Sea Trout

By Alex Clayton · Updated 23 October 2026

A sea trout is not a separate species. It is a brown trout, Salmo trutta, that left its river to feed at sea and came back silver and much heavier. Genetics and food supply decide which young trout make that trip and which stay at home.

For the angler, that one difference sets the size of fish you can expect, when they run, why most are caught after dark, and which licence and rules apply in England, Wales, Scotland and Northern Ireland.

This sea trout UK guide covers identification, local names such as sewin, the life cycle, where and when to fish, the rules by nation and how to release fish well.

Sea Trout Facts at a Glance

The key numbers for a sea trout, with the rules detail further down.

Fact	Detail
Scientific name	Salmo trutta, the same species as brown trout
Other names	Sewin, peal, finnock, mort, white trout (see the names table below)
Typical adult size	35 to 60 cm, about 1lb to 5lb 5oz (0.5 to 2.4 kg)
Diet at sea	Small fish, crabs, shrimps and prawns
River run	Fish enter rivers from April, with most arriving June to October
Best fishing	Mostly after dark on summer nights
Licence in England and Wales	Salmon and sea trout rod licence, not the trout and coarse licence
Status in Wales	30 of 33 main sea trout rivers rated At Risk (NRW, October 2024)

Those sizes describe typical fish, not the upper limit. Seasons vary by nation and by river, so check the table in Sea Trout Rules by Nation before you plan a trip.

Sea Trout, Brown Trout or Salmon? How to Tell Them Apart

If sea trout and brown trout are the same species, how do you know which one is on the end of your line? Often you can tell. Sometimes nobody can without a microscope.

A fresh-run sea trout is the easy case. Time at sea leaves it bright silver along the flanks and usually much bigger than the resident trout in the same river. The difficulty starts once a sea trout has been in fresh water for a while. It colours up, the silver fades, and it starts to look like any large brown trout. At that point, according to the Wild Trout Trust, the only certain way to separate the two is scale reading under magnification, which nobody can do on the bank.

An Atlantic salmon is a separate species, Salmo salar, and the full breakdown of how to tell a sea trout from a salmon has its own page. The distinction matters because salmon rules are often stricter: in England, every salmon caught before 16 June must be released under national byelaw. Read more in our Atlantic salmon guide, or use our fish identification guide for other UK fish.

If you cannot tell what you have caught, treat it as the stricter species and return it.

Sewin, Peal, Finnock and Mort: Sea Trout Names Around the UK

Call it sewin in Wales or peal in the South West, and you are still talking about a sea trout. The names change from coast to coast, which can make older books and club rules confusing.

Name	Where it is used	Notes
Sewin	Wales	Welsh byelaws define sea trout as any fish commonly known as sea trout or sewin
Peal (or peel)	South West England	Regional name
Finnock	Scotland	A first-return fish after less than a year at sea
Mort	North West England	Regional name
White trout	Ireland	Regional name
Herling, whitling	Regional	Regional names

The Welsh definition settles it in law: sewin and sea trout are the same fish. Whatever the local name, the same rules apply.

The Sea Trout Life Cycle: Smolt, Finnock and Returning Fish

About 75% of sea trout survive spawning and go back to sea, according to the Wild Trout Trust. The same fish can run the same river more than once, and it grows bigger between visits.

The cycle runs in four stages:

Smolt. After 1 to 3 years in the river, young trout of 5 to 7 inches (about 13 to 18 cm) turn silver and head to sea in shoals. They go from late March to April, mostly at night.
At sea. The fish feed on small fish, crabs, shrimps and prawns. This rich diet is where the extra growth comes from.
Finnock. Some fish return after less than a year at sea. In Scotland these first-return fish are called finnock.
The run. Adult fish enter rivers from April, with most arriving between June and October. By day they hold in deep or shaded pools, and they become active after dark.

A female carries roughly 800 eggs per lb of her weight, and on many rivers most sea trout are female. Large females carry more eggs and bigger ones, so big hen fish matter most to the next generation.

A brown trout that stays at home lives on what the river provides. A sea trout feeds in the far richer sea, which is why it comes home so much larger.

Where and When to Fish for Sea Trout

Plenty of anglers fish for sea trout in daylight and wonder why the pools look empty. The fish are often there, holding deep or in shade, waiting for dark.

Where to fish

Sea trout run rivers in all four nations. Rivers named in the official sea trout byelaws include:

Wales: the Teifi, Tywi, Dyfi and Cleddau
England: the Border Esk, Eden, Camel, Tamar, Dart, Exe and Torridge
Scotland and Northern Ireland: many rivers and loughs hold sea trout, with access through clubs and estates

Most beats have their own permit rules, so check with the fishery or club before you travel.

When to fish

Specialist sea trout guidance puts the best night fishing at roughly June to August. Start once it is fully dark, because the first hour or two of darkness is often the most productive. After a spate, wait for the river to drop and clear, since coloured water usually means fewer fish. Target pool tails, glides and shallows close to deeper water.

Methods in brief

A typical starting set-up for night fly fishing is a 10ft single-handed rod rated #7 with a #8 floating line, an 8 to 10ft leader of 8 to 10lb, and a single size 8 or 10 fly or a tube. Keep your torch off the water.

