Invisíveis porém detectáveis: a utilização do monitoramento acústico passivo em peixes-boi da Amazônia (Trichechus inunguis).

Abstract

Direct observation of Amazonian manatees (Trichechus inunguis) in nature is very difficult due to their cryptic behaviour and low water visibility. Alternative non-invasive methods are needed to monitor the populations of this species considered vulnerable. Trichechus inunguis is known to emit sounds in captivity, and their vocalizations differ between sex and age, allowing us to potentially estimate the population structure by Passive Acoustics Monitoring (PAM). Bearing this in mind, the objectives of this study are: 1) assess whether Amazonian manatees can be acoustically detected in nature through PAM; 2) determine, in captivity, if there is a relation between the number of acoustic detections and the number of individuals; and 3) verify if the number of acoustic detections in captivity increases in the breeding season, i.e. with the presence of calves. Recordings were held in captivity in 1998 and 2016, and in nature along the Amazon River Basin between June and July 2001 in Brazil and in July 2012 in Peru. The acoustic parameters of vocalizations recorded in nature are similar to those registered in captivity. Recordings in nature occurred after the birth period, so it is possible that the vocalizations are predominantly of lactating females and their calves. The use of playback for recordings in Brazil may have stimulated the vocal response of some individuals. It is possible to monitor populations of T. inunguis in nature through PAM, especially during and after the birth period. However, there was no significant relationship between the vocalizations and the number of individuals, but it was verified in captivity that the presence of calves increases the number of acoustic detections.