Increasing scale and complexity of offshore wind farms (OWF) and on-going concern for European Protected Species (EPS) has led to interest in identifying alternative mitigation strategies to commonly used visual observation and acoustic detection methods for marine mammals. Acoustic Deterrent Devices (ADDs) have been identified as a potentially effective tool for a number of small cetacean species, but research has been lacking for some other key species that occur within prospective OWF sites.
A controlled exposure experiment (CEE) was designed and implemented to test the efficacy of the Lofitech ADD as a potential mitigation tool for the minke whale during piling operations. During August and September 2016, visual tracking of minke whales was undertaken in Faxaflói Bay, Iceland by a team of researchers on the R.V. Song of the Whale. When a focal animal had been identified and tracked for at least 30 minutes, the ADD was deployed at a distance of 1,000 m in order to expose the animal to the ADD signal. The behaviour of the focal animal was tracked during a control, treatment and post-treatment phase in order to understand the potential reactions to the ADD signal. Biological parameters such as inter-sequence interval, inter-blow interval, net swim speed, and measures of path predictability were recorded and later analysed to examine the behaviour in detail. In addition, the variable ‘away speed’ was derived in order to determine both the directionality and the longevity of any effect on the behaviour of the focal animal.
The ADD itself was fully characterised in the field. The unit deployed was found to have a source root mean square (rms) sound pressure level of 198 dB re 1 μPa re 1m, for a fundamental frequency of 14.6 kHz. The pulse length had an average of 752 ms. As noted in the literature, variability among individual units is expected.
A total of 46 minke whales were successfully tracked. Of these, 15 included successful deployments of the ADD. The focal animal moved away from the ADD deployment site in all cases. A significant increase in net swim speed during the treatment phase was observed, with whales increasing their speed by an average of 7.4 kmh-1. The away speed variable showed a significant increase in speed during the second half of the treatment phase, indicating that animals both increase their speed and the directness of their path in relation to exposure to the ADD signal.
The results highlight that the Lofitech ADD is effective at evoking a deterrence response in minke whales, suggesting that such devices could be effective at reducing any potential for injurious effects from exposure to subsea noise generated during pile-driving activity at OWF sites.