Humpback whale mother-calf health assessed using drone tech
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University of Hawaiʻi at Mānoa Hawaiʻi Institute of Marine Biology biologists used drone imagery to understand how nursing humpback whale mothers and their calves fare as they cross the Pacific Ocean. Recent declines in North Pacific humpback whale reproduction and survival of calves highlight the need to understand how mother-calf pairs expend energy across their migratory cycle. The study was published in The Journal of Physiology.
The team used drone cameras to measure calf growth and maternal body condition days after calf birth in Hawaiʻi, and then compared these measurements to the body conditions of humpback females in Alaska feeding grounds, measuring pregnant and lactating (producing milk for nursing) females as well as humpback females whose reproductive status was unknown.
"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," said Martin van Aswegen, Marine Mammal Research Program (MMRP) Ph.D. candidate and lead author of the study.
Size matters
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 Hawaiʻi, 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. Mother humpbacks in Hawaiʻi 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 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.
"The surprising part of this study was our ability to find the same individual mothers and calves over great distances and time periods," said van Aswegen. "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."
Birth rates decline
Studies document a 76.5% decline in mother-calf encounter rates in Hawaiʻi between 2013 and 2018, with birth rates declining by 80% from 2015 to 2016. In Southeast Alaskan feeding grounds, 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 heat wave, 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, resulting in nutritional stress and declines in reproduction.
"This work forms the basis for future studies investigating the energetic demands on humpback whales," said Lars Bejder, MMRP director and co-author of the study. "Our humpback whale health database, including 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 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."
"This study showcases how teamwork across disciplines and institutions helps us uncover the intricate relationships between maternal health, calf growth, and environmental stressors," said Jens Currie, MMRP Ph.D. candidate, chief scientist at Pacific Whale Foundation and co-author of the study.
This work was done in partnership with Alaska Whale Foundation, Pacific Whale Foundation and other partners.
More information: Martin van Aswegen et al, Maternal investment, body condition and calf growth in humpback whales, The Journal of Physiology (2024). DOI: 10.1113/JP287379
Journal information: Journal of Physiology
Provided by University of Hawaii at Manoa