exposing itself to predation and to conditions it cannot control. That it happens repeatedly across unrelated lineages tells you the return is large.
THERE ARE THREE RETURNS, AND MOST MIGRATIONS COMBINE THEM.
FOOD. Productivity is patchy in space and time. Marine feeding grounds can offer growth rates that fresh water cannot, which is the underlying reason salmon go to sea. The reverse also happens: a fish may move to fresh water because the food there is seasonally abundant.
REPRODUCTION. Eggs and larvae need specific conditions — a substrate, a temperature, a current, an absence of predators — and those may be nowhere near where the adult feeds. Spawning migration is a movement to where the offspring can survive rather than where the parent can.
SURVIVAL. Escaping conditions that are becoming lethal: freezing, drying, deoxygenation, or a temperature outside the tolerable range.
THE VOCABULARY, WHICH IS WORTH LEARNING BECAUSE IT SORTS THE WORLD'S MIGRATORY FISH.
DIADROMOUS fish cross between fresh and salt water. Within that, ANADROMOUS fish feed at sea and breed in fresh water — salmon, sea trout, shad, sturgeon. CATADROMOUS fish feed in fresh water and breed at sea — eels, and a smaller number of others. That distinction has a record of its own because the physiological problem it creates is enormous.
POTAMODROMOUS fish migrate entirely within fresh water, which is much more common than the diadromous cases and far less celebrated: barbel moving to gravel, carp moving to shallow weed, and the great river migrations of the Amazon, Mekong and Mississippi systems.
OCEANODROMOUS fish migrate entirely within the sea — tuna, mackerel and herring covering ocean basins.
HOW THEY NAVIGATE is separately remarkable and separately documented, and the salmon case has a record of its own.
AND WHY IT MATTERS TO SOMEBODY FISHING: a migratory fish is present, catchable and countable only during part of its cycle, its numbers depend on conditions somewhere you have never been, and it is uniquely vulnerable at the choke points where it concentrates. Almost every fisheries management measure that looks arbitrary from the bank is a response to one of those three facts.
Three different journeys that the one word hides
"Migration" covers movements with almost nothing in common except that the fish is not where it was, and separating them is what makes the word useful to an angler.
THE SPAWNING MIGRATION IS A ONE-WAY BET. Salmon into rivers, eels to the deep Atlantic, pike into shallow bays, many sea fish onto specific grounds at specific times. It is expensive, it is timed by photoperiod and triggered by temperature or flow, and it concentrates fish in predictable places — which is exactly why it is the migration most heavily regulated and most often closed to fishing. A concentration of spawning fish is the easiest fishing of the year and the least defensible.
THE FEEDING MIGRATION IS ROUTINE AND REVERSIBLE. Fish following prey inshore in summer and back out in autumn, bass moving onto a warming shore, mackerel arriving and leaving, coarse fish moving between deep winter holding water and shallow summer feeding water. This is the migration that decides whether a mark is worth fishing this month, and it moves with prey and temperature rather than with the calendar.
THE DAILY MOVEMENT IS A MIGRATION AT A SMALLER SCALE and is frequently the one that matters most on the day. Fish moving up in the water column at dusk, onto a flat with a rising tide, or out of a weed bed to feed at night. The mechanisms are the same — food, light, safety — compressed into hours.
AND CROSSING THE SALINITY LINE IS A PHYSIOLOGICAL EVENT, NOT A GEOGRAPHIC ONE. A fish moving between fresh and salt water reverses the direction its whole osmoregulatory system operates in, which takes days and is why diadromous fish hold in estuaries rather than running straight through. An angler who understands that estuary pause understands where a large share of estuary fishing actually happens.
THE PRACTICAL CONSEQUENCE. Ask which of the three is in play before concluding that fish have "gone". A spawning migration means they are somewhere specific and probably protected; a feeding migration means the mark is out of season rather than empty; a daily movement means come back at a different hour.
Scientific evidence
Supported by peer-reviewed research or fisheries science. The sources are listed and can be checked.
