Piles of concrete pipe were dropped around 1 turbine foundation off the Dutch coast and tagged cod moved in and stayed, where the flat swept sand around them offers nothing to hide inside at all

The southern North Sea floor is not a landscape. It is a surface.

Sand, gravel and shell, graded flat and kept that way, because a century of bottom trawling removed anything that stood up from it.

The shellfish reefs that once broke that plain are gone.

Cod are not a fish of open sand. They are a fish of cracks, ledges and holes, and they use structure to ambush from and to shelter in.

Which leaves a large predator living over ground that offers it no cover at all.

So the limiting resource out there is not food. It is somewhere to be.

What a hole of the right size is worth

Drop any hard object on that seabed and two things happen in sequence.

First it is colonized. Barnacles, hydroids, anemones and mussels settle within months, and with them come small crustaceans and the fish that eat those.

Second, and this is the part that matters for cod, the object creates interior space.

A boulder makes a shadow. A pipe makes a room, with a diameter that can be chosen to suit an animal of a particular size, an entrance, an exit and a roof.

That is why pipe was used rather than simply more rock. Rock adds surface for food to grow on, and pipe adds refuge.

The cod in this experiment spent a large share of their time not swimming around the structures but inside them.

Four heaps around a single monopile

The site is a wind farm off Zeeland, on the southwestern Dutch coast, and the experiment centered on one turbine.

Four artificial reefs were placed around that foundation, each a heap of concrete pipes in a range of diameters.

Two of the four sat on granite rock scour protection of the kind already laid around turbine bases. The other two did not, which made the layout a deliberate comparison rather than a demonstration.

Mixed diameters are part of the design rather than an accident of supply, because a pipe wide enough to shelter a large cod is no use to a small one and the other way round.

Standing among the heaps are sixteen sets of listening gear, each bolted to a custom frame on a stainless steel tripod about five feet tall so the sand does not swallow the signal.

Packing that many close together is what gives fine scale position rather than mere presence, so a fish can be placed inside or outside a given pipe.

The whole arrangement is small. It would fit comfortably inside a city block.

Sixty four fish and eighteen months of pings

Cod were caught, fitted with acoustic transmitters and released at the site in two batches, 45 in July of 2021 and another 19 in May of 2022.

The tags carried pressure and acceleration sensors as well, so depth and activity came back alongside position.

Tracking ran to January of 2023, about eighteen months end to end, and the results were published in the summer of 2024.

Site fidelity and residency both came out high. Fish that found a reef tended to stay with it and to return to it.

There was no meaningful difference between the reefs with rock scour beds and the ones without, which suggests the pipes were doing the work.

And only a handful of individuals stayed through winter, which is the finding that gets left out most often.

Attraction is not the same as production

This is the oldest argument about artificial reefs and it is not settled by this study.

A structure that concentrates fish has either created new ones or gathered up existing ones from a wide area, and telemetry cannot separate those two.

Concentration on its own can make a population easier to catch rather than larger, which is why reefs and fishing closures usually go together.

The second batch of fish also only had about eight months of tracking rather than eighteen, so the headline period does not apply to all 64, in the way that a seeded stone bed or a tagged marine predator gets reported as one clean number.

The paper itself frames the result as strong attraction rather than proven benefit, describing site fidelity and residency and stopping carefully short.

What happens when the turbine comes down

The practical question sits at the other end of the project life.

Offshore wind consents in the North Sea generally require full removal of everything installed, including the scour rock, which was written to protect the seabed from industrial leftovers.

Twenty five years of colonization turns that clause into an instruction to demolish a reef that the surrounding fishery has come to depend on.

Deliberately adding refuge structures makes the conflict sharper rather than softer, because it makes the habitat intentional, as the project’s own writeup acknowledges.

The concrete pipe was never about the concrete.

It was about the hole in it.