The longest and most comprehensive acoustic study ever mounted in New Zealand has revealed an underwater orchestra of snapping shrimp, popping fish and moaning whales in the Hauraki Gulf.
University of Auckland researcher Rosalyn Putland has just completed a pioneering PhD study, which collected half a million minutes of sound from six listening posts set up around the gulf.
The study yielded a rich soundscape showing that our underwater environment is anything but a still, silent world, with the seafloor-fixed hydrophones picking up everything from the clicking of dolphins to the slow rumble of far-off earthquakes.
"The fundamental question we wanted to answer is, what is the soundscape of the Hauraki Gulf?" she said.
"It's a very quiet embayment relative to a lot of places around the world, and we wanted to establish a baseline of what is actually happening naturally, prior to any human impacts in the future.
"Very little sound data has been collected in New Zealand, and it's been in short-term studies, so it was nice to get this long-term view, with seasonal and annual variations."
Over the three-year period, Putland recorded two minutes of every 20 - "if I listened to all of it I'd still be here 10 years later playing it back" - and used computer models to
process the enormous amount of data.
Among the sounds captured were the moans of the critically-endangered Bryde's whale, which, despite being one of the third most common mammals found in the gulf, had been rarely recorded.
"The whales were a great sound to hear - that's a really low moaning sound and they appeared very frequently, and probably more than we first thought they were going to.
"She was able to tease out a colourful range of other specific biological sounds.
"We have urchins, which make these kind of crackling, crunchy sounds as they feed on rocks, and that's actually quite a fundamental sound here in New Zealand, and we have snapping shrimp, which is found all over the world and pretty much constantly throughout the day and night."
The din of rain and wind above the surface could be heard on the seafloor, as could earthquakes from as far as 750km away - something Putland was able to confirm using GeoNet data.
"We thought, wow, that's a pretty awesome sound, and potentially the whales and fish might actually be used to that sound on a natural basis."
The project's ultimate finding was a positive one, with each biological sound filling its own "niche" in the gulf's soundscape.
"It's been shown in the past that it's quite a healthy environment if all of the niches are filled."
New acoustic studies like Putland's have been allowing scientists to gain some remarkable insights into our marine environment.
Devices deployed in the Cook Strait over six months last year captured vocalisations from Antarctic blue whales, Antarctic minke whales and several different beaked whale species that were rarely seen due to their extensive diving behaviour.
These were likely to be the first recordings of Gray's and strap-toothed beaked whales in New Zealand.
Bryde's whales share secrets with their fins
Meanwhile, research into the long-term survival and abundance of the gulf's Bryde's whale population has used photographs of their fins to aid efforts in improving the management of the small population.
Bryde's whales are classified as nationally critical in New Zealand and appear only on the north-eastern coast of the North Island.
In the Hauraki Gulf, Bryde's whales appear year-round, usually in waters deeper than 40 metres.
The study, led by Massey University's Dr Gabriela Tezanos-Pinto, collated an extensive database of photographs showing distinctive fin features of individual whales over eight years to obtain missing demographic information about the local population.
The study catalogued 72 whales over eight years, of which, 20 returned to the gulf year after year.
Overall, the study estimated that less than 50 whales visit the gulf on each season.
"Our analysis indicates that the local population of Bryde's whales could not sustain the death of more than one whale per year," Tezanos-Pinto said.
"This highlights the fragility of the local population, particularly, when we know that ship-strikes have been previously estimated to cause an average mortality of two whales per year in the gulf."
An adjunct researcher of the Massey's Coastal-Marine Research Group, Tezanos-Pinto said that before the study, detailed demographic information of this population was unknown.
"While we cannot yet make assumptions about the trajectory of the local population over time, our results can now provide baseline information that can be used to monitor trends in the future and improve the management of the local population."
Using photo identification allowed researchers to focus on the individual whales and show whether they were returning to the gulf.
"Our results show that there is a combination of some whales returning, whereas others do not, but new immigrants are coming in as well," she said.
"The data could be used in the future to examine population trajectories over time and the impact of ship strikes on the long-term persistence of the population, but our findings highlight the need for continuous, as opposed to intermittent, photo-identification studies of Bryde's whales so that fluctuations in abundance, movements, female reproductive rates, individual patterns of residency and habitat use can be monitored over time."
The research, just published in the journal Endangered Species Research, was funded by the Hauraki Gulf Research Fund administered by the Department of Conservation and Auckland Whale and Dolphin Safaris.