- FishBase: a global information system on fishes — Froese, R. and D. Pauly, eds. — FishBase Consortium (Scientific institute)Relied on for: Recorded life history, temperature tolerance, depth range, growth and reproductive biology across species.States this directlyPassage read and verified
- Freshwater Fishes of the British Isles — HarperCollins (New Naturalist 75), 1992 (Reference work)Relied on for: Life history, habitat requirement, seasonal behaviour and physiological tolerance of European freshwater fishes.States this directlyCitation checked — the work exists and is described correctly
Where to go next
Fish this applies to
- Atlantic salmonMarine productivity supports growth fresh water cannot, and fresh water offers spawning conditions the sea does not. The salmon's life is that trade made explicit.
- Beluga sturgeonA worked example of migration as an obligation rather than a preference. Beluga must reach gravel far up a large river to reproduce, and no amount of suitable marine habitat substitutes for it. When the Iron Gates closed, the population above it did not adapt; it ended.
- European eelThe eel runs the same trade in reverse, which is why it is the standard counter-example to any account of migration built only on salmon.
- SterletThe control case that makes the point. Sterlet moves upstream to spawn and stays in fresh water throughout, so it never needed to pass the barrages — and it is the one sturgeon of the basin still widely present. The difference between it and its relatives is the migration, not the fishing pressure.
- Twaite shadA shad cannot leap. Where a weir removes access to gravel, the population above it disappears and does not return until the weir does something different — which makes the species the most direct measure of river connectivity in Britain.
What these connections rest on (5)
Scientific evidence — 5 connections
Supported by peer-reviewed research or fisheries science. The sources are listed and can be checked.
Atlantic salmon
A mechanism edge between a record and the biology beneath it. The underlying claims are documented in the fisheries science and reference literature cited rather than restated from practice.
- NASCO conservation and management of North Atlantic salmon — North Atlantic Salmon Conservation Organization (Government fisheries body)Relied on for: The conservation and management of North Atlantic salmon, including life history, run timing, marine survival and identified drivers.States this directlyCitation checked — the work exists and is described correctly
- Freshwater Fishes of the British Isles — HarperCollins (New Naturalist 75), 1992 (Reference work)Relied on for: Biology and life history of British and Irish salmonids, including spawning, juvenile stages and migration.States this directlyCitation checked — the work exists and is described correctly
Beluga sturgeon
A place or a fish joined to the mechanism that explains it, written where the subject is a worked example of that mechanism rather than merely subject to it.
- The IUCN Red List of Threatened Species — International Union for Conservation of Nature (Conservation authority)Relied on for: Conservation assessments of the sturgeons of the Black Sea and Danube basin, the threats recorded against them, and the basis for the extinction-risk statements made.States this directlyCitation checked — the work exists and is described correctly
- The European Environment Agency's assessments of Europe's waters, seas and coasts — European Environment Agency, Copenhagen (Conservation authority)Relied on for: Assessment of river continuity, barriers to migration and hydromorphological pressure in the Danube basin.States this directlyCitation checked — the work exists and is described correctly
European eel
A mechanism edge between a record and the biology beneath it. The underlying claims are documented in the fisheries science and reference literature cited rather than restated from practice.
- ICES Advice on fishing opportunities, catch and effort — International Council for the Exploration of the Sea (Scientific institute)Relied on for: Scientific advice on the European eel including recruitment trends, the state of the stock and the basis for management advice.States this directlyCitation checked — the work exists and is described correctly
- The IUCN Red List of Threatened Species — International Union for Conservation of Nature (Conservation authority)Relied on for: Global extinction-risk assessment, population trend and the threats identified for the species concerned.States this directlyCitation checked — the work exists and is described correctly
Sterlet
A place or a fish joined to the mechanism that explains it, written where the subject is a worked example of that mechanism rather than merely subject to it.
- The IUCN Red List of Threatened Species — International Union for Conservation of Nature (Conservation authority)Relied on for: Conservation assessments of the sturgeons of the Black Sea and Danube basin, the threats recorded against them, and the basis for the extinction-risk statements made.States this directlyCitation checked — the work exists and is described correctly
- The European Environment Agency's assessments of Europe's waters, seas and coasts — European Environment Agency, Copenhagen (Conservation authority)Relied on for: Assessment of river continuity, barriers to migration and hydromorphological pressure in the Danube basin.States this directlyCitation checked — the work exists and is described correctly
Twaite shad
A link from a species to the mechanism or environment record that explains something about it.
