Where a submarine cable runs along a seabed, or where an overhead line crosses an estuary or goes in a tunnel underneath the estuary, electric and magnetic fields impinge on fish and other marine life. This is becoming more common as we build more offshore windfarms and more interconnectors to other countries. In the drop-down boxes below, we explore in more detail what effects, if any, these EMFs may have on marine life.
What fields are produced?
A subsea cable can be either AC or DC. In each case, it produces a magnetic fields that is likewise either AC or DC. So that is the field that it produces directly in the sea: either an AC od DC magnetic field.
Subsea cables, and cables in a tunnel under an estuary, do not produce any external electric field because they are surrounded by a metal sheath that screens the electric field. So there is no direct electric field. But there is an indirect electric field, produced by induction in the sea water from the magnetic field. That happens in two ways. One is that, with both AC and DC, the flow of the sea water through the magnetic field induces an electric field. The other, for AC only, is that the alternation of the AC magnetic field likewise induces an electric field in the seawater. Then, as yet another possibility, an overhead line crossing water will also produce a direct electric field in the surface layers of the water.
This table sets out the possibilities:
|earth’s magnetic field|
|DC submarine cable|
|AC submarine cable or tunnel|
|AC overhead line crossing water|
How big are the fields?
Every cable is different! The geometry and load can vary enormously, which affect the field. We give some typical values for representative designs in the following table, but we stress that these must not be taken as definitive for any particular cable.
Do we expect fish to be sensitive to these fields?
We know that some fish (and other marine species) have developed sensitivity to either electric or magnetic fields.
Many elasmobranchs (sharks etc) have developed exquisite sensitivity to electric fields. They use it to detect prey. It's achieved by having evolved some extremely specialised organs - the ampullae of Lorenzini - that are optimised for detecting extremely low electric fields (a threshold sensitivity of 0.5 µV/m is reported - sometimes stated as being the equivalent of a 1.5 V cell connected across the width of the Atlantic).
Other species also have a sensitivity to electric fields, but without such highly developed organs, and therefore with poorer sensitivity. As well as detecting prey, it can also be an indirect way of detecting the earth's magnetic field (via the induced electric field from water flow) and hence used for navigation.
Other species have developed sensitivity to magnetic fields. They use it as one of the cues to navigating. It's achieved by particles of magnetic material called magnetite, often embedded in their skeletal structure.
So these species, which have a sensitivity to natural fields, must be considered at least potentially sensitive to fields from the power system as well. But there's one big proviso. The sensitivities that have developed, both to electric and magnetic fields, are to DC or static fields. We'd expect them to be much less sensitive to power-frequency AC fields (though how much less sensitive is quite difficult to predict).
Species that are reported to be sensitive to DC electric fields
Species that are reported to be sensitive to DC magnetic fields
What effects might the fields have?
If a fish is using electric fields to detect prey, and it picks up a spurious signal from a power cable, it could be distracted from hunting real prey, and potentially waste energy searching for non-existing prey.
But note that these species already have to cope with the electric field induced by the flow of sea water through the earth's magnetic field, and the field from the power cable is going to be perceived as quite similar - so we probably wouldn't expect the power cable to be a major problem for the fish.
If a fish is using magnetic fields to navigate, for example to return to its spawning grounds, and it picks up a spurious signal from a power cable, this could interfere with its navigation.
But note that several factors mitigate any effects. The magnetic field is just one of the cues the fish uses, and if something goes wrong with the magnetic field, it is quite likely to make more use of other cues to compensate. The perturbation to the earth's field will be transitory only - in the immediate vicinity of the cable - which is only a small fraction of their overall journey. And once they're embarked on swiming up an estuary or river, they don't really need the navigation clues from the magnetic field - they just keep swimming - so river and estuary crossings are unlikely to affect these fish much anyway.
What is the evidence in practice?
What do the experts say?
None of the authoritative scientific review bodies that we normally turn to when we're thinking about EMFs and human health have looked at fish. But there are a couple of quite detailed reviews:
EFFECTS OF EMFS FROM UNDERSEA POWER CABLES ON ELASMOBRANCHS AND OTHER MARINE SPECIES
U.S. Department of the Interior
Bureau of Ocean Energy Management, Regulation and Enforcement
Pacific OCS Region
Scottish Natural Heritage
Commissioned Report No. 401
Both reports comment on the lack of extensive evidence - there are relatively few studies reported, and it is difficult to draw firm conclusions.
The former concludes (in part):
Behavioral responses to electro- or magnetic fields are known for some species but extrapolation to impacts resulting from exposure to undersea power cables is speculative.
The latter concludes (in part):
Current knowledge suggests that EMFs from subsea cables and cabling orientation may interact with migrating eels (and possibly salmonids) if their migration or movement routes take them over the cables, particularly in shallow waters (<20m). The effect, if any, could be a relatively trivial temporary change in swimming direction, or potentially a more serious avoidance response or delay to migration. Whether this will represent a biologically significant effect cannot yet be determined.
National Policy Statement
National Policy Statement EN-3 relates to renewable energy, and has a section which deals with offshore wind. This states:
Mitigation2.6.76 EMF during operation may be mitigated by use of armoured cable for inter-array and export cables which should be buried at a sufficient depth. Some research has shown that where cables are buried at depths greater than 1.5m below the sea bed impacts are likely to be negligible . However sufficient depth to mitigate impacts will depend on the geology of the sea bed.
2.6.82 If it is proposed to install offshore cables to a depth of at least 1.5m below the sea bed, the applicant should not have to assess the effect of the cables on intertidal habitat during the operational phase of the offshore wind farm.
There is some scientific basis for expecting that some marine species might be sensitive to EMFs produced by electricity cables. But no-one really knows exactly what species or exactly what effects the sensitivity might produce. There is no good evidence of existing subsea cables having any dramatic effect on marine life in their vicinity, and, whilst there are reports of relatively minor effects, part of the general uncertainty is that we don't really know exactly what these effects are attributable to.
So we conclude that there probably are effects but, in as much as we can tell on the limited evidence available, they are probably minor effects.
This is, incidentally, a contrast to the situation with human health. With human health, the consensus scientific view is that there probably aren't health effects. But if there are, no-one disputes that the effect in question - childhood leukaemia - would be serious. With marine species, it's the other way round - there probably are some effects, but they are probably quite minor.