Biologists utilized drone imagery to gain insights into the welfare of nursing humpback whale mothers and their calves during their journey across the Pacific Ocean. A recent decline in the reproduction rates and survival of North Pacific humpback whale calves emphasizes the importance of understanding the energy expenditure of these mother-calf pairs throughout their migration.
This week, a pioneering study published in The Journal of Physiology by biologists 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 comprehension of how lactating humpback whales and their calves navigate the Pacific Ocean. The noticeable drop in North Pacific humpback whale reproduction and calf survival signifies a pressing need to understand the energy usage of mother-calf pairs during their migratory journey. Collaborating closely with the Alaska Whale Foundation, Pacific Whale Foundation, and other partners, this research offers unprecedented insights into the life cycle of humpback whales throughout their migratory journey, providing essential baseline data to assess the effects of rapid ocean ecosystem changes on humpback whales.
The researchers harnessed drone imagery to evaluate calf growth and the body condition of mothers shortly after calf birth in Hawaii. They then compared these observations with the body conditions of female humpback whales in Alaska’s feeding grounds, examining both pregnant and lactating females, along with those with uncertain reproductive statuses.
“We employed drone-based photogrammetry to assess the body size and condition of humpback whales in their Hawaiian breeding areas and Southeast Alaskan feeding grounds,” states Martin van Aswegen, an MMRP PhD candidate and lead author of the study. “In total, we conducted 2,410 measurements across 1,659 individuals, including 405 repeat measurements from 137 lactating females, allowing us to monitor changes in maternal body volume during migration.”
The findings reveal that larger female whales had calves that grew larger and at a faster rate. Over a six-month period, lactating mothers showed an average body volume decrease of about 17%, while their calves’ body volume significantly increased by nearly 395%, and their length grew by almost 60%. The research team found that during their time in Hawaii, humpback whale mothers were losing roughly 214 pounds of blubber daily. Over 60 days, this loss translates to about 50 tons of krill or 25 tons of Pacific herring. In the 60 days of lactation, mother humpbacks in Hawaii experienced a 20% loss in body volume, with their lactation energy costs exceeding the total energetic requirement of a year-long pregnancy.
In Southeast Alaska, lactating humpback mothers were observed to gain weight at the slowest rate compared to non-lactating females, averaging about 32 pounds daily. In contrast, pregnant and non-pregnant females gained weight at rates six and two times that of lactating females, respectively.
“The most surprising aspect of this study for me was our capability to trace the same mother-calf pairs over vast distances and durations,” remarks van Aswegen. “For instance, we managed to get 405 repeat measurements from 137 lactating females in Hawaii and Southeast Alaska, with eight mother-calf pairs measured in both locations within the same year. Being able to track the same whales over 3,000 miles apart over approximately 200 days is truly extraordinary and provides incredibly valuable data for our research questions.”
In Hawaii, humpback whales play significant cultural, economic, educational, and environmental roles. Research reveals a 76.5% decrease in mother-calf encounter rates in Hawaii from 2013 to 2018, alongside an 80% reduction in birth rates from 2015 to 2016. In the Southeast Alaskan feeding grounds, a total reproductive failure was documented in 2018, with calf survival plummeting tenfold between 2014 and 2019. These patterns align with the longest global marine heatwave, which disrupted food chains and diminished prey availability across the North Pacific. It is suspected that humpback whales were unable to secure adequate food in their feeding areas, leading to nutritional stress and significant declines in reproduction and population.
This recent study by MMRP enhances our understanding of the energetic needs for humpback whales to rear offspring, accentuating the vital importance of Hawaii as an essential breeding ground. The findings indicate that humpback whales are particularly vulnerable during the early stages of calf development and lactation, underscoring the need for careful management of these waters. This information is crucial for evaluating how human activities may adversely 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 placed on humpback whales,” stresses Lars Bejder, MMRP director and co-author of the study. “Our humpback whale health database, which includes 11,000 measurements from 8,500 individual whales in the North Pacific, is utilized across various projects within the Marine Mammal Research Program and beyond. The findings become even more impactful when integrated with fine-scale behavior and movement data obtained through biologging tags; reproductive and stress hormone data from tissue and breath samples; and tissue data collected from post-mortem examinations. Such studies will be instrumental in predicting the resilience of large baleen whale populations in light of challenges including disturbances, entanglement, vessel collisions, and climate change.”
Protecting humpback whales necessitates collaboration and shared efforts.
“The ability to monitor individual humpback whales across thousands of miles and over several months illustrates the strength of collaboration,” notes Jens Currie, MMRP PhD candidate, Chief Scientist at Pacific Whale Foundation, and co-author of the study. “This research exemplifies how cooperative efforts across disciplines and institutions help us understand the complex interactions between maternal health, calf development, and environmental pressures. Such partnerships are essential as we work to safeguard humpback whales and their habitats amid a changing climate.”