- Freshwater Fishes of the British Isles — HarperCollins (New Naturalist 75), 1992 (Reference work)Relied on for: Distribution, habitat association, identification and life history of the freshwater fishes of the British Isles.States this directlyCitation checked — the work exists and is described correctly
- FishBase: a global information system on fishes — Froese, R. and D. Pauly, eds. — FishBase Consortium (Scientific institute)Relied on for: Distribution, habitat association, depth range, diet and reproductive parameters recorded for the species.States this directlyPassage read and verified
Where it matters
What these connections rest on (1)
Scientific evidence — 1 connection
Supported by peer-reviewed research or fisheries science. The sources are listed and can be checked.
Fly fishing
A method or discipline joined to the mechanism that changes how it is conducted. Written where the method's own practice differs because of the mechanism — a different bait, a different season, a different handling routine, a decision not to eat something — and refused where the mechanism is merely true of the fish being caught.
- Freshwater Fishes of the British Isles — HarperCollins (New Naturalist 75), 1992 (Reference work)Relied on for: Life history, physiological tolerance, seasonal behaviour and feeding biology of European freshwater fishes.States this directlyCitation checked — the work exists and is described correctly
- FishBase: a global information system on fishes — Froese, R. and D. Pauly, eds. — FishBase Consortium (Scientific institute)Relied on for: Recorded environment, temperature and oxygen tolerance, feeding biology and distribution of the species named.States this directlyPassage read and verified
- Sea Angler — Bauer Media (Specialist angling publication)Relied on for: British sea angling practice including seasonality, conditions and the conduct of shore and boat fishing.FishingHQ's interpretation of what it saysCitation checked — the work exists and is described correctly
- Freshwater Fishing — Macdonald & Jane's, 1975 (Specialist angling publication)Relied on for: Comprehensive angling practice across disciplines, including seasonal behaviour and the conditions each method is worked in.States this directlyCitation checked — the work exists and is described correctly
Where this applies
- The Black SeaThe basin where the answer to that question was cut. Sturgeon, shad and other species ran between this sea and the rivers draining a third of continental Europe; the Iron Gates barrages and the other dams ended most of it. A migration is a behaviour that depends on a route existing, and this is the clearest European case of what happens when the route is removed.
- The DanubeThe Danube's sturgeons ran from the Black Sea far up the river, and barrages cut the route. Migration is not a detail of their biology here — it is the thing that was taken away.
- The River SpeyEverything about the fishery is a consequence of anadromy: fish feeding at sea, returning to gravel a long way upstream, and running on water rather than on dates.
What these connections rest on (3)
Scientific evidence — 2 connections
Supported by peer-reviewed research or fisheries science. The sources are listed and can be checked.
The Black Sea
A place or a fish joined to the mechanism that explains it, written where the subject is a worked example of that mechanism rather than merely subject to it.
- The IUCN Red List of Threatened Species — International Union for Conservation of Nature (Conservation authority)Relied on for: Conservation assessments of the sturgeons of the Black Sea and Danube basin, the threats recorded against them, and the basis for the extinction-risk statements made.States this directlyCitation checked — the work exists and is described correctly
- The European Environment Agency's assessments of Europe's waters, seas and coasts — European Environment Agency, Copenhagen (Conservation authority)Relied on for: Assessment of river continuity, barriers to migration and hydromorphological pressure in the Danube basin.States this directlyCitation checked — the work exists and is described correctly
The River Spey
A link from a place to the mechanism record that explains why its fishing works the way it does.
- Freshwater Fishes of the British Isles — HarperCollins (New Naturalist 75), 1992 (Reference work)Relied on for: Distribution, habitat association and ecology of the freshwater fishes of the British Isles.States this directlyCitation checked — the work exists and is described correctly
- FishBase: a global information system on fishes — Froese, R. and D. Pauly, eds. — FishBase Consortium (Scientific institute)Relied on for: Distribution, habitat association and life history recorded for the species.States this directlyPassage read and verified
Practitioner consensus — 1 connection
Widely and independently recommended by experienced anglers and specialist sources. This is real angling knowledge, not proof: controlled evidence for it is limited, and FishingHQ records it as convention rather than as a demonstrated effect.
