For many years the marine environment was thought to be a quiet place, in fact, the famous marine explorer Jacques Cousteau even released a documentary film ‘The Silent World’ in 1956 depicting the ocean as just that. Inquisitive scientists have since unveiled the truth: the ocean is a boisterous place, with a wide range of species producing sound for a range of ecologically important activities. In addition to the naturally occurring sounds in the environment, there are sounds humans create, such as boat noise. Along with a growing number of soniferous (sound-producing) species being identified since Cousteau’s pioneering documentary was first aired, anthropogenic sounds have also been increasing throughout the world’s oceans. You can check out different types of naturally occurring and anthropogenic sounds here.
Here in Aotearoa, we are proud advocates of our marine protected areas that allow nature to thrive naturally, we enjoy going out for a snorkel and seeing what wonders protecting an area can achieve. However, in marine reserves, recreational boats can be abundant due to increased tourism and scientific research and this may significantly impact the natural soundscape. Using hydrophones (underwater recorders) we were able to listen in on the Goat Island Marine Reserve soundscape and compare it with the soundscape just outside the boundary lines. By comparing these different sites, we aimed to determine how boat noise alters the soundscapes within the marine reserve and implications boat noise may have on species residing within it.
We found many different biological sounds in our recordings, with numerous fish species, kina (sea urchins), snapping shrimp, whales, and dolphins all being present at some time during the study. Along with these were some sounds that weren’t able to be identified and which could be new to science! All of the species vocalising had their own acoustic niche, meaning that no two species will produce the same unique sound simultaneously. This indicates a healthy environment as fewer acoustic gaps and clearly partitioned acoustic niches indicate a healthy ecosystem, where species are able to identify and interpret their own sounds. We focused on the New Zealand two spot demoiselle vocalisations to represent how biological sounds in the soundscape can be impacted by boat noise. The number of clicks produced by the two spot demoiselle decreases at dawn and increases at dusk, with the highest number of clicks occurring during the night – these guys are real party animals! The biological function of these clicks has not yet been determined, however, the schooling nature and temporal sound patterns they produce are similar to the New Zealand bigeye, which uses sound for social cohesion. The similarities between these species suggest that like the New Zealand big eye, the two spot demoiselle produce sound for social cohesion.
We also found, not surprisingly, that when boat noise enters the soundscape, ambient sound levels increased significantly. Spectograms allow visual representation of the spectrum of frequencies of a signal as it varies with time. With the use of spectrograms the impact of boat noise on the ambient soundscape was able to be determined most significant in the lower frequency ranges, 10 – 2,500 Hz. With two spot demoiselle clicks occurring at 600 – 2,500 Hz, masking of their communication is likely to occur as a consequence of boat noise. To put masking into context, it would be the same as you trying to have a conversation with a car alarm going off next to you, making it impossible to hear the person you’re conversing with.
When we compared boat noise occurrence inside and outside the marine reserve, we found that Goat Island Marine Reserve does provide some protection from boat noise. Listening stations at the centre of the reserve detected 15% less boat noise than the listening station furthest outside the boundary line. During the popular Leigh fishing competition, listening stations outside the reserve jumped by 5 – 15 dB over natural sound levels. In the centre of the marine reserve, however, there was no change to natural sound levels during this time. With a large increase in sound levels, naturally occurring sounds have a greater chance of being masked. Inside the reserve, however, there appeared to be some refuge from this.
The ocean is a vast area that covers 72% of the earth’s surface. Protection of the ocean through marine protected areas has proven to have many benefits for the environment as well as fisheries. Despite this, there is a lack of implementation of marine reserves, with only 1.89% of the ocean being exclusively no-take zones. An area classed as a no-take marine protected area prevents many activities that contribute to anthropogenic sound including mining, fishing, and drilling. This study suggests that no-take marine protected areas do provide species and habitats with some protection against boat noise, and therefore, mitigates masking of biological sounds. With marine reserves providing refuge for marine species that rely on sound for communication, policymakers must be provided with quantitative targets to further ensure the protection of species residing within them. Notably, these targets will differ based on the objectives of individual marine protected areas. In addition to providing these targets, assessment of the effects of anthrophony (human-produced sound) should be included as a standard part of any the assessment of values of marine areas and in marine reserve design and environmental plans to ultimately ensure minimal human impacts on these ecologically rich areas.
Kendall Leitch is an Environmental Scientist at Streamlined Environmental and completed her MSc on marine acoustics at the University of Auckland in 2019.