The 2011 tsunami that caused a nuclear disaster in Japan has allowed scientists to measure the noise levels in Hawaiian waters without the presence of human activity.
Sounds originating from human activity have made the oceans much noisier. These artificial sounds have caused distress to marine animals and led to detrimental effects to their natural behaviour, sometimes leading to an increase in risky behaviour.
These oceanic noises are so prevalent that it is a challenge for researchers to measure the amount by which the noise has increased from the pre-industrial times.
However, the major earthquake in 2011–the one that took out the Fukushima reactor–has given scientists the rare opportunity to measure noise levels in the oceans sans human activity.
After striking Japan, the earthquake tremors travelled all the way to Hawaiian waters. This caused all human activity and ship traffic to cease.
At that time, a team of scientists from Duke University were at four shallow bays at Hawaii's Kona Coast recording underwater sounds. When the tremors struck, it was the perfect opportunity to find out what the bays sounded like without any human activity.
Their findings have been published in the recent issue of Marine Pollution Bulletin.
"On the tsunami day, underwater sound levels during the loudest part of the day measured 98.8 decibels (re 1 uPa). On days when human activities in and near the bays weren't halted, we recorded sound pulses more than 16 times louder than that," said Heather L Heenehan, a postdoctoral scientist at NOAA's Northeast Fisheries Science Center. She led the study as part of her doctoral dissertation at Duke's Nicholas School of the Environment.
(A "re 1 uPa" is a reference used by scientists to measure relative loudness of sounds recorded underwater. Every increase of 10 decibels is perceived as a doubling in loudness.)
According to a press release issued by the university, noise from boat traffic in the four bays reached up to 125 decibels (re 1 uPa), while pulses from nearby sonar exercises reached 143 decibels (re 1 uPa).
This is a concern as the four bays are home to spinner dolphins. These dolphins rest in the bays during the day, and hunt for food at night. However, during the day, their sleep is disrupted by human activity like ship traffic, sonar, dolphin-encounter boat tours and nearby fish farms. In fact, the study names humans as being responsible for creating the loudest disruptions in the four bays.
According to Heenehan, the new tsunami-enabled benchmark of what the bays sound like without human disruptions gives policymakers, conservationists and local communities a target to aim for when implementing measures to reduce underwater sound levels.