The Danube
A place joined to the mechanism that explains what is distinctive about fishing there, which is what makes a location record transferable rather than local colour.
- The IUCN Red List of Threatened Species — International Union for Conservation of Nature (Conservation authority)Relied on for: Extinction-risk assessment, population trend and the threats identified for freshwater and diadromous species, including barriers to migration and exploitation.States this directlyCitation checked — the work exists and is described correctly
- Code of Conduct for Responsible Fisheries — Food and Agriculture Organization of the United Nations, Rome, 1995 (Standards body)Relied on for: International principles for responsible fisheries, including the management of shared and transboundary waters and of introduced species.States this directlyCitation checked — the work exists and is described correctly
Related reading
- Catadromy, anadromy, and crossing the salinity lineThe vocabulary that sorts the world's migratory fish, and the physiological feat that the two headline categories both depend on.
- What salmon eat at each stage of lifeThe whole point of the marine phase is that the sea produces growth fresh water cannot, so the diet at each stage is the migration argument stated in food.
- Where eels spawn, and why nobody has watchedA migration whose destination is inferred rather than seen is the limiting case of the general question, and it shows how much of what is known about migration comes from indirect evidence.
What these connections rest on (3)
Scientific evidence — 3 connections
Supported by peer-reviewed research or fisheries science. The sources are listed and can be checked.
Catadromy, anadromy, and crossing the salinity line
A mechanism edge between a record and the biology beneath it. The underlying claims are documented in the fisheries science and reference literature cited rather than restated from practice.
- FishBase: a global information system on fishes — Froese, R. and D. Pauly, eds. — FishBase Consortium (Scientific institute)Relied on for: Life history, diet, migration pattern, habitat association and depth range recorded for the species concerned.States this directlyPassage read and verified
- Freshwater Fishes of the British Isles — HarperCollins (New Naturalist 75), 1992 (Reference work)Relied on for: Biology, life history, distribution and conservation status of British and Irish freshwater and diadromous fishes.States this directlyCitation checked — the work exists and is described correctly
What salmon eat at each stage of life
The energetic basis of anadromy is documented in the life-history literature cited rather than restated from practice.
- FishBase: a global information system on fishes — Froese, R. and D. Pauly, eds. — FishBase Consortium (Scientific institute)Relied on for: Diet, habitat association, life history and feeding behaviour recorded for the species concerned.States this directlyPassage read and verified
- Freshwater Fishes of the British Isles — HarperCollins (New Naturalist 75), 1992 (Reference work)Relied on for: Biology, life history and distribution of British and Irish freshwater and diadromous fishes.States this directlyCitation checked — the work exists and is described correctly
Where eels spawn, and why nobody has watched
A mechanism edge between a record and the biology beneath it. The underlying claims are documented in the fisheries science and reference literature cited rather than restated from practice.
- ICES Advice on fishing opportunities, catch and effort — International Council for the Exploration of the Sea (Scientific institute)Relied on for: Scientific advice on the European eel including recruitment trends, the state of the stock and the basis for management advice.States this directlyCitation checked — the work exists and is described correctly
- The IUCN Red List of Threatened Species — International Union for Conservation of Nature (Conservation authority)Relied on for: Global extinction-risk assessment, population trend and the threats identified for the species concerned.States this directlyCitation checked — the work exists and is described correctly
Sources for this page
- FishBase: a global information system on fishes — Froese, R. and D. Pauly, eds. — FishBase Consortium (Scientific institute)Relied on for: Life history, diet, migration pattern, habitat association and depth range recorded for the species concerned.States this directlyPassage read and verified
- Freshwater Fishes of the British Isles — HarperCollins (New Naturalist 75), 1992 (Reference work)Relied on for: Biology, life history, distribution and conservation status of British and Irish freshwater and diadromous fishes.States this directlyCitation checked — the work exists and is described correctly
- NASCO conservation and management of North Atlantic salmon — North Atlantic Salmon Conservation Organization (Government fisheries body)Relied on for: The conservation and management of North Atlantic salmon, including life history, run timing, marine survival trends and identified drivers.States this directlyCitation checked — the work exists and is described correctly
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