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You are listening to the first audio recording of the African manatee in its natural habitat! It is the result of the collaboration between AMMCO and a group of three researchers (Two Americans, Eric Angel Ramos from City University of New York and Athena Rycyk from New College of Florida and a Japanese, Mumi Kikuchi from Wildlife Research Center of Kyoto University) that established the first acoustic record of the African manatee in Lake Ossa, Cameroon.

This new acoustic monitoring methodology now allows AMMCO to detect the presence of manatees from the sounds they emit. To do this, the team trained Clinton, an AMMCO intern pursuing his Ph.D at the University of Yaoundé 1, on the use of passive acoustics to collect manatee vocalizations as a new tool for ecological monitoring of Lake Ossa.

Gone are the days when you had to take inventories of manatees by day and by night to observe their activities. Since January 2020, you just need to install some acoustic recorders to get a glimpse of their life. AMMCO has three acoustic recording systems, two of the AUSOMS-mini AQM-002 brand and one of the LS1 brand (http://loggerheadinstruments.com/). The AUSOMS-mini are small hydrophones of cylindrical shape, they can record acoustic signals from manatees at a maximum distance of 30m for five days. The LS1 is a more powerful passive acoustic monitoring system that can continuously record acoustic signals at a distance of up to hundreds of meters for months.

This acoustic monitoring consists of configuring the waterproof recorders, fixing them on a concrete mass (in order to keep them at the bottom of the water) and deploying them in the lake. After a number of days or months of recording, the recorders are removed from the water and the data are extracted and copied to a computer. Using acoustic signal analysis software (RavenPro), manatee vocalizations and feeding noises are identified and extracted. This is how we get information on the day and time of each vocalization and feeding sounds as well as the number and types of vocalizations, the fundamental, minimum and maximum frequencies and the duration of each vocalization.

Thanks to this new method, AMMCO and collaborators will be able to characterize the vocalization of the African manatee for the first time ever. The method will also help to assess the impact of the proliferation of Salvinia molesta on the distribution of the African manatee in Lake Ossa. This will also allow us to extend the acoustic monitoring to other species of the aquatic megafauna of Cameroon such as cetaceans.

The main difficulty encountered in this process is the lack of software that automatically detects the vocalization and feeding sound of the African manatee from the hydrophone recordings. Therefore, acoustic data analysis involves listening to and observing spectrograms of all recordings, which is very tedious. As a result, it takes either a considerable amount of time or a whole team to analyze long recordings. The collection of vocalizations our team is building will be critical to future development of automated tools to extract the vocalizations.

Bottom line, the hydrophones make it possible to identify the presence of the manatee remotely (without the in-situ presence of an observer) and this for a longer time and at a broader distance. We have recorded more than 800 African manatee vocalizations so far. Some of these vocalizations highlighted the presence of the manatee in Mevia, one of the areas of Lake Ossa highly invaded by salvinia.

It is still very early to draw conclusions. The certainty for now is that, like other sirenians, the African manatee utters vocalizations. Constant monitoring over a sufficiently consistent period, a year or more, combined with other detection methods is necessary to give an opinion on the impact of Salvinia molesta on the African manatee.