A comparison of photo-ID and signature whistle based capture-recapture abundance estimates of common bottlenose dolphin

Providing abundance estimates for cetaceans is essential for their conservation and management. Passive acoustics provide a cost-effective option that is being increasingly explored. One promising approach is to use signature whistles within a capture-recapture framework. In this study, both photo-ID and acoustic data of common bottlenose dolphins (Tursiops truncatus) were collected between August and October 2019 off Northumberland, United Kingdom. A total of 26 surveys were conducted and 5771 h of acoustic data were recorded across three sites. The resulting photo-ID catalog contained 43 noncalf individuals and 45 unique signature whistle types were identified using the SIGnature IDentification method. Each dataset was analyzed using closed capture-recapture models and different modeling approaches were considered for the whistle data, including Spatially Explicit Capture Recapture (SECR). The resulting total abundance estimate from photo-ID data was 60 (95% CI 49 to 73) individuals. Using the photo-ID estimate as benchmark, the whistle data modeling approach that provided the most similar estimate was SECR, resulting in 66 (95% CI 56-104) individuals. The study provides further evidence of the applicability of using signature whistles for estimating abundance of bottlenose dolphins and provides the first abundance estimates for the species off Northumberland, United Kingdom.