Implementing a Novel Approach to Quantify the Age-Structure of Free-Ranging Delphinid Populations Using Unoccupied Aerial System Photogrammetry

Abstract

Evaluating the status of free-ranging wildlife populations is crucial for developing effective conservation and management strategies. Traditional monitoring approaches for cetaceans, are often costly, time-consuming, and lack the statistical power to detect changes in population trajectory. Therefore, alternative monitoring approaches are necessary to evaluate population stability and future trends. Stable populations typically maintain a consistent proportion of calves, juveniles, and adults, whereas deviations from this structure may indicate population growth or decline. Changes in age-structure can influence vital rates, offering insights into future population trends. In this thesis, I used Unoccupied Aerial System (UAS) photogrammetry to develop and implement a novel approach for quantifying the age-structure of free-ranging delphinid populations. I applied this method to two case studies representing extreme scenarios of group living in delphinids: one with average group sizes of <10 individuals and another with groups often reaching hundreds. In Chapter 2, I evaluated the accuracy of UAS-photogrammetry for estimating total body length (TL) of trained dolphins (Tursiops truncatus) using the blowhole-to-dorsal fin distance (BHDF) during surfacing events. I assessed its performance for age classification by simulating UAS-derived BHDF estimates using a 35-year age-length dataset from a free-ranging bottlenose dolphin community in Sarasota Bay, USA. The results showed that the TL of surfacing dolphins was overestimated by 3.3% (±3.1 SE), with 72% accuracy in age-class assignment. In Chapter 3, I combined UAS-photogrammetry data with long-term (>20 years) photo-identification data to assess the age-structure of the critically endangered sub-population of common bottlenose dolphins in the Gulf of Ambracia, Greece. I obtained a representative population age-structure estimate in five days of annual UAS-sampling. Subsequently, I compared the age-structure of the Greek population with two extensively studied non-endangered bottlenose dolphin populations: Indo-Pacific bottlenose dolphins (Tursiops aduncus) in Shark Bay, Australia, and common bottlenose dolphins in Sarasota Bay, USA. The results suggest careful consideration is needed when interpreting age-structure estimates across populations. In Chapter 4, I quantified the age-structure of a spinner dolphin (Stenella longirostris) stock in Hawaiʻi, the world’s most exposed dolphin population to human disturbances. This stock faces long-term viability concerns amid rising human activities and has been a NOAA management priority since 2005. My findings highlight concerns about a low calf proportion, indicating the need for ongoing monitoring and management by NOAA. In conclusion, this thesis demonstrates the use of UAS-photogrammetry as a promising and reliable tool for monitoring the age-structure of free-ranging delphinid species. UAS-photogrammetry has the potential to inform management more rapidly than traditional survey methods, particularly for large pelagic stocks.