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The larvae used in the study came from haddock that were caught wild near IMR's research station in Austevoll, where the experiment was conducted.

Magnetic fields from subsea cables can slow down haddock larvae

Haddock larvae swam more slowly after being exposed to magnetic fields similar to those created by undersea cables connecting offshore wind farms in the laboratory.

Communications Adviser
Presented by: Institute of Marine Research
Published

Offshore wind farms and other marine renewable energy facilities are connected by submarine high-voltage cables. These cables produce magnetic fields that can affect marine organisms that swim or drift near the cables – particularly fish larvae that have less ability to swim away from the cables.

These possible impacts could be particularly relevant for fish larvae, which have limited capacity to swim away from the cables. However, whether there is an effect on fish larvae is currently unknown. Ongoing research at the Institute of Marine Research (IMR) is beginning to fill this knowledge gap.

In their most recent study, a research team at IMR focused on impacts on haddock – an ecologically and commercially important species that is also a popular choice for fish and chips.

Reduced swimming activity

Postdoctoral fellow Alessandro Cresci at IMR.

“We wanted to understand whether haddock larvae, which will drift in areas where offshore wind turbines are planned, are affected by magnetic fields in the intensity range of those generated by the subsea cables that connect the turbines,” says Alessandro Cresci, a postdoctoral fellow at IMR.

The researchers set up an experiment that simulated a scenario where fish larvae swam or drifted past an undersea cable. The behaviour of the larvae was filmed and analysed.

“We found no evidence of attraction to or repulsion from the magnetic field. However, the exposure reduced by more than half the swimming activity of the great majority of the haddock larvae, and substantially reduced their acceleration,” Cresci says. “This reduction in swimming activity could affect the dispersal and survival of haddock larvae in the wild.”

Swims according to its own compass

In earlier research, Cresci and colleagues found that haddock larvae don’t just drift passively with the current; rather, they swim in a north-westerly direction, following an internal magnetic 'compass'.

This directional swimming plays a role in determining where the newly hatched larvae end up after spawning. Therefore, if their swimming activity is reduced as the larvae drift past subsea cables, this could impact their dispersal and lower their chances to survive.

“For this reason, we wanted to understand whether haddock larvae, which will drift in areas where offshore wind turbines are planned, are affected by magnetic fields in the intensity range of those generated by the subsea cables connecting the turbines,” Cresci says.

Realistic simulation in a test tank

The experiment was carried out in a special tank built for the purpose, with electric coils that could induce magnetic fields in one half of the tank. The intensity of the magnetic field was the same as from ordinary subsea cables.

The set-up thus gave a realistic imitation of what larvae will experience when they swim or drift near an undersea high-voltage cable in the wild.

“We placed the larvae one at a time in the middle of the raceway and observed their behaviour for 15 minutes. Half of the larvae could freely swim between the high and low magnetic field zones,” Cresci explains.

Half of the larvae were exposed to the magnetic field; the rest were observed under similar conditions, but with the electric coils switched off, so that they were only exposed to the natural magnetic field of the earth. These served as controls in the experiment.

The experiment was conducted in the dark, with no visual cues that could influence the behavior of the larvae. The behaviour of the larvae was recorded using an infrared-sensitive camera.

A: Experimental setup used to expose Atlantic haddock larvae to a static magnetic field (B-field) gradient. The black squares are a pair of parallel Helmholtz coils. The two grey rectangles with smoothed corners are two raceways in which larvae were swimming. Black dashed lines show the two sides of the raceway. Light and dark grey dashed lines show the intensity of the B-field on each side of the raceway. In the Control group (coils OFF), there was an ambient geomagnetic field in both sides of the raceway. In the Exposed group (coils ON), there was higher B-field intensity on Side 1, and lower intensity (close to the geomagnetic field intensity) on Side 2. B: B-field intensity along the raceway (x axis) with coils ON and coils OFF. In the Control group, the geomagnetic field had the same value along the whole raceway (50 μT). In the Exposed group, the B-field intensity had a gradient going from 150 μT on Side 1, decreasing towards the end of Side 2, to settle at approximately 50 μT at the right end of Half 2. Haddock larvae were free to swim along the whole raceway during the experiment.

Clear differences

The haddock larvae showed clear differences in exploratory behavior: A minority (20 per cent) of the larvae were highly exploratory and swam back and forth in the raceway tank. Most (80 per cent) of the larvae were non-exploratory and kept to one side of the raceway tank. This pattern of individual differences in behavior has been observed in several species of fish.

The effect was different between the two groups:

The exposure to magnetic fields in the range of those produced by high voltage subsea cables reduced the swimming speed of non-exploratory larvae by more than half (60 per cent) compared to the controls. Their acceleration was also strongly reduced (by 38 per cent).

There was no such effect on the exploratory group of fish.

Unique and important findings

This experiment on haddock larvae is part of a larger research project on the impacts of offshore wind farms on fish larvae. The project is assessing the impacts of operational noise from wind farm turbines and magnetic fields from subsea cables on fish larvae.

In a similar study published earlier this year, the IMR team found no significant effect of magnetic fields on larvae of lesser sandeel (Ammodytes marinus), a key species in the North Sea ecosystem. The researchers plan to do an analogous experiment on Atlantic cod larvae.

In addition, they have carried out unique research on the impacts of low-frequency sound in the intensity range of those produced by the turbines on marine fish larvae.

“The expansion of the offshore wind industry is underway, and it’s happening fast. We have made significant advances in the understanding of the possible impacts this might have on early-life stages of fish,” Cresci says.

“Our results provide some of the first evidence of the possible ecological future impacts of this expansion. This type of research should be taken into consideration when evaluating the possible implications of this further industrialisation of the sea for marine life.”

Reference:

Cresci et al. 'Magnetic fields produced by subsea high voltage DC cables reduce swimming activity of haddock larvae (Melanogrammus aeglefinus)', PNAS Nexus, 2022. DOI: 10.1093/pnasnexus/pgac175 Abstract.

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