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Photogrammetry of blue whales with an unmanned hexacopter

2016; Wiley; Volume: 32; Issue: 4 Linguagem: Inglês

10.1111/mms.12328

1748-7692

John W. Durban, Michael J. Moore, Gustavo Chiang, Leigh S. Hickmott, Alessandro Bocconcelli, Gloria Howes, Paulina Bahamonde, Wayne L. Perryman, Donald J. LeRoi,

Advanced Measurement and Detection Methods

Baleen whales are the largest animals ever to live on earth, and many populations were hunted close to extinction in the 20th century (Clapham et al. 1999). Their recovery is now a key international conservation goal, and they are important in marine ecosystems as massive consumers that can promote primary production through nutrient cycling (Roman et al. 2014). However, although abundance has been assessed to monitor the recovery of some large whale populations (e.g., Barlow et al. 2011, Laake et al. 2012) many populations are wide-ranging and pelagic, and this inaccessibility has generally impeded quantitative assessments of recovery (Peel et al. 2015). To augment traditional abundance monitoring, we suggest that photogrammetric measures of individual growth and body condition can also inform about population status, enabling assessment of individual health as well as population numbers. Photogrammetry from manned aircraft has used photographs taken from directly above whales to estimate individual lengths (Gilpatrick and Perryman 2008) and monitor growth trends (Fearnbach et al. 2011), and shape profiles can be measured to assess body condition to infer reproductive and nutritional status (e.g., Perryman and Lynn 2002, Miller et al. 2012). Recently, Durban et al. (2015) demonstrated the utility of an unmanned hexacopter for collecting aerial photogrammetry images of killer whales (Orcinus orca); this provided a noninvasive, cost-effective, and safe platform that could be deployed from a boat to obtain vertical images of whales. Here we describe the use of this small, unmanned aerial system (UAS) to measure length and condition of blue whales (Balaenoptera musculus), the largest of all whales. We used an APH-22 hexacopter (Aerial Imaging Solutions, Old Lyme, CT) to photograph blue whales in a known feeding area in the Gulf of Corcovado and Gulf of Ancud, southern Chile (Fig. 1; Hucke-Gaete et al. 2004). This hexacopter is described in detail by Goebel et al. (2015), and was used successfully during recent boat-based photogrammetry of killer whales by Durban et al. (2015). In our study, this small (<2 kg, 82 cm wingspan) hexacopter was deployed from an 18.6 m wooden-hulled boat. The hexacopter was operated within line-of-sight by a pilot using a 2.4 GHz radio control from the boat, and it was hand launched and retrieved from the foredeck of the vessel by a second person, who functioned as the ground station operator (Fig. 2). When one or more whales were sighted from the vessel, the animals were approached within 300 m, and after a sense of their surfacing behavior had been established, the hexacopter was launched to an altitude of 50–60 m to be ready overhead for the next surfacing. The pilot flew the UAS out to the whales from the ship until the animals were evident in the downward-looking video feed from the onboard camera (Olympus E-PM2 with M.Zuiko 25 mm F1.8 lens), which was transmitted in real time by 5.8 GHz radio link to the ground station. The ground station operator then advised the pilot on fine-scale adjustments to frame the animals as they surfaced, and the pilot used a remote link to trigger the capture of high-resolution (16 MP) still images on the camera's flash memory, which provided a resolution of <2 cm from our operational altitudes (Durban et al. 2015). In the period 22 February to 8 March 2015, we attempted 59 flights over blue whales, 37 of which were successful in collecting whale images. No change in the behavior of the whales was observed when the hexacopter was overhead. Whale dive times typically lasted 8–12 min, and flights averaging 12.1 min (maximum = 17.3 min) were conducted in an attempt to overlap with at least 1–2 surfacing bouts. The total distance flown during a flight averaged 1,286 m (maximum = 3,336 m), but the distance to the pilot was less (typically <300 m and always <500 m) as the boat was continuously maneuvered to enable line-of-site piloting. Flights were restricted to times when wind speeds were <8 m/s (15 knots). Image measurements were calculated as previously described, using altitude and focal length to scale from image pixels to real measurements (Fearnbach et al. 2011) using altitude estimates from the air pressure sensor on the hexacopter (Durban et al. 2015). The accuracy of this approach was validated by seven measurements (on four different days) of the overall length of the research ship of approximate whale length (18.6 m). From altitudes of 50–56 m, the average measurement bias was just 0.03 m (range = 0.8 m), representing <0.002% (range <5%) of the total length of the boat. Estimates of individual whale length showed similar consistency. Whales were measured for body length from rostrum tip to tail notch in images that showed a flat surfacing orientation (Fig. 3). Individual whales were identified based on unique pigmentation patterns and distinctive scars (e.g. Gilpatrick and Perryman 2008). A total of 22 individual whales were measured in 1–7 images, and six whales with 4–7 repeat measurements all showed variability of <5% (range = 3.0%–4.3%) around average body lengths ranging from 18.9 m to 22.1 m. We selected the longest body length measurement image of each whale as the most robust to negative bias, which could occur if the whale was angled towards the surface or had a rounded back during surfacing (e.g., Fearnbach et al. 2011). The 22 whales ranged in length from 14.4 m to 23.6 m (Fig. 4). The two shortest animals in the data set (estimated lengths of 14.4 m and 15.5 m) appeared from field observations to be dependent calves; the two presumed mothers had lengths of 22.7 m and 22.2 m, respectively. These measurements were consistent with lengths of blue whales previously measured using photogrammetry from manned aircraft (Gilpatrick and Perryman 2008), particularly for those whales photographed in the Eastern Tropical Pacific (females with calves averaged 22.4 m in length), which have been genetically linked to the whales feeding off southern Chile (Torres-Florez et al. 2014). Body width measurements were also taken at the point on the whales' body that equaled 40% of the body length from the rostrum, where widths were variable in whales of adult size (Fig. 3, 4). These measurable differences between whales likely indicated individual variability in body condition (Fig. 3). This was the first study to use an unmanned aircraft to obtain quantitative photogrammetric measurements from large whales. It demonstrated the utility of a small hexacopter to be safely and efficiently deployed from a large boat platform to quickly position a camera above whales, which spent limited time at the surface. This builds on the successful use of the same aircraft from a smaller boat to study gregarious killer whales (Durban et al. 2015). Although hexacopter operations have a limited scale compared to wide-ranging manned aircraft, this study again demonstrated the hexacopter to be noninvasive, with a limited sound footprint (Goebel et al. 2015) that enables photographs to be obtained from lower altitude than manned aircraft without disturbing the whales. As a result, high resolution images can be coupled with onboard estimates of altitude to resolve differences in whale morphometrics with high precision (within centimeters). The data we collected demonstrate the potential for obtaining repeated estimates of length and width to monitor changes in growth and body condition of blue whales over time, and this utility should also extend to other whale species. As large whales recover from exploitation and approach carrying capacity (e.g., Laake et al. 2012) we will see them respond to variability in the environment (e.g., Perryman et al. 2002). Monitoring individual growth and condition will therefore be important for understanding population dynamics and monitoring responses to environmental change, and we anticipate that portable UAS platforms, like the hexacopter used here, will become key research tools. We thank María Francisca Cortés Solari, Rafaela Landea Briones, MERI Foundation, and the Woods Hole Oceanographic Institution Access to the Sea Fund for funding this work. We also appreciate the support of Pesquera Los Elephantes, Thomas and Jose Montt, and the crew of the M/V Centinella. This research was conducted under Chilean Permit MERI-488-FEB-2015.