Lockdowns good for marine mating
Study finds ocean dwellers communicate better without noise pollution from boats
Marine mammals and fish in the Hauraki Gulf have been proven to change their behaviour during lockdown as the noise of recreational boats temporarily diminished.
A combined study by University of Auckland and Canada’s University of Victoria researchers showed the ability of bottlenose dolphins and bigeye fish to clearly hear each other more than doubled during lockdown 2020.
Noise pollution is known to affect marine life, which use sound to communicate.
“The impacts are multiple,” said marine scientist Associate Professor Craig Radford: “depending on whether your vessel is a gin palace or a smaller boat and the speed and proximity to animals.
“If males and females can’t hear each other for mate selection, you are going to affect reproduction.”
Acoustic data was collected between February 2020 and May 2020 using seafloor mounted acoustic recording stations at five sites in the Hauraki Gulf. Recorders captured two minutes of sound every 10 minutes which equated to six samples per hour or 144 samples a day.
The samples were then split into pre-lockdown and during lockdown.
Without small boats, the Gulf became much quieter at all five acoustic monitoring sites, particularly at frequencies below 1 khz.
Median sound pressure levels were down by 8 decibels and 10 decibels on the first day and vessel noise levels dropped by almost half, before dropping even further — to 8 per cent of normal levels.
The research found calculated communication ranges for dolphins and bigeyes significantly increased as a result, particularly when the monitoring site was further from the city. The maximum median range which dolphins were able to hear each other in the Rangitoto Channel for example was calculated at 400m prior to lockdown but rose to 565m once lockdown began.
Further away from the city, at the Ahaaha Rocks off the northern coast of Waiheke Island, dolphin communication ranges increased from 2.9km to nearly 4km and for bigeye fish from 4m to 70m.
The study showed the ability of dolphins and bigeyes to clearly hear each other more than doubled during lockdown.
Radford said further study was planned, but had been postponed due to the latest lockdown, to look at the impacts inside and outside of marine reserves from different sized vessels travelling at different speeds.
A study using bait pots had already found that fish inside marine reserves swarmed around boats in expectation of being fed while fish outside a reserve scattered and took some time to gain confidence to return.
When holiday weekends dawn the Coromandel’s harbour entrances resemble highways of boats.
Radford said unlike the “cocktail party effect” — the ability of humans to isolate a single voice in a crowded, noisy room, and to focus on the signal content of that one voice — not all marine creatures can.
Fish don’t have the ability to increase their sound and can’t be too close together because it could be classified as aggressive behaviour.
“There is evidence some fish can do that but the vast majority can’t increase the level of their call,” he said.
The study is of interest to those on the Coromandel who are lobbying for further protections of our marine life.
Thomas Everth in Whitianga said he’d witnessed boat ownership rise postlockdown as Kiwis bought boats because they could not take their usual overseas holiday.
“In summer it was horrendous,” he said.
“With the high tech gear that’s available these days to recreational fishers too, what has a chance?”
Associate Professor Radford says the sheer number of recreational vessels in normal times is not offset by the fact they are often only present for short periods of time.
“New Zealand is a long way behind the rest of the world when it comes to noise effects,” said Radford.
He and colleagues, including MAF, had got Maritime NZ onboard to consider noise effects on marine life, and Radford believed Maritime NZ was the right organisation to work with on guidelines.
But it was a slow process. “Research into noise generated by smaller boats has been somewhat neglected because of the larger scale noise generated by ships, but key data from this study provides strong evidence that small vessels, where there are enough of them, directly influence sound levels and are definitely having an impact.”
The research is published in Global Change Biology and was done in collaboration with Dr Matthew Pine from Canada’s University of Victoria.
"With the high tech gear that’s available these days to recreational fishers too, what has a chance?" Thomas Everth