Understanding the population health status of long-lived and slow-reproducing species is critical for their management. However, it can take decades with traditional monitoring techniques to detect population-level changes in demographic parameters. Early detection of the effects of environmental and anthropogenic stressors on vital rates would aid in forecasting changes in population dynamics and therefore inform management efforts. Changes in vital rates strongly correlate with deviations in population growth, highlighting the need for novel approaches that can provide early warning signs of population decline (e.g., changes in age-structure). We tested a novel and frequentist approach, using Unoccupied Aerial System- (UAS) photogrammetry, to assess the population age-structure of small delphinids. First, we measured the precision and accuracy of UAS-photogrammetry in estimating total body length (TL) of trained bottlenose dolphins (Tursiops truncatus). Using a log-transformed linear model, we estimated TL using the blowhole-to-dorsal-fin-distance (BHDF) for surfacing animals. To test the performance of UAS-photogrammetry to age-classify individuals, we then used length measurements from a 35-year dataset from a free-ranging bottlenose dolphin community to simulate UAS-estimates of BHDF and TL. We tested five age-classifiers and determined where young individuals (<10 years) were assigned when misclassified. Finally, we tested whether UAS-simulated BHDF only or the associated TL estimates provided better classifications. TL of surfacing dolphins was overestimated by 3.3% ±3.1% based on UAS-estimated BHDF. Our age-classifiers performed best in predicting age-class when using broader and fewer (two and three) age-class bins with ~80% and ~72% assignment performance, respectively. Overall, 72.5-93% of the individuals were correctly classified within two years of their actual age-class bin. Similar classification performances were obtained using both proxies. UAS-photogrammetry is a non-invasive, inexpensive, and effective method to estimate TL and age-class of free-swimming dolphins. UAS-photogrammetry can facilitate the detection of early signs of population changes, which can provide important insights for timely management decisions.

We physically measured TL (i.e., the tip of the rostrum to the tip of the natural notch created by the overlapping fluke lobes, hereafter referred to as the notch) and BHDF for 18 bottlenose dolphins under human care at two facilities in Hawaiʻi, USA. The distance from the center of the blowhole to the anterior insertion of the dorsal fin (BHDF) is an established proxy for TL in bottlenose dolphins (Cheney et al., 2018; van Aswegen et al., 2019). Six adult males ranging from 11.5 to 34.5 years of age (mean = 23.6 ±7.9 years) at Dolphin Quest Oʻahu (DQO); HI, USA, were measured in June 2019. Six females and six males ranging from 4.0 to 49.0 years of age (mean = 17.4 ± 14.8 years) at Dolphin Quest Hawaiʻi (DQH); HI, USA, were measured in August–October 2019. The date of birth (DOB) of the 14 individuals born in facilities is known. The other four individuals (two males and two females) were born in the Gulf of Mexico. The age of these animals was based on the size that they were when collected. Dolphins were measured in a stationary and straight position for all measurements. TL was collected on the ventral side of the dolphin in an inverted position using a tape measure attached to a rigid PVC pipe. The base of the measuring pipe was placed onto a rigid plate aligned with the tip of the rostrum to allow for straight-line measurements. BHDF measurements were made from the center of the blowhole to the insertion of the dorsal fin using a soft measuring tape. One measurement set (consisting of 2–3 replicates per measurement) was collected on the day or within a week of the UAS sampling. To increase sample size, 4–6 additional replicates were collected within the next seven months (total of 7–10 TL and BHDF measurements per animal). DQH measurements per animal were collected on the same day.

A DJI Inspire-2 quadcopter was used to collect aerial imagery. The Inspire-2 was equipped with a DJI Zenmuse X5s digital camera (20.8-megapixel, Micro Four Thirds format; calibrated following Dawson et al. (2017)) with an Olympus M.Zuiko 25 mm f/1.8 lens. A LightWare SF11/C laser altimeter (Dawson et al., 2017) was attached, providing an accuracy of 0.1 m and resolution of 1 cm. Despite the precision, some inaccurate altitude readings were recorded. To correct these errors, a custom-made smoother was applied to the original data. The Inspire-2 recorded videos in 4k resolution (3840x2150 pixels). Consecutive flights using both platforms (n=24 flights in total) were conducted at five altitudes (16m, 20m, 30m, 40m and 50m).

Dolphins were sampled under two scenarios: stationary and positioned flat and straight in the water and with the slight arching that occurs when surfacing naturally while swimming. Stationary animals were supported by husbandry staff under the caudal region to maintain the body straight and the fluke flat. Photogrammetry of stationary behaviors was collected to compare UAS measurements of TL and BHDF (Fig. 2) with the respective physical measurements.

A target of three images was selected per individual, altitude, and behavior (i.e., stationary and surfacing) combination. Images were extracted using VLC Media Player Software (VideoLAN). For surfacing dolphins, video stills and photographs were selected when both the blowhole and dorsal fin insertion were visible and when the individual’s body was as straight and horizontal as possible (i.e., minimal body arch). Available images of sufficient quality varied by platform. In total, 144 video stills (75 stationary and 69 surfacing) of sufficient quality were used. Due to weather or the lack of images of sufficient quality,  individual F was removed from the analyses.

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