Seasonality of acoustic detections and distribution patterns of dolphins on the Northwest coast of Portugal

This thesis investigates the acoustic ecology of cetaceans along the northwest coast of Portugal, focusing on three delphinid species: short-beaked common dolphins (Delphinus delphis), bottlenose dolphins (Tursiops truncatus), and Risso’s dolphins (Grampus griseus). These species rely on vocalizations, such as whistles and echolocation clicks, for communication, recognition, and foraging. Utilizing Passive Acoustic Monitoring (PAM), the study analyses the distribution patterns of these dolphins and the background noise within the study area, examining correlations with human activities and infrastructures, and assessing their influence on dolphin presence and vocal behaviour. Additionally, it sought to compare acoustic data with visual observations. Results indicate a seasonal presence of dolphins, with a higher presence during summer compared to winter. Whistle production was more frequent in the northern regions of the study area and peaked during the summer months, correlating with higher encounter rates. Echolocation clicks were predominantly recorded in areas devoid of snapping shrimp presence, and those areas with low and high presence of clicks were also aligned with high dolphin encounter rates. Background noise exhibited a seasonal pattern, with peak levels occurring in winter. Among various human activities, only the density of vessel routes significantly contributed to the background noise. Despite this, neither dolphin presence nor their vocalizations appeared to be affected by the background noise levels recorded in the study.