Cetaceans spend a large part of their lives underwater where they are difficult to detect by observers. Acoustics would complement visual observation data over the vast expanse of the Agoa Sanctuary by limiting human presence.
The technique used by the Sanctuary's teams is passive acoustics, which directly records ambient sounds and offers several advantages for monitoring marine mammal populations:
- detection of discrete species,
- continuity of acquisition (day/night, interseasons, etc.) regardless of weather conditions and in areas that are sometimes difficult to access,
- recording of acoustic signals which allows to 'replay' the recordings for a posteriori analysis and use for awareness,
- recording of other sources of noise that may have an impact on marine mammals (maritime traffic).
For several years the Agoa Sanctuary has been testing equipment to record underwater sounds. The establishment of an acoustic observatory will allow the acquisition of data in order to better understand human-generated noise, but also to characterize species’ and populations’ residency within the Sanctuary.
Jason HighBlue hydrophones
Within the framework of the CARI'MAM Interreg' project, the Agoa Sanctuary is currently coordinating the deployment of hydrophones in 14 territories of the Wider Caribbean Region (Martinique, Guadeloupe, Haiti, St. Eustatius, Aruba, Bonaire, St. Bart, Anguilla, St. Martin, Bermuda, St. Vincent and the Grenadines, Turks and Caicos Islands, Bahamas, Jamaica, and the Dominican Republic).
Twenty hydrophones have been deployed since December 2020 to record the sounds emitted by all species of marine mammals passing nearby. They were specially built for this mission by the University of Toulon. Immersed at a depth of 20 meters for one year, they will be brought up every 40 days to recover the data and be maintained. They are able to detect sounds inaudible by the human ear and will also capture noise pollution related to human activities. These data will then be analyzed by a deep learning software.
This action is the result of an intense collaboration between marine mammal specialists of the Caribbean through the CARI'MAM (Caribbean Marine Mammals) network.
In late 2015 and early 2016, C-POD recorders were deployed in a pilot phase in Guadeloupe. Their objective was to acquire data on the delphinids of the Agoa Sanctuary by recording their clicks.
It is the first time that this device was deployed in tropical waters which are frequented by many species emitting clicks. Two species were acoustically identified: bottlenose dolphins (Tursiops truncatus) and rough-toothed dolphins (Steno bredanensis). Thanks to the collected click recordings, a resting period and geographical area could be identified for these dolphins. However, identifying the number of individuals is not possible with C-PODs and it is relevant to couple these recordings with opportunistic or standardized visual observations.
This test also showed that moving the recorders away from the coast was necessary in order to be more efficient. Indeed, West Indian coastal waters contain a lot of parasite noises (such as clapping shrimps clicks for example). The C-PODs being programmed to automatically record all detected click, whatever its origin, the memory cards of the recorders are quickly full and it would be advisable to bring them up very regularly.
Acoustic monitoring and noise pollution
In addition to marine mammal monitoring, setting up hydrophones allows to listen to the noise generated by human activities.
Active acoustic monitoring methods, such as sonar, also make it possible to study the sea floor. However, their use contributes to noise pollution in the marine environment. Indeed, the principle of active acoustics is to emit a sound from a device. This sound will reverberate on the animals and the surrounding terrain and return to the device which will record it.