How the technology works

CONVENTIONAL high-power acoustic deterrent or harassment devices (ADDs and AHDs) are widely used to keep seals away from fish farms, writes Dr Thomas Götz.

These devices often aim to induce pain or scare target animals by emitting sound at very high source levels and duty cycles (that is, the time that sound is produced)

The long-term success of these devices is often limited and habituation, a decrease in responsiveness to the signal, can be a significant problem in contexts where food motivation is involved2.

ADDs have also been highlighted as a conservation concern as they cause large-scale habitat exclusion in non-target species, such as harbour porpoise and dolphins.

They also have some potential to cause hearing damage in target and non-target species, particularly when noise pollution is almost continuous because farmers use many devices1.

An alternative can be found by harnessing an autonomous reflex arc in the brainstem, the acoustic startle reflex, which has been found to cause flight and avoidance behaviour without a decrease in responsiveness over time in the majority of tested seals3.

This approach only requires low noise doses by using brief, isolated sound signals emitted at low duty cycles.

Target specificity can be achieved by choosing a frequency band where hearing sensitivity of seals is higher than in non-target species, such as harbour porpoise4.

The method has empirically been shown to be successful in keeping seals away from a fish farm while not adversely affecting the behaviour and distribution of harbour porpoise4.

In a consecutive study, a startle reflex based system reduced seal predation by ~91-97 per cent on a fish farm in Argyle over the course of one year, a result that was also confirmed in a second, shorter term test on a farm in the same area5.

As the device emits much lower noise doses than ADDs in a frequency band where porpoise have lower auditory sensitivity, there is no risk of hearing damage in target and non-target species4.