Characterisation of the underwater noise caused by marine traffic in the Bay of Gibraltar
María Renee Contreras Merida
The introduction of sound to the ocean from human activities is a form of pollution commonly known as underwater noise. This noise can be categorised into two sources, impulsive and continuous sounds (also known as ambient noise), depending on the time scale it persists,. Low-frequency ambient noise in the ocean, mainly from ships, has been increasing since the nineteenth century. The rise in noise levels in the ocean have been reported to impact on marine fauna, in the form of masking important biological signals, inducing stress and different behavioural responses. As international regulations continue to be developed, several regional or local legislations have been created to regulate many pollutants and achieve a Good Environmental Status, including underwater noise. Efforts to monitor and manage vessel noise pollution are embodied in the MSFD descriptor (11.2), which mandates member states to ensure underwater noise levels do not surpass good environmental status thresholds. One of the most common methodologies in these guidelines is the passive acoustic measurements undertaken with hydrophones deployed either by surface-based or bottom-mounted systems.
Gibraltar is considered a relevant marine area due to its rich diversity in species and habitats, presenting several pelagic fishes and cetaceans migrations through the Strait of Gibraltar, crossing from the Atlantic Ocean to the Mediterranean Sea and vice versa. Within the Bay of Gibraltar, a no-take, no anchoring zone called Seven Sisters was created in the Marine Conservation Zone in Rosia Bay due to high marine biodiversity importance. This British overseas territory presents high traffic flow density, in which elevated continuous underwater noise is likely to impact all the marine fauna and ecosystems present in this area. As there is currently no sufficient evidence to assess underwater noise trends in such relevant areas, this study was created to develop the first characterisation of underwater noise caused by marine traffic in Seven sisters, Rosia Bay, Gibraltar. To achieve this, the project was carried out deploying an RTSys EA-SDA14 hydrophone to collect continuous underwater noise and develop recommendations for management and mitigation of underwater noise in this area based on the obtained results. Using PAMGuide in RStudio, different noise level metrics (PSD, SPL, TOL) were estimated for three different time frames (morning, midday and evening) of fifteen minutes each, for a day of the week and a day of the weekend. Spectrograms and statistical analysis were obtained for five files of three minutes each. Direct observations were acquired for every vessel in the Bay during the deployment period. The noise levels between the different time frames for each day, using the broadband SPL values were further analysed through an ANOSIM in Primer7.
Based on the results of the acoustic analysis in this study, marine traffic noise was located in different frequency bands ranges (20 to 500 Hz, above 1,000 Hz up to 10,000 Hz). This study is the first underwater noise measurement caused by marine traffic in the Bay of Gibraltar, the broadband noise levels present in the Bay of Gibraltar were reported around 113 to 138 dB re 1 μPa during a weekday, and around 115 to 140 dB re 1 μPa during a weekend day. From the observational data, the noise levels in the weekday were produced by more commercial vessels (n=34) than recreational vessels (n=10). Meanwhile, for the weekend, an almost equal amount of recreational (n=44) and commercial (b=41) vessels were observed during the acoustic recordings. Further analysis in the ANOSIM demonstrated significant differences (p<0.01) between almost all of the different time frames of both days.
To fulfil the MSFD requirements, the noise levels were also reported for the 1/3-octave band frequencies, 63 Hz and 125 Hz, as well as adding a third frequency (2,000 Hz) recommended by authors to include recreational vessels. For the 63 Hz third-octave frequency band, values ranging from 95 to 125 dB re 1 μPa, and from 98 to 125 dB re 1 μPa were recorded for the weekday and weekend day, respectively. In the 125 Hz third-octave frequency, values of 97 to 120 dB re 1 μPa for the weekday and from 100 to 125 dB re 1 μPa for the weekend day, were reported. The added 2,000 Hz frequency presented values of 95 to 118 dB re 1 μPa during the weekday and 100 to 120 during the weekend day. Due to high noise levels impacts on marine fauna by masking biological signals, stressing and inducing behavioural responses, it is recommended for Gibraltar’s waters to implement mitigation strategies such as defining allowable harm limits of noise and a maximum limit of bunkering vessels in the Bay, promotion of proper maintenance of the hull and machinery, stakeholder engagement, speed limit and temporal restrictions when needed.
Keywords: Underwater noise, Marine traffic, noise mitigation, Gibraltar.
Primary Supervisor: Dr Awantha Dissanayake
Secondary Supervisor: Dr Nathan Merchant
Centre for Environment, Fisheries and Aquaculture Science –Cefas
In conjunction with:
Department of Environment, Sustainability, Climate Change and Heritage –DESCCH