The aim of this project was to quantify the effects of boat noise, like those used by the tourism industry, on soundscape and dolphin and fish communication. The parameters of the project were changed due to COVID (see Endnote) making it uniquely possible to observe the impact of the reduction of boat noise on cetacean activity.
This collaborative scientific research project was run in conjunction with University of Victoria in Canada, University of Auckland, and Herriot-Watt University in Edinburgh.
Background on the Hauraki Gulf
The biodiversity of this marine park is regarded as internationally outstanding and is consequently protected through its own legislation, the Hauraki Gulf Marine Park Act 2000 (Pine et al. 2016). At least 25 species of marine mammals visit the Hauraki Gulf (nearly a third of all marine mammal species worldwide) throughout the year of which 6 are considered to be resident (Hauraki Gulf Forum 2011, 2014).
Noise pollution from vessel traffic has the potential to substantially degrade cetacean habitats around the Hauraki Gulf Marine Park (among other factors). Cetaceans (whales, dolphins and porpoises) depend on underwater sound for activities that are critical to their survival. Those activities include, but are not limited to, ensuring group cohesion, maintaining communication between individuals, locating prey, and predator/hazard avoidance. Their ability to communicate and sense their environment using underwater sound is highly reliant on the ambient acoustic environment, whereby the biologically-important signals must be audible over the background ambient sound level.
Vessel traffic in the Hauraki Gulf Marine Park is capable of causing ambient sound levels to rise to the point where communication between marine mammals may be masked creating the potential for deleterious impacts on marine mammals. Approximately 1,400 ships per annum transit the Gulf to gain access to the country’s largest port (Constantine et al. 2015), as well as 132,000 recreational boats being owned by Auckland’s residents (Beca, 2012).
In addition to these two types of marine traffic, a variety of other vessel-types operate daily, from charter fishing vessels and ferries to cruise liners, harbour cruise boats, tugs and barges, trail-suction hopper dredgers, coastal bunkers, as well as marine mammal tourism vessels.
Vessel noise is highly variable and can be of high intensity, with the capacity to impact cetaceans primarily through elevating ambient sound levels and thereby inducing auditory masking which is the most pervasive impact of vessel noise (McWhinnie et al., 2017) and capable to extending over large areas of the Hauraki Gulf for noisy vessels (Pine et al. 2016; Putland et al. 2018). The noise from most motorised vessels overlaps with the frequency range of cetaceans and can frequently propagate over several kilometres (Pine et al. 2016; Halliday et al. 2017; Pine et al. 2018).
Passive Acoustic Monitoring and Noise Modelling
Our aim was to get passive acoustic monitoring data over 2 years to study vessel traffic volumes and marine mammal detections near Auckland City (where only sighting data exists and was sparse – i.e. only collected during daytime, or when people were on the water).
Five passive acoustic monitoring sites were set up in the inner Gulf – inside the Rangitoto Channel, off Auckland’s North Shore, Shearer’s Rock near Tiritiri Matangi Island, the middle of the Hauraki Gulf and the Aahaha Rocks near the Noises Islands.
The monitoring sites, each containing a SoundTrap (the deeper water at the Noises and Mid-Gulf sites also had an acoustic release for retrieval), recorded the soundscape continuously. This meant that vessel activity and counts of vessel transits through the Rangitoto Channel could be quantified which could not be done if recorders were running on duty cycles.
Noise modelling would be done on AIS data of ships and tourism vessels to get cumulative sound levels over each month. Modelling would also be used to model communication ranges of dolphins when exposed to vessel noise compared to times without vessel noise present.
The arrival of COVID
When COVID arrived in 2020, NZ was one of the first countries to enter into strict L4 lockdown. This lockdown extended to the oceans, meaning non-essential vessels were banned from operating while shipping was dramatically reduced.
