Anthropogenic noise is an established global pollutant, with mounting evidence that noise from human activities (e.g. transportation, construction, and resource extraction) can impact behaviour, physiology, and fitness in a broad range of taxa. Establishing noise as a stressor and pollutant has been an evolving process where research methods, techniques, and investigative scope continue to change and develop. However, there still exists many unanswered questions, such as impacts on local community structures and populations. Therefore, evolution within the field of anthropogenic noise research (e.g. new study systems and technology) can be used to help researchers explore logistically challenging questions.

This thesis presents an integrated body of research on the impacts of noise to aquatic life at varying levels of biological organisation: starting with impacts on individuals, moving to effects on interspecific interactions and local communities, and ending with population-level assessment. Importantly, I have strived to incorporate aspects of noise mitigation in each section.

In Chapter One, I introduce anthropogenic noise, the importance of underwater sound to taxa, and outline the evolution of noise-related research. Then, I present data showing that real motorboat noise negatively affects egg-tending and nest defence in male damselfish on the Great Barrier Reef, but also provide evidence for the potential benefits of a simple, yet effective, noise-mitigation strategy (Chapter Two). Following this, I provide evidence that playback of SCUBA noise hinders interspecific behaviour and alters community structure near ecologically important cleaning stations on the Mesoamerican Barrier Reef (Chapter Three). Moreover, by establishing acoustic disturbance as a mechanism for the negative impacts of diver presence, I am also identifying a potential avenue for mitigation. In Chapter Four, I develop and present an energetic and spatially explicit individual-based model that uses realistic projections of noise-pollution levels in the Northeast Atlantic to explore the impacts of noise to an important fish stock. Using the model, I further unpick the population-level impacts of noise to the fish stock and use findings to develop and test a targeted noise-mitigation strategy to improve population-level outputs (Chapter Five). Finally, I discuss the implications of findings, present important areas for further research, and critically discuss the management of noise in our oceans (Chapter Six).

Motorboats are a pervasive, growing source of anthropogenic noise in marine environments, with known impacts on fish physiology and behaviour. However, empirical evidence for the disruption of parental care remains scarce and stems predominantly from playback studies. Additionally, there is a paucity of experimental studies examining noise-mitigation strategies. We conducted two field experiments to investigate the effects of noise from real motorboats on the parental-care behaviours of a common coral-reef fish, the Ambon damselfish Pomacentrus amboinensis, which exhibits male-only egg care. When exposed to motorboat noise, we found that males exhibited vigilance behaviour 34% more often and spent 17% more time remaining vigilant, compared to an ambient-sound control. We then investigated nest defence in the presence of an introduced conspecific male intruder, incorporating a third noise treatment of altered motorboat-driving practice that was designed to mitigate noise exposure via speed and distance limitations. The males spent 22% less time interacting with the intruder and 154% more time sheltering during normal motorboat exposure compared to the ambient-sound control, with nest-defence levels in the mitigation treatment equivalent to those in ambient conditions. Our results reveal detrimental impacts of real motorboat noise on some aspects of parental care in fish, and successfully demonstrate the positive effects of an affordable, easily implemented mitigation strategy. We strongly advocate the integration of mitigation strategies into future experiments in this field, and the application of evidence-based policy in our increasingly noisy world.

Coral reefs worldwide are increasingly damaged by anthropogenic stressors, necessitating novel approaches for their management. Maintaining healthy fish communities counteracts reef degradation, but degraded reefs smell and sound less attractive to settlement-stage fishes than their healthy states. Here, using a six-week field experiment, we demonstrate that playback of healthy reef sound can increase fish settlement and retention to degraded habitat. We compare fish community development on acoustically enriched coral-rubble patch reefs with acoustically unmanipulated controls. Acoustic enrichment enhances fish community development across all major trophic guilds, with a doubling in overall abundance and 50% greater species richness. If combined with active habitat restoration and effective conservation measures, rebuilding fish communities in this manner might accelerate ecosystem recovery at multiple spatial and temporal scales. Acoustic enrichment shows promise as a novel tool for the active management of degraded coral reefs.