The ocean is a noisy place. Many marine species make sounds to communicate, hunt, and navigate within their three-dimensional world, making our seas surprisingly acoustically diverse. For example, herring maintain the cohesion of large shoals by releasing air (aptly named ‘Fast Repetitive Ticks’ or FRTs) which make loud and distinct popping noises. Crustaceans will communicate through rasping and snapping sounds by vibrating or rubbing parts of their hard exteriors together. Coral reefs hum with calls and noise from hundreds of species of fish, very like the dawn chorus you can expect to hear in a mature woodland. Some of the ocean’s most well-known vocalists, whales and dolphins, produce clicks, squeals, grunts, moans and even form lengthy and complex songs to reach each other across ocean basins.
The noises that animals make are known as bioacoustics, and in the seas, these calls combine with natural and manmade sources to form an overall ‘soundscape’, the cacophony of sounds that can be found in a particular location. Soundscapes can change over time; indeed, our oceans are far noisier since the Industrial Revolution and the invention of the diesel engine. Ship noise contributes heavily to soundscapes across the world, and it is thought the effective communication range for many marine species has reduced as a result. This would be like trying to hear your friend talking in a busy pub, and similarly as we may do in such a situation, many animals have increased the volume and characteristics of their calls to compensate.
The soundscape can also be used as a measure of biodiversity. Typically, more complex soundscapes represent a broader range of vocalising species, which may indicate how biodiverse and healthy an area might be. The Community Association of Lochs and Sounds (CAOLAS) are using this fascinating metric to study the effect of re-introduced native oysters in Loch Aline, Morvern. Research in other locations suggests that large, well-established oyster reefs have a complex soundscape, representing the greater numbers of fish and crustacean species that find shelter within and upon the interlocking shell structures. Using underwater microphones known as hydrophones, CAOLAS are recording the soundscapes at the oyster re-introduction site, as well as at other locations to enable a comparison over time.
The re-introduced oyster bed is in its early stages, with the first individuals released in 2021. Reef formation is beginning as the oysters start to reproduce on their own, and it is expected this will improve local biodiversity as more habitat is available for young fish and other species. Some research also suggests that young free-swimming oyster larvae are more attracted to complex and noisy reef-like soundscapes, eventually settling where adults can be found in favourable conditions. To encourage the settlement of oyster larvae at the re-introduction site, CAOLAS is investigating whether recorded oyster reef underwater playbacks could help guide them towards the existing oyster bed. CAOLAS may find themselves holding their very own oyster-only music festival in the coming years- no noise complaints please!