How Atlantic Salmon Farming Works in Norway: Hatchery → Sea Pens

Atlantic Salmon Farming in Norway: From Hatchery to Sea Pens

Atlantic salmon farming in Norway follows a predictable, regulated cycle: ~6–12 months in freshwater hatcheries to smolt size (~100–300 g), then ~12–18 months in sea pens to market weight (~4–6 kg). Typical pens are about 50 m in diameter and 20–40 m deep. Operations are licensed under MAB (maximum allowable biomass) and governed by NYTEK/NS 9415 technical standards.

Sognefjorden—often called the King of the Fjords—runs ~205 km inland and drops to ~1,308 m at its deepest. Lærdalselvi, a tributary river, is famous for big wild salmon and is widely known as the Queen of Salmon Rivers.

Salmon ladders have extended the salmon-carrying stretch of Lærdalselvi from ~25 km to 41 km, and about one-third of the river’s water is regulated for hydropower—both key to understanding today’s management. In 1974, an estimated ~20,000 salmon were caught in Sognefjorden and ~7,000 in Lærdalselvi, underlining how productive this system once was.

The inner Sognefjord has National Salmon Fjord status (aquaculture is pushed to the outer fjord), and Lærdalselvi is a designated National Salmon River. After Gyrodactylus salaris was detected in 1996 and the river was repeatedly treated, Lærdalselvi was declared healthy and reopened to tightly regulated fishing in 2017.

The main challenges

• Sea lice & aquaculture impacts. Lice from open-net farms can raise mortality in wild juveniles; escaped farmed fish can interbreed with wild stocks. These remain high-priority risks in national assessments.

• Gyrodactylus salaris (parasite). Once devastated multiple rivers, including Lærdalselvi; national programmes report broad eradication successes by 2017.

• Hydropower regulation. Flow changes and barriers reduce habitat and migration. Analyses link hydropower impacts to a substantial share of salmon rivers in Norway.

• Acidification (acid rain). Historically wiped out ~18 southern salmon stocks; long-term liming has helped restore several rivers.

• Invasive pink salmon. Non-native, spreading in many northern rivers; large removal efforts and traps are now deployed.

Snapshot facts

• Sognefjorden: ~205 km long; ~1,308 m deep. Wikipedia

• Lærdalselvi ladders: salmon stretch ~25 → 41 km; ~⅓ of flow used for power. National Wild Salmon Center

• 1974 catches: ~20,000 in Sognefjorden; ~7,000 in Lærdalselvi. National Wild Salmon Center

• Average/record fish in Lærdal: ~7.5 kg average; 26.5 kg record.

• How long do farmed Atlantic salmon stay in sea pens? About 12–18 months, site and season dependent.

• How big are typical pens in Norway? Around 50 m in diameter and 20–40 m deep.

• Do salmon jump to breathe air? No. They breathe with gills; surfacing can refill the swim bladder or relate to parasite avoidance.

• What is MAB? Maximum Allowable Biomass—licence-based cap on live fish weight per site.

Illustration: ChatGPT / OpenAI, based on ecological sources (Norwegian Directorate of Fisheries, ICES, FAO reports on Atlantic salmon), August 2025.

Atlantic Salmon Farming in Norway

Summary: Atlantic salmon spend ~6–12 months in freshwater hatcheries until they reach ~100–300 g (smolt), then ~12–18 months in sea pens to reach ~4–6 kg for harvest.

Typical pens are ~50 m in diameter and 20–40 m deep (a 35×20 m pen exists but is small).

Operations are regulated by MAB (biomass limits) and technical standards (NYTEK and NS 9415) to prevent escapes and protect welfare.

Salmon don’t jump to breathe—they use gills—but may surface to gulp air for the swim bladder or react to sea lice.

Wild river vs. fish farming

Lærdalselvi in Vestland is called the “queen of salmon rivers.” Salmon ladders there extended the salmon-carrying stretch from 25 to 41 km. Farming uses a different, controlled system — but the fish is the same species (Salmo salar). Takeaway: rivers teach us salmon biology and migration; farms use that biology to grow food safely and predictably.

