By Alison Ballance
Half a million minutes of underwater recordings (and counting) later, PhD student Rosalyn Putland is well on the way to her goal of creating an underwater sound map of the Hauraki Gulf. Along the way she’s recorded dolphins, whales, fish, urchins – lots of urchins – as well as lots of boats, big and small. Rosalyn says sound is a very important sense for marine animals.
Sound travels much further through water than through air. It’s much faster, roughly four times faster in water than in air. And that’s why it’s such an important sense for animals in the marine ecosystem. We use visual cues on land, we usually see things before we hear them, but it’s completely the other way round for anything in the ocean.
Bottlenose dolphins and common dolphins both feature in the underwater recordings, which are made by a network of six remote hydrophones anchored to the sea bed at depths between 8 and 50 metres. The hydrophones record for 2 minutes every 20-minutes, for up to two months at a time, before the data needs to be downloaded. Rosalyn Putland, from the University of Auckland’s Institute of Marine Science, based at the Leigh Marine Laboratory, says there are a lot of quiet recordings, as well as daily dawn and dusk pulses of sound made by feeding sea urchins rasping their mouthparts. Rosalyn says that with bottlenose dolphins “there’s lots going on: whistle, clicks and what I like to call kissing sounds. They’re harmonic sounds which cover the whole frequency range.” Common dolphins don’t use the ‘kissing’ sounds.
She has recorded what she thinks are Brydes whale calls, and describes them as short low moans, less than a second long.
The value of recording 24-hours a day is shown in one of Rosalyn’s more unusual findings, that a lot of dolphin vocalisations are happening in the middle of the night, “when we tend not be out on a boat recording.”
Rosalyn says that her sound map will help pinpoint noisy areas in the Hauraki Gulf where there is lots of boat noise.
"The big thing with sound is that unfortunately there aren’t any barriers in the ocean. So we have marine reserves … but they don’t stop sounds coming in. Sounds can travel great distances around the ocean, especially the low frequency sound of boats … so those large tankers can be heard many kilometres away, and that can have an impact [on marine animals]. One key species we’re worried about is the Brydes whale which vocalises down in those low frequencies. And so if we’re having too much boat noise in an area, is that potentially going to have an influence on those Brydes whales in the future."
Former marine science PhD student Shariman Ghazali was the first to record sounds made by New Zealand fish such as big eyes and gurnard.
Brydes whales featured in an our Changing World special on the marine spatial planning process currently underway for the Hauraki Gulf.
Listen to the story Underwater Soundscape of the Hauraki Gulf or download a podcast: