The seabed here is mud and sand, flat in every direction for miles.
Then four steel legs come down through it, each one wider than a car.
They arrived cold and bare, painted and welded, with nothing living on them at all.
Now the same legs carry a crust of barnacles, mussels and anemones thick enough to change their outline.
Fish that have no business on hard ground gather around them anyway.
So what is pulling them in, when everything they normally live on is the sand?
What a steel leg looks like to a fish that lives on sand
Bare steel underwater does not stay bare.
A film of bacteria goes down first, then hydroids, barnacles and mussels build over it, and within a couple of seasons the surface is a thicket.
That thicket is not food for a haddock.
What lives inside it is: amphipods and worms and small crustaceans in densities the open sand never reaches.
The fish are not moving onto the structure.
They are feeding at the edge of it and going back to the bottom they prefer.
These particular foundations help by accident.
They were installed without any rock armor around the base, so the sediment right up against the legs stays soft and workable.
A flatfish can lie in its own habitat and still be inside the feeding halo.
The structure standing in up to 200 feet of water
The foundations in question are jackets, four legged lattice towers of welded tube rather than a single driven pile.
Each one puts four separate columns onto the seabed instead of one, which multiplies the colonized surface and the number of sheltered corners at the bottom.
They stand in water between roughly 115 and 200 feet deep, carrying 84 turbines of seven megawatts apiece.
The legs are open to the current on every side.
Sediment scours out slightly at the base and settles again, and the crust keeps thickening on the steel above it.
None of that was the point of building them.
It is what happens anyway when you leave several thousand tons of colonizable surface standing in a productive sea.
What the cameras recorded and where they sat
Baited camera frames were lowered at four distances from each of 24 turbine foundations: about 100 feet, 200 feet, 390 feet and 790 feet.
That is 96 deployments in all, run across ten days in July of 2024.
The cameras logged 2,626 individual fish.
Flatfish made up 64 percent of them and haddock another 30 percent, so 94 percent of everything seen was one of two groups.
Abundance and biomass both fell as the cameras moved away from the steel.
At the closest station there were roughly 1.5 times as many fish and 1.6 times as much biomass as at the farthest.
Haddock did not fall off in a straight line the way the flatfish did.
Their numbers dropped steeply inside about 200 feet and then flattened, which is the signature of a tight halo rather than a broad gradient.
What the numbers do not say
Body length was measured too, and it did not move.
Neither haddock nor flatfish were any longer near the legs than far from them, which matters because more fish in one place is not the same as more fish overall.
A structure can concentrate animals that were already there and produce nothing new, and counting them at close range cannot tell the two apart.
The authors of the new paper say so directly, and they fence the result in further.
One wind farm, one summer, one year.
Only jacket foundations, so nothing here transfers to a monopile field without testing it.
The farthest camera sat 790 feet out, which is still well inside the array.
Whether the effect holds through winter, or across the wider North Sea build, is simply not in the data.
What it means for the farms still being planned
A wider comparison from the same team is more encouraging.
Earlier work at this farm and a neighboring one measured fish within 165 feet of the jackets against reference sites more than 1,600 feet away.
Abundance and biomass were higher at the foundations, and haddock there ran half an inch longer, sometimes closer to three quarters.
So at the scale of the whole farm the fish were bigger, while inside the farm they were not.
That is a real finding and an unfinished one.
It matters because the decisions being made now are structural: jacket or monopile, rock armor or bare seabed, and whether a decommissioning rule that requires clean removal is steadily deleting marine habitat it was written to protect.
Those choices get locked in for decades on the day the steel goes down.
The fish, meanwhile, have already decided where they want to be.
They are within a hundred feet of it.
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