Researching spinner dolphin population demography in Hawai’i using drones
Written by Fabien Vivier
In September 2021, we conducted boat-based surveys off the Kona coastline in search of spinner dolphins (Stenella longirostris). This field effort was part of my PhD thesis research focusing on quantifying the age structuration (proportion of calves, juveniles, and adults) of the spinner dolphin population off the Kona coastline. Please click on the following hyperlinks if you would like to know more about my work and previous blogs.
During the first two weeks of the expedition, we used one research vessel (operated by the excellent team of Captain Zodiac) and surveyed either the northern (up to ~75 km of coastline) or the southern (up to ~80 km of coastline) part of the Kona coastline every day. During the second two weeks of the expedition, we used two boats – also operated by Captain Zodiac. This time around, the objective was to survey as much of the Kona coastline as possible from North to South (see Figure X) each day. To accomplish this, I was joined by my colleague Liah McPherson, and additional team members to help conduct surveys. Each of the two boat teams was composed of one Unoccupied Aerial System (UAS, drone) pilot (either Fabien or Liah), two team-members (for photo-identification, data taking and UAS-related tasks) and one boat captain.
Every day, the team woke up around 04h15 and left the research house at 05h00 am to meet the boat crew at the Honokohau marina, where the boat(s) were getting prepared and readied by Captain Zodiac. Once at the marina, all research equipment was transferred to the boat and the boat was organized prior to a 05h30 / 05h45 am departure. Once at sea, we started scanning and searching for spinner dolphins as the coastal waters were surveyed within 250 m from shore. Once with a group of animals and depending on weather conditions / distance to Kona International Airport (where UASs cannot be operated for safety reasons), the team usually started with operating the drone at a maximum of 120 m over the animals to estimate the size of the group encountered. Once a group estimate was obtained, the drone was operated a lower altitude over the group to record videos of the animals when they surfaced. This type of imagery will allow for the estimation of the body length of each animal based on its blowhole to dorsal fin insertion distance (BHDF) as BHDF represents a good proxy for total body length. Such relationships were initially tested on free-ranging bottlenose dolphins (Tursiops truncatus) and on animals under humane care as part of my PhD work aiming at calibrating this UAS-photogrammetry method. Depending on group size, weather conditions and group cohesiveness, between one and several drone flights were required to collect all the data needed. Meanwhile, other team members worked closely with the boat captain to approach the animals safely and respectfully to photograph their dorsal fin, which are unique for each individual. Once a group was fully sampled, the encounter was terminated and the team resumed their search along the coast for other groups of animals, until the weather turned bad, the entire coastline was surveyed or no more spinner dolphins were encountered. On their way back to the marina, the boat teams usually searched for other species of cetaceans in deeper waters. On most days the teams were back on land between 14h00 – 15h30 and drove back to the research house for post-fieldwork chores. At the research house, research equipment needed to be cleaned and readied for the next day, drone batteries were recharged, notes were transcribed, and data were offloaded and backed up on separate hard drives. By the end of each day, lights were typically off by 19h45, as everyone needed to rest.
During the initial two weeks of this field expedition, we surveyed approximately 1,000 km of coastline per week, and ~860-900 km per week once two vessels jointly surveyed the whole coastline. On average, we encountered between 250 and 320 dolphins per day. A total of 31 h 29 min was spent operating the drone during 137 drone flights. A total of 72 groups of spinner dolphins were encountered of which over 50,000 photographs were taken. Preliminary analyses suggested that group size varied between 1 and 225 animals, with an average group size of ~50 individuals. Not only spinner dolphins were encountered during this trip, but also two pantropical spotted dolphins (Stenella attenuata, commonly mixed with some spinner dolphin groups), nine groups of bottlenose dolphins, two groups of short-finned pilot whales (Globicephala macrorhynchus), one group of false killer whales (Pseudorca crassidens) thanks to tagging effort from the Cascadia Research Collective, two groups of Cuvier’s beaked whales (Ziphius cavirostris), three groups of dwarf sperm whales (Kogia sima) and four sightings of oceanic white-tip sharks (Carcharhinus longimanus).
Overall, this field effort was a great success, and important information was collected towards my PhD thesis research. This success is the result of a team effort, and I would like to thank all the persons who joined and / or contributed to the project for the month: Lars and Aude for their intellectual, financial, logistic and overall support, Cormac Booth and Alex Brown from the SMRU consulting for their support with the sampling method, my lab mates Liah and Kirby, for their support in the field, the Captain Zodiac crew – Colin, Deron, J-C and Tim for the field logistics and efforts towards the project, Kyleigh, Madison, Brittini, Amanda and Aleigh, our volunteers, for their help with the data collection and daily tasks. Thank you also toto Trisha, Dawn and Whitney, the Hilo team, for committing a few days of their schedule every week.
The last two days of the expedition were for many the highlights of the trip, with many species encountered in one day. However, there will never be a better way of ending a field season than with the visit to an erupting volcano. On the third to last day of our field season, the Halemaʻumaʻu crater, at the summit of Kīlauea volcano started erupting. The team could not have left the Big Island without witnessing this with their own eyes. Even though it was still relatively early in the process of the eruption, the team had a fantastic experience.
Now it is time for the data processing phase of the project to start. This includes cataloguing and fin-matching all the animals that were photographed. Drone videos will need to be watched and rewatched to accurately estimate group sizes, and extract length measurements from the surfacing animals. Later, the age structuration of each group encountered will be estimated, based on the length of the animals, and results will be compiled into a PhD. chapter and manuscript and communicated to management agencies and local communities.
Special thanks to Colin from Captain Zodiac for his support and great work during this season.