One unexpected consequence of the COVID-19 outbreak and the subsequent shutdowns is a quieter planet. Scientists around the world have reported less noise above and below ground and even underwater as a result of decreased human activity. This is a pleasant respite for people troubled by noise pollution, which can cause damage to the inner ear, hearing loss, stress, anxiety, depression, high blood pressure and heart disease. It also may give some sea creatures a break as several recent studies have shown how noise and vibration caused by ships, motorboats and port activity, adversely impact marine life.

A team from the University of Exeter studied the effect of noise on the defensive camouflage behavior of shore crabs. To protect themselves from predators, the crabs change color to blend in with the rocks around them. Over eight weeks, lead study author Emily Carter observed dark-shelled juvenile shore crabs that she placed in white tanks and exposed to underwater sounds of a cruise ship, a container ship and an oil tanker. A control group heard natural water sounds at a similar amplitude to the ship sounds. The shells of the crabs exposed to the ship noises lightened to blend into the tank color only half as much as those in the control group.

"Color change in shore crabs is a slow, energetically costly process that's controlled by hormones that activate specialized pigment cells across their shell," said Carter. "Stress consumes energy and disrupts hormone balance, so we believe that the stress caused by ship noise either drains the crabs of the energy required to change color properly or disrupts the balance of hormones necessary to make that change."

In addition, the crabs hearing the ship noises grew and molted much slower and half of those crabs did not respond at all to a simulated shore bird attack, while those that moved to hide did so very slowly.

An international research team working in French Polynesia studied 40 pairs of clownfish exposed to motorboat noise and natural reef sounds. The team found that the motorboat noise caused hormonal changes in the fish that led to behavioral changes, including hiding in their anemone hosts, reluctance to move into open water to feed and greater aggression toward the domino damselfish that share the anemone.

Those fish exposed to the noise were also unable to respond appropriately to additional stressors, such as predators and climate change, which further increased their risk. They showed higher than normal levels of cortisol, a stress hormone, and of testosterone and 11-ketotestosterone, associated with the observed behavioral changes. These findings could be used to develop noise-mitigation tools.

Researchers studying harbor seals in Alaska's Glacier Bay National Park found that the roar-like sound male seals emit to mark territory and attract a mate is being obscured by the noise from cruise ship engines. The two sounds are the same very low frequency.

To determine this, the researchers recorded underwater sounds in Glacier Bay during the months that coincided with the harbor seal mating season. The microphones were able to capture 545 roars from four seals as well as the noise of passing cruise ships, which accounted for a large portion of the noise on the recordings. The seal roars did not change in volume, frequency or duration when the ships were present, even though the ship noise was louder. The team concluded that the females could not hear the males above the sounds of the cruise ships and that the seals were already roaring as loud as possible.

Governments, industry and researchers find solutions

In 2018, the Global Ocean Observing System, a UN project that coordinates ocean monitoring efforts, added noise to its list of essential variables at the request of the International Quiet Ocean Experiment (IQOE), a scientific program that promotes research to gain a better understanding of ocean soundscapes and the effects of sound on marine organisms.

In 2019, the Vancouver Fraser Port authority began offering discounts on harbor dues to commercial cargo ships that adhere to best practices for reducing underwater noise along with other environmental impacts. Ships with EnergoProfin, a device made by Finnish company Wärtsilä that significantly reduces propeller noise, qualify for a 23% reduction in port fees. In Europe, the Noise Management in European Ports (NoMEPorts) document provides guidance and best practice for noise management.

Following the 9/11 terrorist attacks, commercial shipping transport essentially stopped, providing a unique moment of near silence in which to study ocean environments without the usual man-made noise. The slowdowns of activity as a result of COVID-19 offer the same opportunity. IQOE is requesting ideas on how to best use the information gathered during this time.