In the past, monitoring marine mammals is usually based on visual methods (i.e. land, boat or aerial observations).  However, such methods depend entirely on detection during the limited time in which the animal comes to the surface.  Many marine mammals, primarily cetaceans and sirenians, spend 95% of their lives below the surface of the water.  Thus, it can be particularly difficult to visually find and track these species.  Furthermore, visual observations are further dependent upon good weather and high visibility conditions, and are restricted to daylight hours.

Passive acoustic monitoring (PAM) provides an efficient platform for monitoring vocal marine mammals independent of weather conditions, visibility or time-of-day.  This is a relatively new direction in marine mammal research, but one which is becoming increasingly interesting for both population management and risk mitigation.

Some of the potential applications of passive acoustic monitoring include:

• Abundance Estimation:  Where the total number of animals in a given area is estimated.  The number of calls recorded may give an indication as to the number of animals using the area.  However, there are still problems to be overcome with this technique, primarily how to account for individuals who produce multiple calls (to avoid over-estimation) or alternatively for relatively quiet individuals (to avoid under-estimation).
• Mitigation:  When carrying out noisy industrial activities (e.g. seismic surveys, pile-driving), it is necessary to know when marine mammals are present.  If the animals are in close proximity when work begins, the resulting increase in sound levels has the potential to cause substantial damage.  Thus, by listening for their vocalisations prior to and during operations, there is the opportunity to reduce the risk to marine mammals by halting work until the animals have left the area.
• Habitat and Behavioural Analysis:  The presence of animals in different parts of their range and the time they spend there can help us to understand the importance of different habitats. In some cases, it is also possible to differentiate between foraging and socialising (e.g. odontocetes:  presence of echolocation clicks vs. social sounds), thus gaining information on the behavioural state of animals using that habitat.
• Monitoring Rare and Elusive Species:  Some species, such as beaked whales or the endangered Vaquita, are very discrete when surfacing which can pose problems for research surveys.  Acoustic monitoring can be used to detect the presence of these species more easily than visual surveys.

However, as our technology improves we are becoming increasingly aware of what a noisy place the ocean is.  The seas are already full of natural physical and biological sounds; these are now being added to by the increasing occurrence of anthropogenic (human-made) noise.

There is currently a substantial amount of research going into cataloguing underwater noise.  The sounds produced by marine mammals are still being discovered, and the challenge lies in isolating these sounds from general ocean noise.  As well as listening to marine mammal sounds, we can also visually inspect them using a variety of acoustic monitoring software.