By: Jørn J Fremstad // Norwegian Institute for Nature Research

“Salmon make sounds. We don’t yet know whether these sounds are meaningful, but experiments have shown that the soundscape changes when the fish are hungry or afraid of something, for example. Acoustic monitoring can revolutionise the way we learn about nature, and it’s cost-effective too,” says researcher Carolyn Rosten at the Norwegian Institute for Nature Research (NINA).

Together with colleagues at SINTEF Ocean, she is investigating how sound can be used to monitor the natural environment both quickly and easily. This is done using small recorders fitted with a microphone or hydrophone, also known as “acoustic loggers”. Listening to what the salmon can tell us is just one of many possibilities offered by the new method and tool.

Tested using feed and humans

The experiments on salmon were carried out both at NINA’s research station at Ims in Rogaland and in aquaculture net-pens at SINTEF ACE. At Ims, the researchers observed changes in the soundscape in the fish tank when a school class was visiting. In the pen at SINTEF ACE, sounds were measured before and after feeding.

“A big group of kids inevitably make quite a lot of noise, and we measured marked differences in the frequencies of the sounds the salmon made before, during, and after the children’s visit to the fish tank. It was as if the salmon felt threatened by these large, noisy creatures and were relieved when they left. We also discovered how long it took for the fish to calm down,” says Rosten.

Potential savings for aquaculture

“In the pens, we found that hungry salmon make sounds at different frequencies compared with salmon whose bellies are full. This discovery could help aquaculture industry cut costs, as outlay on feed is one of the industry’s biggest expenditures,” she continues.

Sound can be used to monitor species and environments both on land and in the water. An audio recording could be enough to detect invasive or threatened species, or to describe periodic changes in plants and animals, ecological conditions, or human impact on nature.

Croaking frogs and singing birds

“We can for example detect when frogs are starting to croak, or the arrival of migratory birds in the spring. We can also record sounds that don’t belong in a particular area, in order to get an early warning of invasive species,” says Rosten.

It is possible to monitor both individual species and entire habitats. Acoustic loggers can be set up either as a self-contained system, or as a supplement to existing monitoring systems. They continuously collect data in real time and transmit the information via the mobile network to a website that can be monitored by researchers, environmental managers, and even the general public if desired.

Works even in harsh conditions

Acoustic monitoring is also an effective method when visibility is poor, such as in murky water, dense forest, in fog, or at night. The loggers are equipped with a battery and solar panel, and should be able to function unattended for at least six months. As the system requires little maintenance, the researchers avoid having to make repeated visits that can cause disturbance in the area being monitored. These capabilities make acoustic monitoring particularly suitable for harsh Norwegian conditions.

“The digital revolution opens up many possibilities. Using the technology and analytical methods that have now become available, we can already monitor nature on a broad scale using sound. By developing the technology further, we will identify even better solutions for Norwegian nature monitoring programmes,” says Carolyn Rosten.