Karen Boswarva looks into the invisible electromagnetic fields of energy that animals and humans use to navigate the sea
OCEANSCIENCE
Natural magnetism
Karen Boswarva looks into the invisible electromagnetic fields of energy that animals and humans use to navigate the sea
As divers, we teach or were taught how to use a compass. I’ ll never forget the dry skills exercises, often in a car park, in the rain. Hood placed on backwards, a few spins for extra disorientation, before setting off towards your goal. We practise this skill as it’ s integral to finding our way around the underwater landscape, but how often do we consider how it actually works?
We know the key component, the compass needle, acts like a rotating magnet by aligning itself with the Earth’ s electromagnetic field( EMF). We also know that if you get too close to objects that emit their own EMFs( like the metal on wrecks) then Earth’ s EMF is disrupted, sending us round and round in circles.
In fact, EMFs are all around us, as physical fields produced by electrically charged objects, a combination of both electric and magnetic forces that vary over space and time. They can be naturally occurring( lightening) or come from man-made sources such as power cables and mobile phones.
Earth’ s EMF, also called the geomagnetic field, extends from the Earth’ s interior out into space. Generated by electric currents in the molten core, it protects the planet from harmful solar radiation. Acting like a giant shield, it deflects charged particles carried by solar winds. Natural phenomena such as the Northern Lights occur when charged particles from the sun deflect off the Earth’ s magnetic field and react with gases in our atmosphere.
EMF is measured in units of Tesla. Earth’ s EMF ranges from 25 to 70 µ T( microtesla), with a variation of 0.002- 0.005 µ T / km.
Some animals are capable of detecting and utilising Earth’ s EMF. They use it for orientation, navigation, and migration. This includes many marine animals such as fish, sharks, marine mammals, crustaceans and sea turtles. These animals are called magneto-sensitive or electrosensitive, as the levels of EMF they can detect are believed to be in the nano tesla range.
All elasmobranchs( sharks, skates, and rays) possess an additional sense called electroreception. Using specialised sensory organs called ampullae of Lorenzini located around their heads, they detect the smallest electric fields emitted by animals. Other marine animals such as loggerhead turtles and sockeye salmon use Earth’ s EMF to navigate the vast ocean and find their way back to natal nesting beaches and spawning rivers.
Our understanding of how marine species utilise Earth’ s EMF is developing as we learn more about how different species react to changes in EMFs. Spurred on by the recent
“ All elasmobranchs( sharks, skates, and rays) possess an additional sense”
advancement in subsea infrastructure, researchers believe that EMF from power cables can cause local changes in the electric and magnetic environment, which may influence the natural behaviours of magnetosensitive marine animals at various life stages.
EMFs consist of an electric( E-Field), a magnetic( B-Field) and an induced electric field( iE-Field). Water displacement from currents and animal movement generates the iE-field. In power cables, the E-field is contained in the cable’ s armouring, while the B-field is emitted into the environment.
Tests conducted in a laboratory and in-situ have shown that some marine animals display no behavioural changes to increases in the B-Field surrounding power cables. By contrast, other animals exhibit increased attraction or disturbance depending on on their stage of development, exposure level( burial depth and distance from the source) and duration of exposure. These findings are important for mitigating against the effects that human pressures place on the environment, but they also provide us with a greater understanding of the diversity of animals that make use of these incredible forces of nature. �
Sharks are thought to use their electroreceptors both to hone in on prey at close range and to navigate by electromagnetic fields in the sea
PHOTO: SIMON ROGERSON
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