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Underwater sound from anthropogenic sources such as seismic surveys, marine renewable device installations and naval exercises has the potential to disturb and cause injury to a variety of marine species. There is particular concern for the potential effects upon marine mammals, which utilise sound to communicate, hunt and navigate. Observed effects include behavioural changes and reduced sighting rates, with unknown consequences for individuals or populations over time. Potential effects on marine mammals include sound induced damage to auditory systems, altered diving behaviour potentially resulting in decompression sickness, stranding and death. The aim of the thesis was to develop a framework of best practice measures relating to mitigating the potential effects of underwater sound on marine mammals during offshore exploration and development operations. In order to mitigate the potential effects of underwater sound, regulatory guidelines have been developed and implemented around the world, principally for seismic surveys. These guidelines limit the activation of seismic sources when in proximity to marine species, and involve the use of specially trained personnel on survey vessels known as Marine Mammal Observers (MMOs). A critical review of the guidelines identified variation in the level of precaution applied to measures, such as the distance at which species can be sighted before reducing sound output from the operation. MMOs collect sighting information for all encounters with marine species, resulting in large volumes of data detailing species occurrence and behaviour. A sample dataset was found to be subject to variation in quality due to the different academic backgrounds and training levels of personnel. The data highlighted significant differences in relation to sighting distance while the sound source is active or inactive, though lacked detail with regard to behaviours, likely due to the lack of ethological expertise among the observers. A questionnaire was conducted to determine any differences of opinion regarding current mitigation practice and the underlying issue between stakeholder groups. There was no difference of opinion between stakeholders regarding the importance of underwater sound compared to other environmental issues facing marine species such as fisheries by-catch and overfishing. Areas of consensus were evident, with most stakeholders finding current mitigation practice to be only ‘somewhat’ effective, and that sightings data collected by MMOs should be better utilised, with it being more useful for adding to our knowledge of marine mammal distributions than for determining the effects from operations. A framework for enhancing the collection, use and dissemination of MMO data is described with recommendations for the development of a Global Positioning System (GPS) enabled smartphone/ tablet based field data collection system, linked to an internet based geographical information system to enhance species distribution analysis. By coupling this with a simplified mitigation methodology, the outcome would enhance the risk management of operations in relation to where species are known to occur, with mitigation aimed at reducing exposure at critical times or in critical habitats. Simplifying mitigation and enhancing data collection and use will benefit stakeholders in managing essential operations responsibly.

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This thesis describes the development and testing of the Combustive Sound Source (CSS), which is a broadband underwater sound source. The CSS is being developed as a clean, safe, and cost effective replacement to underwater explosive charges, which exhibit an inherent danger to marine life and researchers using the charges. The basic operation of the CSS is as follows. A combustible mixture of gas is held below the surface of the water in a combustion chamber and ignited with an electric spark. A combustion wave propagates through the mixture and converts the fuel and oxidizer into a bubble of combustion products, which expands due to an increase in temperature, and then ultimately collapses to a smaller volume than before ignition, producing a high intensity, low frequency acoustic signal. The thesis begins by discussing the background, history, and purpose of developing the CSS. It continues by describing the current apparatus and the essential components and convenient features added to the latest mechanical design. The general operation is discussed along with a description of an experiment conducted to determine the acoustic output and robustness of the current CSS. The results of this experiment are presented in terms of the effect of volume, ignition depth, oxidizing gas, combustion chamber size, and repeatability of acoustic signatures. Discussion of apparatus robustness is presented to suggest improvements for future CSS designs. / text

Soundscape Ecology, the physical combination of sounds at a particular time and place, is a rapidly growing field. As acoustic technology advances, several possible future uses of passive acoustic monitoring (PAM), such as biodiversity counts and monitoring of habitat health, are being explored. This thesis is divided into two chapters; each is a stand-alone paper. The first chapter provides a review of soundscape ecology, ambient sound, current recording methods and data analysis used in PAM studies, and identifies several major future recommendations for the field. One of these recommendations is to standardize recording methods and indices used during analysis in long-term studies. The second chapter analyzes a 55-minute continuous recording on a coral reef in Tunicate Cove, Belize in 1996 by Professor P. Lobel. This recording was then subsampled with several intermittent recording schedules to explore the amount of acoustic information lost as periods of active and inactive recording vary. The continuous recording consisted of a high frequency band (3-4 kHz), which may correspond to abiotic sounds, and a low frequency band (0.1-0.5 kHz), which generally corresponds to biotic sounds. Two recording schedules, 30 seconds every 4 minutes and 2 minutes every 10 minutes, were significantly correlated with the continuous recording. The statistical significance of the other five recording schedules varied among the three parameters tested in this study (average power (dB), average entropy, and aggregate entropy).