This allowed us to study the cumulative effects of small boat traffic on shallow water soundscapes near Auckland City in a very unique way – through the collection of novel baseline data. It also illustrated the power of long-term Passive Acoustic Monitoring (PAM) because it was the only form of monitoring that was able to happen during the total ban. Other survey and monitoring methods could not happen, and acoustics was the only type of data that could be used to study lockdown effects.
The PAM programme showed that the immediacy and severity of the lockdown restrictions had a dramatic effect on the marine soundscape, with ambient sound levels dropping as much as 8 dB re 1 μPa over the first 12 hours.
On the first day of lockdown (26 March 2020), the presence of vessel noise in the Rangitoto Channel dropped to 34%, down from 63% the day before (25 March). By the 1st of April, daily vessel presence had dropped to 8% within the Rangitoto Channel, capturing only port-related vessels passing through.
Out near the Noises Islands, vessel activity dropped even further, with no vessels being detected some days. The ambient soundscape and broadband noise levels were the lowest ever recorded in New Zealand, reaching just 102 dBrms re 1 μPa (over a whole day) inside the Rangitoto Channel – levels below the maximum levels recorded during the remote Arctic summer (these range between 73 and 103 dBrms re 1 μPa (Insley et al. 2017)).
With dolphins (and some fishes) heavily relying on their vocalisations and hearing, the sudden drop in ambient sound levels led to them experiencing an immediate increase in estimated communication ranges by up to 65%.
Dolphins were detected throughout the weeks leading up to and during the lockdown period. A hotspot for dolphins (bottlenose and common dolphins) is near the Noises Islands, and changes in detection durations before and during lockdown were most noticeable there than at other sites. For example, in the 31 days before lock-down, dolphins were acoustically present off the Noises Islands for 1341 minutes compared to 2031 minutes in the first 31 days of lockdown. Notwithstanding, dolphins were detected for longer durations at all monitoring sites during lockdown than before it.
The key value in the data collected during 2020 is that it provided hard evidence for the impact of small vessel noise on underwater soundscapes and how marine animals will have to adapt to ever-growing noise pollution.
Statistical testing of the data collected before, during and after lockdown, has revealed relationships between the vessel activity and the ambient noise levels. For example, generalised linear models revealed 2 dB increases in daily sound pressure levels for every 10% increase in vessel presence over 24 hour periods – a measured cumulative effect that vessel noise has on the soundscape. These relationships were not obtainable before lockdown.
These data empirically demonstrate that small vessels, when in sufficient numbers, directly influence ambient noise levels and are not an acute noise source with limited impact as sometimes believed by regulators. Data from during the lockdown has shown that the current volume of small vessel traffic operating within the HGMP is enough to raise the median daily ambient sound levels by least 6 dBrms re 1 μPa in habitats at least 45km from downtown Auckland. Closer to the city, the contribution of vessel traffic to ambient sound levels is even higher – approximately 8 dBrms re 1 μPa within the Rangitoto channel. These data are unprecedented in showing how small vessels have already contributed the ambient sound levels within the HGMP, providing statistically strong relationships that can be used to better predict future noise levels from a recreational vessel fleet that is expected to reach 183,000 by 2041 – meaning an approximate 3 dBrms re 1 μPa increase to existing daily SPLs could be expected.
Recording of High Speed, Foiling Vessels
Endnote: The impact of COVID on the project
The COVID pandemic enhanced the power and conclusions of this project. COVID lockdowns meant we could really investigate how small vessel noise changes a soundscape without theory but through observation. We were therefore also able to study rather than only model the impact of small vessel noise on the acoustic wellbeing of the Gulf’s cetaceans.
COVID and the lockdowns provided unprecedented novel data that allowed us to quantify the effects of boat noise, like used by the tourism industry, on soundscape and dolphin and fish communication. This resulted in us publishing these results and presenting these findings in a series of media stories, radio interviews and podcasts, as well as Matt Pine becoming a scientific consultant on BBC’s The Year Earth Changed documentary with David Attenborough.
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