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Acoustic Monitoring

Marine mammals use sound for communication, foraging, and navigating in a world that is sometimes almost entirely dark. Underwater noise, like that produced by maritime traffic, can be fatal for a number of marine species. In the section for marine mammal research, we use acoustic monitoring to investigate the effect of underwater noise on harbor porpoises and to collect information about their general behavior.

Harbour porpoises use the echo created by their sonar-like clicking sounds to sense the world around them. Using sound, they are able to navigate and hunt prey even when the water is dark and unclear. These sounds are short and at such a high frequency level that the human ear can’t register them. This process of seeing with sound is called biosonar; it is fundamentally equivalent to the sonar used on fishing boats. This is also why the sonar from boats can be so disturbing for porpoise navigation, as they often use the same frequency level. In Denmark, marine biosonar is only produced by harbor porpoises and is so unique that the sound is easily recognized when recorded. The acoustic data logger records the sound, and, by using special programs, we can transform the sound into a frequency that is recognizable for the human ear. These programs can also use the frequency level to differentiate biosonar from underwater noise. Although the program sometimes registers boat sonar as biosonar, the visible sound pattern is easily distinguishable between these two sources. You can watch recordings from our live underwater listening stationto see what sound looks like.

Click detectors

Click detectors record short signals; for every click, a limited amount of information is saved. They are used to record and register porpoises, dolphins and other toothed whales. The old T-Pods (Timed - Porpoise Detectors) have been replaced with the C-POD (Cetacean - Porpoise Detector), which will also soon be replaced by a new system called Soundtrap. This system is currently undergoing test trials.

Recording systems

The recordings systems either record sound continuously or in time intervals (for example, 30 minutes every hour). They are used to register and characterize underwater noise, including maritime trafficking, construction, and sounds produced by the oil industry. Recording systems can also be used to monitor sounds from animals, like sonar clicks from porpoises, toothed whales and dolphins, seals’ barking, and baleen whale songs.

These systems can be mounted on the seabed, where they can record sound for days, months or even years, like the listening station in Little Belt, Denmark. These units can be composed of only one hydrophone, or a multiple-setup (array), and can be deployed from boats.