Eavesdropping on the Māui dolphin

By Andrew Wright, Marine Advisor

Māui dolphin occurrence and distribution has been studied by DOC, MPI and many others through a variety of means for approaching two decades.

Māui dolphin. Photo: Martin Stanley

Hundreds of hours on boats and in the air have provided invaluable information on the dolphins. But there remain a number of niggling questions.

For example, how far offshore do the dolphins go and how often? Also, where do the dolphins go in winter and at night?

Finding answers is proving to be a tricky business. Intensive year-round visual surveying is challenging and expensive. Yet even that may still not be able to address all the issues as it is limited to periods of good weather and daylight.

Pictured with a CPOD, DOC diver Kath Blakemore is clearly content in her work. Photo: Scott Baker, Oregon State University

One option to fill the gaps is to use sound instead of light.

To explore the viability of this, the Marine Species and Threats team partnered with Auckland Council to deploy two acoustic recording devices, called CPODs, in a pilot project for up to four weeks in February 2016. The results were encouraging.

The first was deployed (and recovered) by divers on the first navigational pile coming into Manukau Harbour, near Cornwallis Point. This unit had to be recovered early ahead of pile replacement, following a vessel strike just days before the deployment.

The second CPOD was deployed on an anchor-based mooring a little to the north of Hamilton’s Gap (just south of Manukau Harbour entrance, to the West of Auckland) in 10 metres of water. This was the real test, as some believed that the local conditions would sweep the unit away. We had prepared, but would it be enough?

Deployed over-the-side of a small boat with a surface buoy, the intent was to recover it in the same way.

Fortunately, the anchoring system meant that it remained in place, a little too well perhaps as the anchors were also buried by sand. Commercial divers were ultimately needed to drag the whole rig free.

No Māui dolphin were recorded on the Manakau Harbour deployment, which was not unexpected given low detection rates from earlier studies. (The predecessor to the CPOD, the TPOD, was deployed here by Otago University between 2005 and 2008 and detected Māui dolphins on only 7 days during that period.)

The vertically-challenged navigational pile for the Manukau Harbour deployment. Photo: Andrew Wright

However, Māui dolphin were frequently recorded at the Hamilton’s Gap location, reaching peak average detection rates of almost once every 4 minutes coinciding with the sun being at its highest point.

Although the data set was limited, there also seemed to be some tidal influences, with high and falling tides attracting more Māui dolphin at the peak of the day than other periods.

Frequencies of different detection events. Human hearing lies between 20 Hz and 20 kHz. Example outputs from CPOD software, Chelonia Limited, UK

More research will be needed to confirm that this is not merely an artefact of the short deployment.

More data and a specifically designed array will also be able to examine the night-time low periods: where are the dolphins going?

This pilot also recorded bottlenose dolphins and killer whales, which could be distinguished based on the frequencies they use and other properties of their clicks.

It also recorded a mystery signal that, after much investigation, turned out to be most likely a type of tag that is generally deployed on large fish so they can be tracked. Malcolm Francis from NIWA believes this indicates that great white sharks tagged in Australian waters are visiting the area. At least we now know where Bruce has been instead of reprising his role in Finding Dory.

As pilots do, it raised more questions than provided answers, but the data offers a tantalising first glimpse into Māui dolphin activity at night. It was a success however: acoustic monitoring of Māui dolphins works and clearly can contribute crucial information to recovery efforts.

Sometimes eavesdropping on others can have conservation benefits.

Two trains of Māui dolphin clicks displayed in the CPOD software. Image: Chelonia Limited, UK