Where the porpoises actually are: specific places in the inner Oslofjord

We already have a general guide to porpoises in the Oslofjord. This one is more specific: given the resident population is small and shy, where exactly do we see them, and why those places?

How many, really

The number you will see everywhere is 70 to 80 individuals year-round in the inner fjord. It is a useful figure but worth understanding. It comes from informal counts and captain observations, not from a formal survey. The scientific phrasing is more cautious — Havforskningsinstituttet will say “a few hundred at most, present year-round.”

For context: the full Skagerrak population is around 14,000 and the wider North Sea / Kattegat population over 345,000. What makes the Oslofjord group distinctive is not its size. It is that these animals appear to be resident. They do not migrate. They spend their entire lives in a stretch of water you can sail across in an hour.

The five places I watch for them

After six seasons working this fjord, here is where I actually look.

1. Drøbaksundet

The narrows where the inner fjord constricts to about a kilometre wide, just south of Oscarsborg. Tidal flow through the gap concentrates herring and sprat. A group of up to ten porpoises is regularly reported here, especially through summer. If I am going to see porpoises on an inner-fjord trip, it is most often right at the passage past Drøbaksundet.

2. East of Nesoddtangen

The deep-water corridor between Nesodden and Lysaker. The bathymetry drops to over 150 metres here. Porpoises surface in ones and twos along this line, usually in the morning and evening feeding windows. From Steilene you can watch both directions and catch them moving through.

3. The Steilene basin

Same deep bowl that makes Steilene a seal colony — over 160 metres below your keel. It holds fish, and the porpoises hunt through it. I have seen a pair rolling past the lighthouse on a dead-calm August morning, outlined by their own ripples, thirty metres off the bow. Steilene works for both mammals at once.

4. The Hovedøya — Gressholmen — Lindøya triangle

Shallower than the others but productive. The channels between these three inner islands host summer herring shoals, and porpoises move in to hunt them. Not every day; you need calm water and the right time.

5. Dyna fyr and the Bygdøy sound

Dyna is the small lighthouse off the western entrance to Oslo havn, lit in 1874. The sound around it has strong tidal currents and small-fish congregations. Porpoises turn up here occasionally — less reliable than Drøbaksundet, but close enough to the city that a morning coffee with binoculars on Bygdøy beach has produced sightings.

When to look

Early morning before 08:00 and late afternoon after 17:00. Porpoises feed mostly during the dawn and dusk vertical migration of their prey — herring and sprat rise from depth as the light fades and drops back down at sunrise. Midday is the worst time to look.

Calm water is essential. A porpoise breaks the surface for less than a second and rolls with minimal splash. In chop you simply cannot see them. The best sightings happen on the days that look unremarkable to humans — overcast, windless, glassy.

June through September is the core season when the prey fish move into warmer shallower water and the porpoises follow. I see them less often in midwinter, though they are still here.

What to look for

A porpoise surface event lasts about one second. What you see is a small dark back rolling forward through the water, a brief grey arc, and a tiny triangular dorsal fin. Then they are gone. They exhale audibly at close range — a sharp puff — which is where the English name “porpoise” comes from (French porc poisson, pig-fish). The Norwegian nise comes from Old Norse hnísa, meaning “to sneeze.”

They do not leap. They do not bow-ride. They do not jump. If something is jumping out of the water in the Oslofjord you are looking at a dolphin — rare here — not a porpoise.

The tell when you are in the right place: you will see one surface roll, and then nothing. Fifteen to twenty seconds later, a second roll, fifty to a hundred metres from the first. That is a single porpoise travelling in a straight line underneath you. Wait. Another will appear.

Why they are still here

The inner Oslofjord has been through almost every insult humans invent. Untreated sewage. Industrial heavy metals. The cod stock collapsing to near zero. Decades of shipping noise. Somehow the porpoises stayed.

Two reasons, partly. First, they do not eat cod. They eat herring and sprat, and those remain in the fjord even as the cod is gone. Second, the deep basins — Vestfjorden, the Drøbak channel, the troughs east of Nesoddtangen — provide quiet refuge water away from the busiest boat lanes. Porpoises echolocate at 130 kHz, a frequency above most engine noise. They can hunt in traffic, as long as the fish are still there.

The grim arithmetic

The biggest threat to Norwegian porpoises is gillnet bycatch. Havforskningsinstituttet estimates that about 2,900 porpoises drown in Norwegian coastal nets every year on average, mostly in the cod and monkfish fisheries. For the inner Oslofjord’s small resident group, even one bad fishing season could do serious damage. The 2019 commercial cod-fishing ban from the Swedish border to Telemark (Kragerø) has reduced net pressure in the area, which is a quiet and under-reported piece of good news for the porpoises.

If you want to see one

From land, Drøbak harbour on a calm summer morning is your best free option. Look across the narrows with binoculars.

From the water, your odds go up massively. A sunset cruise in summer catches the late-afternoon feeding window and reaches both the Steilene basin and the Drøbaksundet approach; it is the tour that most often produces a porpoise sighting for us. A full-day cruise gives you enough range to wait through the dead midday and return to the evening window with coffee still warm.

No guarantees. Wildlife is wildlife. But the porpoises are here. Of the six seasons I have worked the Oslofjord, I have had porpoise sightings in five of them. The sixth was just unlucky timing.

One last thing. The porpoise is silent from your point of view — it surfaces, rolls, disappears. From its point of view the Oslofjord is not silent at all. It is full of 130 kHz clicks echoing off every boat hull, every mooring chain, every herring scale. The fjord you think is visual — islands, skylines, sunsets — is to a porpoise a mapped soundscape they have been reading for millions of years before any of us arrived.

Idle the engine inside Steilene around slack tide and listen. The exhale is a sharp puff. The inner fjord is small enough that the same dozen animals work the same patches week after week — Drøbaksundet, the Steilene basin, the channels east of Nesoddtangen.