Do sea lions fall silent when fish have something to say?

Coastal soundscapes are complex systems composed of many biological sounds, among which fish often play a central role. Marine mammal vocalizations can also contribute substantially, either when animals pass through an area or because their loud calls can be heard from offshore. Sea lion colonies, known for their intense aerial vocalizations, may also represent a major sound source around breeding areas. However, how colonies shape the underwater acoustic environment remains poorly understood. This study focused on the barking of California sea lions (Zalophus californianus) at Los Islotes (Gulf of California), to quantify diel and seasonal patterns of vocal activity and its interaction with the surrounding biophony. We developed an AI-based detector trained on BirdNET to identify major biophony components. Barks dominated the diurnal soundscape, while fish sounds dominated just after the end of twilight. Their diel peak coincided with a reduction in sea lion vocalizations. Fish sound abundance increased from April–May to June–July. Bark abundance remained stable. The negative diel correlation between bark and fish sound abundance highlights temporal partitioning among taxa, illustrating the integration of aquatic–aerial soundscapes and demonstrating the potential of passive acoustics to study pinniped vocal behaviour and its interplay with reef biophony.