Feasibility study: quantifying the body condition of humpback whales in Alaska

Written by Martin van Aswegen

I recently participated in a collaborative project involving the Marine Mammal Research Program (MMRP) from the University of Hawaii at Manoa and the Alaska Whale Foundation (AWF) in southeast Alaska. During the course of the season, we conducted a feasibility study trialling the use of unoccupied aerial vehicles (UAVs) to obtain morphometric measurements of North Pacific humpback whales. More specifically, the aim of the study was to quantify the body condition of these whales on their Alaskan feeding grounds. The overall aim of the project is to quantify the bio-energetic demands of humpback whales migrating between the Alaskan feeding grounds and the Hawaiian breeding grounds. Over the coming years, the MMRP and the AWF will be conducting morphometric measurements of humpback whales in both of these important whale habitats.

The Location: Wild Alaska

I joined the AWF team, consisting of Dr. Andy Szabo, Dr. Fred Sharpe and their team of dedicated international volunteers, for six weeks between early July and mid-August 2018. Our research station was situated in Baranof Warm Springs, a small seasonally-occupied community situated on the east coast of Baranof Island, adjacent to the deep, productive waters of Chatham Strait (Fig. 1). We were nestled near the base of a waterfall, with the glacially fed Baranof Lake sustaining an ever-present roar within the bay. The lush, temperate rainforests surrounding our location are pristine, hosting one of the highest densities of brown bears in the world. Suffice it to say, we were working in a pretty awe-inspiring place.

Figure 1. Map of the approximate study area covered in July and August 2018 (blue shade). The relative location of Baranof Warm Springs is indicated by the black dot. Map courtesy of Róbert Szűcs (Grasshopper Geography - grasshoppergeography.etsy.com).

The AWF has studied humpback whales in southeast Alaska since 1996, developing a long-term database on the demographics (age, sex, reproductive class) and behavioral traits of these animals. We wanted to determine the feasibility of identifying individual whales using the UAV, by linking photo-identification images taken from the research vessel with the corresponding fluke displayed in the UAV video. This is easier said than done, as humpback whales are usually identified by the unique patterns on the bottom of their tail fluke, an area not typically visible from a UAV hovering overhead. By measuring whales with known life history, we can sample the same individuals repeatedly over an extended period of time, allowing us to investigate fine-scale changes in body condition throughout the feeding season. Where possible, we targeted mother-calf pairs as well as whales known to participate in specialised foraging traditions such as krill foraging, hatchery depredation and cooperative bubble-netting. The ability to link long-term life history and behavioral data with UAV measurements allows us to investigate the body condition of whales across age, sex and reproductive class, as well as foraging tradition.

Methodology

For this study, we used a DJI Inspire 2 quadcopter equipped with a Zenmuse X5S camera capable of capturing 20.8 MP still images and 60fps 4K video (Fig. 2A). The UAV was hand-launched and retrieved from the research vessel, with flight times typically ranging between 15 and 20 minutes. I operated the UAV by means of a handheld controller, with a high-resolution screen providing a live feed of the UAV’s point of view.

Figure 2. The Inspire 2 quadcopter used in the field (A). Note the Zenmuse X5S camera on the front with the range finder positioned on the bottom of the drone. Image B shows the Inspire 2 positioned above a fluking humpback whale. Images: Rocio Prieto Gonzalez under permit 19703.

Positioned directly above surfacing whales (Fig. 2B and Fig. 3), the UAV recorded video footage with a range finder simultaneously measuring fine scale shifts in UAV altitude and orientation. The UAV altitude and camera focal length are used to scale the image, allowing us to convert the length of a whale (in pixels) into an absolute length (measured in metres). By measuring the total length and width (in 5 % intervals) of an individual whale, we were able to calculate its body volume or relative body condition, enabling us to establish an index of health. For a full description of the methodology, please see Christiansen et al. 2018 (Marine Ecology Progress Series).

Figure 3. An example still frame of a humpback whale from video footage captured by the Inspire 2 quadcopter. On a calm sunny day, it is possible to capture the entire body outline of the whale, allowing for accurate quantification of body length and width.

A new perspective…

The ‘bird’s eye view’ perspective delivered by the UAV also provided us with a unique insight into the behavior displayed by humpback whales in southeast Alaska. These whales are renowned for an extraordinary foraging tactic as complex as it is awe-inspiring – cooperative bubble-net foraging. As the name suggests, these whales work together to trap herring inside a cylindrical net of bubbles, pulsed from the blowholes of one or more whales. The panicked herring are driven into the bubble net by sound - a loud feeding call produced by only a handful of whales in southeast Alaska. As the bubbles slowly pop on the surface, multiple whales (up to 25 individuals) lunge upward from within the bubble net, engulfing their prey (Fig. 4). The whales then regroup and respire on the surface, before diving in preparation for the next lunge. To see this process in action, please watch the video accompanying this blog.

Dr. Sharpe and the AWF have studied this complex behavior for more than two decades, using photo identification, focal follow and acoustic analysis to unravel the social, behavioral and ecological intricacies involved. Of the few thousand whales visiting southeast Alaska during the summer feeding season, approximately 60 non-related individuals are believed to be regular participants in this bubble-netting behavior. By repeatedly filming these events from beginning to end, we hope to investigate a number of questions relating to social structure, allocation of roles and foraging ecology.

Figure 4. An aerial perspective of lunging humpback whales.

Preliminary sample sizes are encouraging!

After a relatively quiet start, whale numbers increased between late July and mid-August. By the end of the field season, we had conducted 90 UAV flights over 13 boat days. While data processing is ongoing, we have measured approximately 100 individuals so far. We only encountered three mother-calf pairs throughout the season, continuing worrying trends of record-low sightings observed over the last few years. We obtained body condition measurements for approximately 50 regular bubble-net participants across 14 sightings. This represents approximately 83 % of the known bubble-netting subpopulation in southeast Alaska, with about a dozen individuals repeatedly measured over a period of three weeks.

What now?

Following the successful field season, we are currently developing a UAV sampling protocol that will be used in both southeast Alaska and Hawaii, integrating the many lessons learned during this encouraging feasibility study. This will provide a standardized field procedure for future fieldwork. Sampling effort will now be focused on comparing humpback whale body condition at key stages throughout the year, including early- and mid-foraging season in addition to pre-southbound/post-northbound migration. We will be conducting similar measurements of humpback whale body condition on the Hawaiian breeding grounds in early 2019.

We are currently preparing for a two-week sampling trip in November 2018. This field work will be conducted in Sitka Sound, where anecdotal evidence indicates an increase in the number of whales forgoing their southbound migration to Hawaii.

Permits

All research activities were conducted in accordance with NMFS ESA/MMPA Permit No. 19703. The UAV operator holds a current unmanned aerial system (UAS) license certified by the Federal Aviation Administration (Cert No. 4153234).