Biologists utilized drone technology to investigate the journey of nursing humpback whale mothers and their calves across the Pacific Ocean. Recent drops in calf survival and reproduction rates for North Pacific humpback whales underscore the importance of understanding the energy expenditure of mother-calf pairs during their migration cycle.
In a pioneering study published this week in The Journal of Physiology, scientists from the Marine Mammal Research Program (MMRP) at the University of Hawaii at Manoa Hawaii Institute of Marine Biology (HIMB) employed drone imagery to enhance our understanding of how nursing humpback whales and their young thrive as they navigate the Pacific Ocean. The declining rates of reproduction and calf survival among North Pacific humpback whales highlight the critical need to explore how mother-calf pairs utilize energy throughout their migratory journey. This research, conducted in collaboration with the Alaska Whale Foundation, Pacific Whale Foundation, and other partners, offers unprecedented insights into the lifecycle of humpback whales across migration, delivering essential baseline information to comprehend how swift changes in ocean environments are affecting these majestic creatures.
The research team harnessed drone imagery to assess calf growth and the body condition of mother whales shortly after calf birth in Hawaii. They compared these findings with the body conditions of humpback females in the feeding areas of Alaska, evaluating pregnant and nursing whales, as well as those of uncertain reproductive status.
“We employed drone-based photogrammetry to measure the body size and condition of humpback whales in their Hawaiian breeding areas and Southeast Alaskan feeding zones,” states Martin van Aswegen, MMRP PhD candidate and lead author. “We collected a total of 2,410 measurements from 1,659 whales, including 405 repeated measurements from 137 nursing females, allowing us to monitor changes in maternal body volume throughout migration.”
The findings indicate that larger female whales tend to give birth to larger, faster-growing calves. Over a period of six months, nursing females experienced an average body volume reduction of about 17%, while their calves’ body volume surged nearly 395% and their length increased by almost 60%. In Hawaii, it was discovered that humpback mothers typically shed roughly 214 pounds of fat each day, equating to about 50 tons of krill or 25 tons of Pacific herring within 60 days. During this 60-day breastfeeding period, mother humpbacks lost 20% of their body volume, and the energy consumed during lactation surpassed the total energy required for their year-long pregnancies.
In the Southeast Alaskan feeding grounds, nursing mothers gained weight at the slowest pace compared to non-nursing females, adding only about 32 pounds daily. In contrast, pregnant and non-pregnant females gained weight at rates six and two times faster than nursing mothers, respectively.
“What astonished me most was how we managed to locate the same mothers and calves over extensive distances and durations,” mentions van Aswegen. “We gathered 405 repeated measurements from 137 nursing females in Hawaii and Southeast Alaska, with eight pairs measured in both regions within a year. Tracking the same whales across 3,000 miles over approximately 200 days is genuinely incredible, yielding invaluable data for our research questions.”
In Hawaii, humpback whales represent vital cultural, economic, educational, and ecological elements. Research has documented a startling 76.5% decrease in mother-calf encounters from 2013 to 2018, with birth rates plummeting by 80% between 2015 and 2016. In Southeast Alaska, studies revealed a total reproductive failure in 2018, with calf survival rates decreasing ten times from 2014 to 2019. These trends coincided with the longest global marine heatwave, which altered food webs and diminished prey availability throughout the North Pacific. It is believed that humpback whales struggled to find enough food in their feeding territories, resulting in nutritional stress and significant reductions in reproduction and population numbers.
The findings of this study enhance our comprehension of the energy demands placed on humpback whales when raising offspring and emphasize the crucial role Hawaii plays as a vital breeding ground. We now understand that humpback whales are particularly susceptible during the initial stages of calf growth and lactation. As such, it is vital to manage these waters with care. This information is critical for evaluating how human actions could negatively affect not only mother-calf pairs but also the overall survival of the humpback whale population.
“This research lays the groundwork for future investigations into the energetic demands faced by humpback whales,” notes Lars Bejder, MMRP director and co-author of the study. “Our health database for humpback whales, containing 11,000 measurements from 8,500 individual whales in the North Pacific, is being employed in various projects both within the Marine Mammal Research Program and internationally. This data gains more significance when integrated with detailed behavioral and movement data (gathered from biologging tags), reproductive and stress hormone data (obtained from tissue and breath samples), and post-mortem tissue data. Such studies will help us predict the resilience of large baleen whale species against threats including disturbances, entanglements, vessel collisions, and climate change.”
Understanding and safeguarding humpback whales is a collective endeavor that requires close collaboration.
“Our ability to monitor individual humpback whales over thousands of miles and months demonstrates the strength of teamwork,” states Jens Currie, MMRP PhD candidate, Chief Scientist at the Pacific Whale Foundation, and co-author of the study. “This research highlights how interdisciplinary collaboration helps us unveil the complex relationships between maternal well-being, calf development, and environmental challenges. Such partnerships are crucial as we work to protect humpback whales and their habitats in a changing climate.”
