Using drones, researchers assess the health of humpback whale mother-calf pairs across the Pacific Ocean
In a groundbreaking study published this week in The Journal of Physiology, biologists at the Marine Mammal Research Program (MMRP) at the University of Hawaii at Manoa Hawaii Institute of Marine Biology (HIMB) used drone imagery to advance understanding of how lactating humpback whales and their calves fare as they traverse the Pacific Ocean. Recent declines in North Pacific humpback whale reproduction and survival of calves highlight an urgent need to understand how mother-calf pairs expend energy across their migratory cycle. This work, done in close partnership with Alaska Whale Foundation, Pacific Whale Foundation and other partners, provides unprecedented insight into the life history of humpback whales across their migratory cycle, and provides key baseline data for understanding how rapid changes in ocean ecosystems are impacting humpback whales.
The team used drone imagery to measure calf growth and maternal body condition days after calf birth in Hawaii, and they then compared these measurements to the body conditions of humpback females in the Alaska feeding grounds, measuring pregnant and lactating females as well as humpback females whose reproductive status was unknown.
“We used drone-based photogrammetry to quantify the body size and condition of humpback whales on their Hawaiian breeding and Southeast Alaskan feeding grounds,” explains Martin van Aswegen, MMRP PhD candidate and lead author of the study. “A total of 2,410 measurements were taken from 1,659 individuals, with 405 repeat measurements from 137 lactating females used to track changes in maternal body volume over migration.”
The research shows that larger females produced larger, faster-growing calves. Over a 6-month period, lactating females decreased in body volume by an average of about 17%, whereas the calves’ body volume increased by nearly 395% and their length increased by almost 60%. In Hawaii, the team discovered that on average, humpback whale mothers lose nearly 214 pounds of blubber per day. Over a 60-day period, this is equivalent to losing roughly 50 tons of krill, or 25 tons of Pacific herring. Mother humpbacks in Hawai’i lost 20% of their body volume over 60 days of lactation, and the energy they used lactating surpassed the total energetic cost of their year-long pregnancies.
In the Southeast Alaskan feeding grounds, lactating humpback mothers were found to have the slowest rates of weight gain compared to non-lactating females, gaining about 32 pounds each day. Comparatively, pregnant and nonpregnant females gained weight at six and two times the rate of the lactating females, respectively.
“For me, the surprising part of this study was our ability to find the same individual mothers and calves over great distances and time periods,” shares van Aswegen. “For example, we obtained 405 repeat measurements from 137 lactating females in Hawai’i and Southeast Alaska, with eight of those mother-calf pairs measured in both locations within the same year. To measure the same whales over 3,000 miles apart over a period of roughly 200 days is truly remarkable and provides such valuable data for the questions we were asking.”
In Hawai’i, humpback whales are important cultural, economic, educational, and environmental pillars. Studies document a 76.5% decline in mother-calf encounter rates in Hawaii between 2013-2018, with birth rates declining by 80% from 2015 to 2016. In the SE Alaskan feeding ground, research reveals total reproductive failure in 2018, with calf survival decreasing tenfold from 2014 to 2019. These observations coincided with the longest lasting global marine heatwave, which shifted food webs and reduced availability of prey throughout the North Pacific. It is believed that humpback whales were unable to acquire sufficient food in their feeding grounds, resulting in nutritional stress and notable declines in reproduction and abundance.
This new study by MMRP refines our understanding of the energetic requirements for humpback whales to produce offspring, and it also highlights the important role Hawai’i holds as a critical breeding habitat. We now know that humpback whales are highly vulnerable during stages of early calf growth and lactation, which means it will be essential to carefully manage these waters. This information is key for considering how human activities could adversely impact not just humpback whale mother-calf pairs, but the survival of the humpback whale species.
“This work forms the basis for future studies investigating the energetic demands on humpback whales,” emphasizes Lars Bejder, MMRP director and co-author of the study. “Our humpback whale health database, comprising 11,000 measurements of 8,500 individual whales in the North Pacific, is being used across several projects within the Marine Mammal Research Program and abroad. These data become even more powerful when used in conjunction with fine-scale behavior and movement data (from biologging tags); reproductive and stress hormone data (from tissue and breath samples); and tissue data derived from post-mortem events. These studies will be used to better predict the resilience of large baleen whale species in the face of threats, including disturbance, entanglement, vessel collision, and climate change.”
Understanding and protecting humpback whales is a shared effort, requiring close partnership.
“Our ability to track individual humpback whales across thousands of miles and over months speaks to the power of collaboration,” notes Jens Currie, MMRP PhD candidate, Chief Scientist at Pacific Whale Foundation, and co-author of the study. “This study showcases how teamwork across disciplines and institutions helps us uncover the intricate relationships between maternal health, calf growth, and environmental stressors. Such partnerships are vital as we strive to protect humpback whales and their habitats amidst a changing climate.”
