First Description of the Underwater Sounds in the Mediterranean Monk Seal Monachus monachus in Greece: Towards Establishing a Vocal Repertoire

Abstract

The Mediterranean monk seal Monachus monachus is one of the most endangered pinnipeds in the world, and is classified as “Endangered” by the International Union for the Conservation of Nature. Any additional knowledge about the species is invaluable to its effective conservation. In the present study, we deployed an autonomous underwater recorder in an important reproductive area of the Mediterranean monk seal in Greece to describe its underwater vocal repertoire. Over the 330 h of continuous recordings, 9231 vocalizations were labelled as potentially produced by Mediterranean monk seals, and 1694 good quality calls were analyzed. We defined 18 call types divided into three main call categories: harmonic, noisy, and pulsative calls. We also described the soundscape in which this endangered species lives and found that human activities around the two main pupping caves had a strong impact on the sonic environment of these seals: the noise level produced by boat traffic was high, and occurred on an hourly (25 to 50 min/hour) and daily basis (10.8 to 16.9 h/day). Such high levels of noise might not only impair the communication of the species, but also impact its survival, as chronic noise can induce physiological stress.

First assessment of passive acoustics as a tool to monitor the endangered Mediterranean monk seal in the Madeira Archipelago (Portugal)

“…On the other hand, hiccups were found to have higher peak frequency and frequency quartiles than growls. The mean values of all the acoustic variables evaluated for the /growl/ sound type in the present study fell within the range of values obtained by Charrier et al (2023). In the case of the /hiccup/ sound type, its duration overlaps part of the range of values also reported by Charrier et al (2023), while the peak frequency and frequency quartiles Q25, Q50 and Q75 fell within the range described by those authors.…”

“…The use of PAM in this study allowed, for the first time, the detection of underwater sounds putatively produced by the Mediterranean monk seal in the Madeira Archipelago. These sounds resemble acoustically and aurally those described by Charrier et al (2023) for M. monachus. Moreover, the values of the acoustic variables mostly fell within the range reported by the same authors, fitting the characteristics of Mediterranean monk seal's calls.…”

“…Considering the 18 call types reported in the literature for this species in a Mediterranean population (Charrier et al, 2023), at least two of them, the /growl/ and the /hiccup/ types (noisy sounds restricted to frequencies below 700 Hz) seem to be present in the Madeira Archipelago population. However, the putative growl sounds detected in the Madeira Archipelago are shorter and do not reach the higher frequency components of the Mediterranean population.…”

“…A total of 61 putative seal sounds were identified based on the qualitative and quantitative similarity to underwater sounds reported for the Mediterranean monk seal (Charrier et al, 2023). Considering the aural and visual characteristics from the spectrograms and the values of the acoustic variables measured, the sounds were classified into two types of low-frequency noisy sounds without a clear harmonic structure and named as /growl/ and /hiccup/ (Figure 2; audio recordings are provided as Videos S1 and S2).…”

“…However, to use passive acoustics, details of the vocal behaviour must be available for the species (Sills & Reichmuth, 2022). Recently, Charrier et al (2023) published a comprehensive analysis of the underwater vocal repertoire of M. monachus in a Mediterranean population, providing crucial baseline data and information that can be used as reference for underwater acoustic monitoring of this endangered marine mammal. Phocids, like other pinnipeds, are highly vocal during the breeding season, producing vocalizations usually associated with territory defence by males, mate selection and mother-pup interactions (Charrier, 2021;Charrier & Casey, 2022), including the production of underwater vocalizations (Charrier et al, 2023).…”