Seals at Steilene: the only colony in the inner Oslofjord

Steilene is a five-island cluster in the middle of Vestfjorden, between Bygdøy and Nesoddtangen, hosting the only permanent seal colony in the inner Oslofjord. Around 100 harbour seals (steinkobbe, Phoca vitulina vitulina) haul out on the outer skerries year-round. The Steilene lighthouse has operated since 1837. The colony was nearly wiped out twice by phocine distemper virus in 1988 and 2002, and recovered both times.

Five islands, one colony

Steilene sits in the middle of Vestfjorden, the deepest basin of the inner Oslofjord — over 160 metres straight down in parts. Five main islands make up the group. Fyrsteilene with the lighthouse. Storsteilene, the biggest. Landsteilene and Persteilene, joined by a narrow spit. And Knerten, the smallest, which is a bird reserve. The seals use all of them, though they prefer the outer skerries where the rock warms early and the swell is low.

The species is steinkobbe — the harbour seal, Phoca vitulina vitulina. Bulls grow past 150 centimetres and over 100 kilos. Cows are smaller. They live up to thirty-five years in the wild and they eat small fish — tobis, sprat, young pollock, herring, about twenty prey species in all. They are gregarious, social on land, and famously shy of boats. Grey seals (havert) sometimes show up as vagrants from the Swedish colonies further south, but they do not breed here. Steilene is steinkobbe territory.

The virus that almost finished them

Twice inside living memory the Steilene colony was nearly wiped out.

In 1988 an outbreak of phocine distemper virus — PDV, a cousin of the human measles — swept through European harbour seal populations starting at Anholt in the Kattegat. It killed more than 23,000 seals across the North Sea and Skagerrak in a single summer. Harbour seals along the Norwegian Skagerrak coast — Steilene included — were nearly wiped out. In 2002 it happened again, just as the population was recovering. The Skagerrak colonies were hit even harder the second time.

What we see today is effectively the grandchildren of those survivors. The Norwegian mainland population is back up to about 7,500 (Havforskningsinstituttet’s most recent full count), and the outer Oslofjord population, mainly at Hvaler and Færder, has grown from ~600 in 2016 to about 1,300 in 2022. The inner-fjord Steilene group has been slower to recover. No one has put a tight current number on it, but it sits in the low hundreds.

When to see them

Late August through September. That is the moulting window, when the seals shed last year’s fur and spend more of the day hauled out on the rock warming up. That is also when Havforskningsinstituttet runs its count flights, because that is when every seal in the colony is visible at once. On a calm, sunny day at Steilene in September, low tide, you can see fifty seals on one slab of rock.

Pupping is mid-June through the first week of July. Peak date is right around Sankthans, the summer solstice. Pups are born on seaweed at low tide and swim within hours of birth. It is one of the most compressed nativity scenes in nature — from uterus to open water in a day. The skerries they use for pupping are the outer ones, south-facing, sheltered from wind. We give those a wide berth in June.

Best visibility is two hours either side of low tide, calm wind, clear sky. Wait on the ferry lane and look toward the lighthouse rocks with binoculars. If you see dark commas on the slab, those are seals.

The rules, clearly

Knerten is a nature reserve. It was established by royal decree on 15 December 1978 and runs a strict no-entry window — no landing, no boat inside the 50-metre marine buffer — from 15 April to 15 July. The rule exists because eider, terns and other seabirds nest on the ground there, and a single person walking through flushes the colony and breaks the breeding year. Ignoring the ban is a criminal offence. Statens Naturoppsyn enforces it.

There is no universal numerical distance rule for seals in Norwegian law, but the principle is ikke forstyrr — do not disturb. If a group of seals flushes off the rock into the water at your approach, you are too close. Fifty metres is the minimum anyone should be operating at; a hundred is better. Pups are born with no fat reserves and every stress event burns energy they can’t afford to lose.

Why they chose Steilene

Bathymetry. The Vestfjorden basin is the deepest hole in the inner fjord — over 160 metres — and the deep water funnels cold, oxygenated Skagerrak water up past Drøbak and into the basin, where it mixes and feeds the herring and sprat shoals that feed the seals. Steilene also sits just far enough offshore that the ferry and boat traffic flows around it rather than through it.

Ask a Nesodden sailor and they will tell you the colony has always been there, as long as there has been rock above water. The 1980s and 2000s were the interruption, not the norm. The seals were here before the lighthouse.

I read which rocks they’ll be on from the wind direction. South-easterly, they prefer the lee side of Storsteilene. Calm and warm, they spread across the outer skerries. Stay 50 metres off and they hold position; come in closer and they slip back into the water.

If you want to see them

From Nesodden you can sometimes see them with good binoculars from the Alværn waterfront looking west. From the water is better. A kayak tour from Tjuvholmen or Lysaker — see our piece on Mad Goats — gets you close enough to spot them, though the approach has to be slow and the wind has to be kind. From a private boat you can circle the group at the respectful distance and see the detail of individual animals.

If you want to actually spend a morning watching them properly — binoculars, anchored in a safe line of sight, time enough to let the colony settle after your arrival — that is what a private cruise is built for. The three-hour tour is enough to reach Steilene, observe the colony at distance, continue past Dyna fyr, and come back around Bygdøy. We know the angles that keep you out of the flight path and still give you a clean view.

One thing to understand about the seals. They have been here through every change Oslo has gone through. Through the cod fishery, through the harbour pollution, through the shipping boom and the post-war decline and the urban cleanup. Twice a virus tried to erase them. They came back twice. Whatever we do to the rest of the fjord, they continue.