We recently came across this interesting article that highlights the significant effects of the use of acoustic tagging. The fact that the seals seem to have honed in on these tags so readily does lead to the question of how much detrimental effect tags will have on a shark’s predation success rate. This is especially relevant to the Great White shark whose diet largely consists of seals.
Food for thought…
Ecologists’ attempts to track shoals of fish are being thwarted by seals which have learnt to recognise the sounds of the shark acoustic tagging devices. Grey seals are skewing the results of scientific studies because they swoop in and eat fish fitted with acoustic tags.
British biologists have now realised that electric tags fitted to large fish act as a ‘dinner bell’ to seals. They think the predators have learned that the low electronic noises emitted by the devices will lead them to a substantial meal. It could have a ‘profound effect’ on research studies designed to monitor behaviour and movements of fish, the scientists admitted.
Projects monitoring the movements of salmon and lingcod have already suffered disproportionate losses of juveniles and researchers now believe the tags could have been acting as homing beacons for hungry seals. They fear further research may be compromised unless a different type of tag is used, one that seals cannot hear.
Researchers at St Andrew’s and Cumbria universities issued their warning after carrying out a series of tests that demonstrated that seals can learn to use the tags to pinpoint a meal. Amanda Stansbury, of the University of St Andrew’s, said: ‘Seals can exploit new sounds such as fish tags and use them to their advantage. If the predators go after the tag it would skew the results.’
SEALS REVISITED BOXES WITH THE ACOUSTIC TAG SIGNIFICANTLY MORE OFTEN THAN BOXES WHICH INITIALLY CONTAINED UNTAGGED FISH
Ten young grey seals, three males and seven females born on the Isle of May in the Firth of Forth were used in experiments to test whether they learnt to associate the ultrasound frequencies of the tags with food. The biologists measured the speed at which the seals found tagged herring hidden in boxes, compared to those with no tags. Their research paper, published today by the Royal Society, said: ‘Seals revisited boxes with the acoustic tag significantly more often than boxes which initially contained untagged fish. ‘Seals learnt the relevance of the acoustic cues and adjusted their foraging strategy to revisit profitable foraging spots.’
‘Our findings present a novel way of looking at anthropogenic noise that illustrates how animals exploit cues when they become available.’ Warning that the adaptation could be costly to studies using tags to monitor fish populations they wrote: ‘Acoustic fish tags are being used extensively in mark-recapture studies to assess fish survival. ‘Research agencies worldwide invest significant resources in acoustic tagging studies to assess fish stocks and determine survival rates. ‘As acoustic tags could make a fish more vulnerable to predation, tagging can lead to erroneous conclusions in such studies.
This concern is supported by observations of decreased survivorship rates for acoustically tagged juvenile salmon compared with those with similar tags that produce no sound signal.
‘Similarly, tagged predator species may experience a decrease in foraging success. Acoustic tags are becoming more widely used on sharks and could make them more detectable by prey species such as seals.’
You may also be interested in some of our other articles regarding diving, such as Great White shark cage diving in Gansbaai, as well as, Diving with Great White sharks in Hermanus.