The humpback whales and Caño Island (Costa Rica): a study of the structure of the songs of the Southern population and possible anthropogenic interactions

Acoustic communication is a fundamental component of the reproductive ecology of humpback whales (Megaptera novaeangliae). The marine protected waters surrounding the Caño Island Biological Reserve in Costa Rica serve as a critical wintering habitat and acoustic arena for the Southeastern Pacific humpback whale population, known as Breeding Stock G (BSG). The purpose of this study was to provide a comprehensive quantitative baseline of the vocal repertoire and hierarchical song structure of the BSG population wintering in this region between July 19 and September 29, 2024. Acoustic data were collected using a manual hydrophone, and 15 distinct song sessions comprising 5292 discrete sound units were analysed using Raven Lite 2.0. Fine-scale fundamental frequency (F0) metrics were extracted, and units were classified both aurally and spectrally to decode the song's syntax. Results revealed a highly structured composition heavily reliant on Mid-Frequency emissions (68% of the total repertoire), aurally dominated by the "Short moan" (30.8%) and the "Modulated cry" (13.1%). The fine-scale classification captured remarkable intra-seasonal cultural evolution. Specifically, a profound spectral shift was documented in late August, characterized by a sudden surge in High-Frequency vocalizations (34.5%) and a near abandonment of Low-Frequency sounds (0.9%). By late September, the repertoire shifted again, returning strictly to Mid-Frequency (75.5%) and Repetitive sounds, with a complete absence of High-Frequency units. The rapid modification of unit codes demonstrates active horizontal cultural transmission within the acoustic arena. Crucially, the drastic shift toward higher frequency registers in late August coincides temporally with the peak of the whale-watching and ecotourism season in the Osa Peninsula. This acoustic anomaly is hypothesized to be a behavioural compensation mechanism employed by singing males to avoid low-frequency acoustic masking generated by vessel engine noise. Ultimately, these findings highlight the extraordinary plasticity of humpback whale cultural transmission, while underscoring the urgent need to implement acoustically informed conservation strategies and passive acoustic monitoring (PAM) within the marine protected area to mitigate the impacts of anthropogenic noise on this endangered population.