Acoustic Identity: Linking Signature Whistles and Visual Identification in a Threatened Dolphin Population

Signature whistles (SW) are a significant element of the vocal repertoire of delphinid species. They encode self-identifying information and allow social cohesion between related and allied individuals through often complex, stereotyped whistle contours. SW recordings allow researchers to explore the presence of individual animals, understand social dynamics, and estimate population size without visual observation. This represents the first study aiming to match distinct SW contours to individual Burrunan dolphins (Tursiops australis) in the Gippsland Lakes, Victoria, Australia. Simultaneous photographic identification and in situ hand-held hydrophone recordings collected in 2021-2024 contained 57 individual dolphins and 236 whistles, resulting in 22 unique SW contours being extracted following modified SIGnature IDentification (SIGID) criteria. Conditional probability analyses revealed that most SW contours were associated with clusters or pairs of dolphins, rather than individuals, reflecting the species' fission-fusion social structure. Cluster analysis supported these associations, highlighting the difficulty in isolating SWs in a population where individuals are not observed in isolation. Additionally, social structure, philopatric behaviour, and the acoustic environment of transient individuals are suggested to influence signature whistle production and complexity. Although direct signature whistle-to-individual matching remains limited, the study demonstrates that population-level and subgroup-level monitoring by passive acoustic methods is achievable. This approach provides a critical acoustic tool for conservation management of the small, threatened Gippsland Lakes Burrunan dolphin population and offers a foundation for future study of individuals through passive acoustic monitoring.