Spinner or worm can work in coloured water where the river rules allow it. In Wales, sea trout bait is limited to a single worm on a single barbless hook with a gape of no more than 8 mm, and bait is banned before 1 May on 23 named rivers. Full detail is in our sea trout fishing techniques guide, alongside other game fish.

Best for: anglers happy to fish late on summer nights. Skip if: you can only fish in daylight in low, clear water.

Sea Trout Rules by Nation: Licence, Seasons and Keeping Fish

The easiest mistake is buying the wrong licence. In England and Wales, the trout and coarse licence covers non-migratory trout only. Sea trout need the salmon and sea trout licence, and every holder must send in an annual catch return, even if they did not fish (GOV.UK).

Prices change each April, so check GOV.UK or our fishing licence guide. Under 13s need no licence, and 13 to 16 year olds get a free junior one.

Nation	Licence	Season	Keeping fish
England	Salmon and sea trout licence (required across England except the River Tweed, and on the Border Esk)	Set by regional byelaw and varies by river. Close season examples: Northumbria and Yorkshire 1 November to 2 April; North West 1 October to 31 March; Southern area 1 November to 30 April	North West: 30 cm minimum. Southern area: 38 cm minimum. Eden: kill no more than 2, Border Esk: no more than 4, between 1 April and 30 September, with no females killed 10 to 30 September. Camel: daily limit 2 and none over 50 cm
Wales	Salmon and sea trout licence	Most rivers 20 March to 17 October. West Wales rivers (Teifi, Tywi, Taf, Cleddau, Aeron, Ystwyth, Rheidol, Nevern) 1 April to 17 October. Anglesey 1 May to 17 November	No removal before 1 May on 23 named rivers. None over 60 cm from any Welsh river. Severn in Wales: all sea trout released
Scotland	No rod licence. You need a permit or written permission from the owner of the fishing rights	Set by district. Check with the district salmon fishery board	Check with the board or club
Northern Ireland	Check nidirect for DAERA rod licence and permit rules	Varies by water. Example: River Bush 1 March to 20 October	Salmon and sea trout caught in the DAERA rod licensing area must be returned unharmed

Further rules:

England national byelaws: one rod at a time for sea trout on rivers and streams, foul-hooked sea trout released immediately, and no selling of rod-caught salmon or sea trout (GOV.UK national byelaws).
Wales hooks: barbless or de-barbed for sea trout, and no trebles outside the Dee, Severn and Wye.
Welsh byelaws are due to change: the 2017 rod and line byelaws cease to have effect on 31 December 2026. Check Natural Resources Wales for their replacement and for current Wye and Usk rules.
South West England: close seasons are set river by river, such as Tamar 1 October to 2 March and Exe, Dart, Taw and Torridge 1 October to 14 March (GOV.UK South West byelaws).

Licence, season, then the river’s own keep rules: check all three before you cast. Our fishing seasons by nation page has the wider picture.

Catch and Release: Handling Sea Trout and the State of Stocks

The 2023 Welsh rod catch was 5,669 sea trout, the lowest since records began in the 1970s. Natural Resources Wales rates 30 of its 33 main sea trout rivers At Risk, 3 Probably at Risk and none Not at Risk. These are the latest figures found, published on 7 October 2024.

Release pays off with sea trout because about 75% survive spawning and the big females carry the most eggs. A released hen fish can come back to spawn again.

Handle every fish you return like this:

Use barbless single hooks, compulsory for sea trout in Wales and quicker to remove everywhere.
Keep the fish in the water while you unhook it.
Hold it upright, facing into the current, until it swims off under its own power.
If a sea trout is foul hooked in England, release it immediately. That one is the law, not just good practice.

If the river’s rules let you keep one, consider returning the bigger hen fish anyway.

Frequently asked questions

Is sea trout the same as salmon?

No. A sea trout is a sea-run brown trout, Salmo trutta. An Atlantic salmon is a separate species, Salmo salar, and salmon rules are often stricter. If you are not sure which fish you have caught, treat it as a salmon and return it.

Is sewin the same as sea trout?

Yes. Sewin is the Welsh name for sea trout, and Welsh byelaws define sea trout as any fish commonly known as sea trout or sewin. It is called peal in South West England, and first-return fish in Scotland are called finnock.

Do I need a licence to fish for sea trout?

In England and Wales, yes: the salmon and sea trout rod licence, plus an annual catch return even if you did not fish. Scotland has no rod licence, but you need permission from the owner of the fishing rights. In Northern Ireland, check nidirect for the DAERA licence and permit rules.

Can you keep a sea trout?

It depends on the nation and the river. All sea trout must be returned in the DAERA rod licensing area of Northern Ireland and on the Severn in Wales. Wales bans keeping any sea trout over 60 cm, or any before 1 May on 23 named rivers. English rivers have regional size and kill limits.

Can you eat a sea trout you catch?

Only if the river's rules let you keep it. Check the season, size and kill limits for that water first, using the rules table above. In England it is illegal to sell rod-caught sea trout or to keep one with the intention of selling it.