Soundscape ecology of the Hauraki Gulf, NZ

Abstract

Many marine taxa rely on sound for important cues about their environment, but there is an increasing awareness that anthropophony may be causing homogenisation and fragmentation of the natural soundscape. The overall aim of this thesis was to investigate the effect of anthropophony on marine life in an ecologically significant embayment. A passive acoustic monitoring array, consisting of six different listening stations, was set-up around the Hauraki Gulf, New Zealand from October 2014 to June 2016. Broadband sound pressure levels (50- 24,000 Hz) were consistent throughout deployment, ranging from 90 – 110 dB re 1μPa. Biophony from different taxa exhibited both temporal and frequency partitioning, as predicted by the acoustic niche hypothesis. Automated detection packages were effective at identifying temporal trends in biophony. There were significant diurnal and seasonal patterns across a range of taxa with increased crepuscular activity by urchins, bigeye vocalisations detected more in summer, dolphin echolocation increased in spring and summer during the night, and Bryde’s whales vocalised more during the day. The use of acoustic detections to ascertain temporal and spatial patterns of soniferous animals has a wide range of ecological applications that can be enhanced with precise source location information. Uncertainty in acoustic localisation calculations was investigated in relation to GPS accuracy and array geometry. Sound can be localised to within 5 m accuracy by using an unevenly spaced right-angled array of hydrophones. The comprehensive detection and localisation techniques developed allowed an improved appreciation of the complex interplay between geophony, biophony and anthropophony in the marine soundscape. In the Gulf, vessel sound exceeded natural variability of the soundscape in 3.9 – 18.9% of recordings collected across the six listening stations and raised the ambient soundscape by up to 60 dB between 50 – 10,000 Hz. Individual vessel transits also reduced the effective communication space of bigeyes and Bryde’s whales, by up to 44 m and 8.3 km, respectively. Importantly, the reduction beyond the natural variability of the soundscape could inhibit social communication and group dynamics of soniferous marine animals. Vessel sound is predicted to increase in coastal embayments throughout the world. Moving forward, information about the Hauraki Gulf marine soundscape provides decisive evidence for management about how to prioritise mitigation of anthropophony in the coming years.