The two farm stages

Freshwater: hatchery → smolt

• Time: usually 6–12 months (many say “about one year”).

• Size at sea transfer: ~100–300 g (200 g is common).

• When: often spring and autumn. Your note “April to January” fits within these common windows.

Sea: net pens (open water)

• Growth: from ~0.1–0.3 kg to ~4–6 kg in ~12–18 months, depending on site and season.

• Harvest: when fish reach market size (often 4–6 kg).

How big are the pens?

• Think of a pen as a giant underwater net bag held by a floating ring.

• Typical sizes in Norway today: ~50 m diameter (160 m circumference) and ~20–40 m deep.

• Your numbers 20 m (depth) + 35 m (diameter) describe a smaller pen. Farms use different sizes based on site conditions.

How many fish per pen?

• Norway uses MAB – Maximum Allowable Biomass. It caps the total live weight at each site.

• Sites must also follow technical rules (NYTEK) and the NS 9415 standard for safe design, mooring and nets — to avoid escapes and keep fish safe.

• After harvest, sites are fallowed (left empty) for at least two months to break pathogen and lice cycles.

• Bottom line: it’s not a fixed “number of fish,” but a licensed biomass plus strict technical and welfare standards.

Do salmon jump to get air?

• No. Salmon breathe with gills.

• They sometimes break the surface or leap to gulp air into the swim bladder (a buoyancy organ) — or because of parasites near the surface.

• Modern farms even use “snorkel pens” or air domes so fish can refill the swim bladder without staying in the lice‑rich top layer.

Sources

• Lærdalselvi (National Wild Salmon Center): facts about the river, salmon ladders (25 → 41 km).

• Nofima: average smolt size in Norway (around 180–200 g; larger smolt trends).

• Mowi – Salmon Farming Industry Handbook (2024–2025): typical harvest weights, cycles, and fallowing practice.

• Scientific papers & engineering guides: common pen sizes (~160 m circumference ≈ 50 m diameter), depths 20–50 m; rules on MAB; NYTEK + NS 9415 standard; snorkel/air‑access studies for swim bladder.

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Fjords 101: how they form

A fjord is a long, narrow sea inlet with steep sides. It began as a glacier‑carved, U‑shaped valley that was later flooded by the sea.

How fjords form — step by step

1. Snow → ice → moving glacier. Over many cold seasons, snow turns to ice. Gravity drives valley glaciers toward the coast.

2. Rock carving. Thick ice plus rock fragments at the base grinds and plucks the bedrock. Erosion is strongest in the middle of the valley, so it becomes over‑deepened (deeper than the coast) and takes on a U‑shape. Side valleys get left “hanging,” which is why fjords have so many waterfalls.

3. The threshold (sill). Near the sea, the glacier is thinner and erodes less. It also dumps moraines (rocky debris). Together, these build a shallow sill at the fjord mouth. Fjords with a clear sill are called threshold fjords. If there’s no shallow sill at the mouth, we call it an open fjord.

4. Flooding & rebound. When the climate warmed after the last Ice Age, the ice melted, sea level rose, and the valleys filled with seawater. At the same time the land slowly rebounded upward after being weighed down by ice.

5. Today’s water layers. Rivers add fresh water at the surface; salty ocean water sits below. In threshold fjords, deep water is renewed only when dense ocean water spills over the sill—so the deepest basin can go low in oxygen. Open fjords exchange more freely.

Quick examples

• Sognefjorden — “King of the Fjords.” About 205 km long and ~1,308 m deep. A classic threshold fjord: a shallow outer sill (~150–200 m) guards a very deep interior.

• Hardangerfjorden — glaciers & branches. About 179 km long, branching around the Folgefonna ice cap. Textbook U‑shapes, hanging valleys and waterfalls.

• Geirangerfjord & Nærøyfjord — UNESCO. Iconic scenery and clear glacial landforms. Nærøyfjorden has a very shallow sill at Bakka, which restricts deep‑water renewal and can lead to low oxygen.