The use of acoustic devices to warn marine mammals of tidal-stream energy devices

Summary

Given the current pace, diversity and magnitude of tidal turbine developments in Scotland, there is a place for forward thinking about our options to address a mammal-turbine collision problem should one prove to exist within the deploy and monitor strategy. Though there are currently no commercial sonic devices on the market specifically designed for this context, an existing Acoustic Harassment Device (Ace Aquatec MMD, Table 2) is already being marketed with this as a potential application. Furthermore, there are a wide variety of different devices developed, tested and applied for other applications with the intention of changing the behaviour of marine mammals and fish, particularly in aquaculture or fisheries contexts.

It is clear, however, from the current study that tidal-stream sites present specific differences to other marine habitats particularly in terms of the high (temporally and spatially) variable levels of background underwater noise that they experience. Off the shelf acoustic warning devices are therefore unlikely to function as they might elsewhere. If acoustic warning is deemed to be an appropriate mitigation tactic worth exploring, there are many options available for tuning existing sonic equipment or designing something from scratch. These options range from simply providing animals with a heads-up warning that they are approaching an obstacle, to providing enough information to allow them to successfully manoeuvre around the operating structure, to encouraging them to avoid the footprint of a device or even an entire array. Within this diversity of options, this report outlines seven key attributes that should be considered as requirements in the design envelopes of any warning device(s).

As this topic moves forward, and as with fisheries and aquaculture related acoustic device application, it is likely that several companies will progress ideas in parallel. Inevitably they are likely to pick different combinations of sound characteristics for their products. However, if we want marine mammals to also learn from any near-miss experiences, then some level of stimulus standardisation is required. Administrative organisations with appropriate oversight will need to take this lead.

Ultimately, and in the face of a potentially attractive mitigation option, it must not be forgotten that any active acoustic warning also represents a new source of sound pollution that is specifically intended to alter the behaviour of marine mammals. Given that we do not yet know whether there is actually a real (rather than perceived) mammal-turbine collision problem, we should consider carefully whether or not it is appropriate to deliberately add extra-noise to the sea simply as a precautionary measure. Nevertheless, while further information on the collision issue is likely to emerge as turbines are deployed over the next few years, the acoustic warning option should continue to be explored as it would be immediately needed if a problem becomes apparent.