The hearing abilities of elasmobranch fishes were examined in response to several types of stimuli using auditory evoked potentials (AEP). Audiograms were acquired for the nurse shark, Ginglymostoma cirratum, the yellow stingray, Urobatis jamaicensis, in a controlled environment using a monopole underwater speaker. A dipole stimulus was used to measure the hearing thresholds of the horn shark, Heterodontus francisi, and the white-spotted bamboo shark, Chiloscyllium plagiosum. The dipole experiments yielded much lower thresholds than any other experiment, suggesting that this type of sound specifically stimulated the macula neglecta by creating a strong velocity flow above the head of the shark. A shaker table was created to measure the directional hearing thresholds of the C. plagiosum and the brown-banded bamboo shark, C. punctatum. This experiment showed that these sharks could sense accelerations equally in all directions suggesting that they have omnidirectional ears. The results also yielded higher thresholds than with the dipole, suggesting that the macula neglecta was not stimulated as the sharks were being accelerated. An audiogram was also acquired for the Atlantic sharpnose shark, Rhizoprionodon terraenovae, using a monopole speaker in the field. This experiment revealed that the hearing thresholds did not appear to be masked by ambient noise levels, and resulting thresholds yielded the lowest levels detected by any elasmobranch using AEPs. Taken together, these experiments show that sharks are most sensitive to low frequency sounds in the near field and use both their otoconial endorgans as well as the macula neglecta to sense particle motion.

Ocean environments with sloped seafloors can give rise to sound paths that do not remain in a constant plane of propagation. Numerical modelling of sound fields in such environments requires the use of computer models that fully account for out-of-plane sound propagation effects. The inclusion of these three-dimensional effects can be computationally intensive and the effects are often neglected in computer sound propagation codes. The current state-of-the art in sound propagation modelling has seen the development of models that can fully account for out-of-plane sound propagation. Such a model has been implemented in this research to provide acoustic consultants JASCO Applied Sciences with an important tool for environmental noise impact assessment in complicated marine environments. The model is described and validation results are shown for benchmark test cases. The model is also applied to study three-dimensional propagation effects in measured data from a realistic ocean environment. Particular analysis techniques assist in the interpretation of the modelled sound field for this physical test environment providing new insight into the characteristics of the test environment. / Graduate

Wave energy has the potential to contribute considerably to the UK's energy mix. The marine environment is already subjected to many anthropogenic pressures. There is a need to develop the industry as sustainably as possible. A key concern is the potential for underwater noise to affect marine life. A wave energy converter (WEC; BOLT Lifesaver, Fred Olsen Ltd.) was deployed at the Falmouth Bay marine renewable energy test site (FaBTest). The underwater sound levels were recorded at this site for a two week baseline period, a five-day installation period and intermittent operational and non-operational activity from March 2012 - November 2013. The recordings were also analysed for the Marine Strategy Framework Directive (MSFD) indicator third octave bands of 63-Hz and 125-Hz for shipping noise. The median and modal sound levels in Falmouth Bay were found to be loudest in the frequency range 100 Hz - 1 kHz and affected by local shipping activity. During installation activity, the sound levels were louder at all frequencies recorded as compared to similar periods with no installation activity, with a mean difference of 6.9 dB Hz-1 in the range 10 Hz to 48 kHz. Long term marine renewable energy construction projects may affect the MSFD indicator bands. There was little overall difference in the average sound levels for the operational and non-operational periods as the median PSD levels were louder by an average of 0.04 dB Hz-1 during the operational activity as compared to the non-operational activity. The results of this study indicate that the effect of a single WEC device on the overall sound levels in Falmouth Bay is relatively low considering the substantial presence of shipping in the area. However, in the immediate vicinity of the device (<200 m), the sound produced was found to be of significance to marine animals. It therefore requires considering in future deployments, particularly at a site with little anthropogenic activity.