The Challenges of Being a Mother Humpback Whale

By Jean Souza

A humpback whale swims with its calf. Photo by Ed Lyman/HIHWNMS, NOAA permit 782-1719

Field research conducted by two Hawaiʻi-based University of Hawaiʻi researchers, PhD student Martin van Aswegen and Dr. Adam Pack, are providing new insights into the stresses of mother humpback whales with calves.

Their research projects were presented last November during a research symposium hosted by the Hawaiian Islands Humpback Whale National Marine Sanctuary. They are among the recipients of research grants awarded by the National Marine Sanctuary Foundation. This is a summary of their findings.

Gathering Place

Humpback whales in Hawaiʻi come from all over the North Pacific, including Russia, the Aleutian Islands, the Northern Gulf of Alaska, Southeast Alaska and British Columbia. They travel more than 8,000 miles roundtrip between their northern feeding grounds to their breeding grounds in the Hawaiian Islands. While in Hawaiʻi, the whales fast and live off the fat reserves stored in their bodies from eating massive amounts of krill and small schooling fish they find in their northern feeding grounds.

What Happens in the Feeding Grounds Affects the Breeding Grounds

A humpback whale and its calf are seen here swimming, Photo by J. Moore/HIHWNMS, NOAA permit 15240

It’s not always easy for whales to find their prey and consume sufficient prey to maintain good health. Humpback whales are well adapted to take advantage of seasonally abundant food, store the energy as fat layers, or blubber, in their bodies, and fast during epic migrations to and from their breeding areas closer to the tropics.

Some years, the number of whale sightings in Hawaiʻi are lower than normal, or they look slimmer, or fewer calves are observed. Warming ocean conditions in their feeding grounds can lead to fewer prey, changing locations where prey may be found, and/or resulting in less nutritious prey. In 2016, a trifecta of oceanic events occurred that affected humpback whale prey, including a strong El Niῆo, a strong shift in the Pacific Decadal Oscillation, and the 2014-2016 Pacific marine heatwave (known as the “Blob”) that hit the North Pacific. In those situations, humpback whales may decide to stay on their feeding grounds and forgo the annual migration to the breeding grounds to continue to search for and consume food, or they may shorten their stay in Hawaiʻi, or they may decide to go to other breeding areas.

Mothers with Calves

Hawaiʻi continues to be an important breeding, calving and nursery area for the humpback whales of the North Pacific. After a 12-month gestation period, female whales give birth while in Hawaiʻi. The mom-calf relationship is a strong one, as the mother is the sole source of food for the calf, providing fat-rich milk for its first six-to-ten months of life.

Moms with calves tend to prefer the shallower waters off Hawaiʻi where there are fewer male whales, and where there may be fewer predators. Males looking to breed prefer non-lactating females, though males will sometimes investigate the ovulation status of a mother with a calf.

In competitive groups, males vigorously compete with each other for access to the female. If the female has a calf, the calf may become separated from the mother in the speed and jostling of a competitive group. The mother will then need to locate the calf after the competitive group has dissolved.

The energy expended by the mother during competitive group behavior compounds the energy demands of the calf, all while fasting. Mom-calf pairs have an extended stay in Hawai‘i of as many as 50 days, compared to the average humpback whale stay of 14 days. This also contributes to the energetic stressors on mothers.

Drones as a Whale Research Tool

A humpback whale and its calf. Photo by J. Moore/HIHWNMS/NOAA/MMHSRP, permit #20311

Martin van Aswegen has been using drones since 2018 to measure the size and body condition of humpback whales in the breeding grounds off the island of Maui and in the feeding grounds of Southeast Alaska.

He studies humpback whales for eight to ten months each year at both the breeding and the feeding grounds. With collaborators in Hawaiʻi and Alaska, he uses drones with customized altimeters that have a resolution and accuracy of about half inch. These drones can measure the entire length of an animal and the width of that animal at 20 points along its length. The widths are then converted to body volume for each whale. Length and body volume ratios were determined for each class of animal — calf, juvenile, adult and lactating female. The project poses the question: Does the measured whale have a body volume above or below what we would expect at this point in time in Hawaiʻi or Southeast Alaska, based on its length? The results for lactating females were startling.

Moms are Challenged

All adult humpback whales are expected to lose body mass while in Hawaiʻi. During a 36-day period in Hawaiʻi, a lactating female — a mom — lost about 11 percent of its body volume or about 180 pounds per day.

All whales had been expected to increase body volume in Southeast Alaska. The findings indicate mothers are always not doing so. One whale experienced an additional 21 to 28 percent reduction in body volume during a period of about 171 days between the breeding ground and the feeding ground.

The body condition of many moms continued to have lower than expected body volumes even after a whole season in the feeding area. Moms had not been able to locate and consume sufficient prey to offset the lactation costs. When lactating, the mom transfers most of her energy to the calf. This continues until the calf switches to a krill/fish diet in months six-to-ten. At best, females were plateauing. Very few females with calves were gaining weight throughout the feeding season.

Biopsy Samples and Stress Hormones as Tools to Study Stress in Moms

A humpback whale accompanied by its calf. Photo by J. Moore/HIHWNMS, NOAA permit 20311

The research led by Dr. Adam Pack prioritized the understanding of the natural stress response of mothers and how this is related to their body condition. His geographic focus is the four-island region in Hawaiʻi consisting of the islands of Maui, Molokaʻi, Lānaʻi and Kahoʻolawe. This area has the greatest density of humpback whales in Hawai‘i as well as the greatest density of calves. His collaborators are the UH Marine Mammal Research Program, including Martin van Aswegen, Hawaiian Islands Humpback Whale National Marine Sanctuary, Pacific Whale Foundation, Hawaiʻi Pacific University, University of Alaska-Fairbanks, and Alaska Whale Foundation.

Biopsy samples of skin and blubber the size of a pencil eraser are obtained from humpback whales using a hollow-tipped dart shot from a crossbow. The stress hormone concentrations in the blubber are good indicators of long-term stress.

Dr. Pack found that the number of escorts with a mom-calf pair may be a short-term stressor, but produces no significant difference in cortisol concentration — a female stress hormone.

However, elevated cortisol concentrations in moms with yearlings, versus a mom with a calf of the year, do suggest significant stress. This correlates with Martin van Aswegen’s findings of poor body condition in mothers because the moms have been offloading their energy reserves to their calves since they were conceived and while on the feeding grounds.

“Balloons with Tails”

The fat-rich mother’s milk allows calves to grow about 46 percent during a 36-day period off Maui. Its body volume triples. The calf continues to grow on the feeding grounds where a calf had a 336 percent increase in body volume over 171 days. This led Martin van Aswegen to say they were looking like “balloons with tails.”

Conclusions

In the words of Dr. Pack, a healthy mother is key for ensuring the health and success of her calf. In these studies, the goal is to promote a healthy population of humpback whales for all generations in the waters we all call home.

Baseline data on body condition and natural stress levels are important for the analysis of challenges that humpback whales, especially mothers, are facing including the impacts of human actions and natural events.

Body condition varies across different whale classes in Hawaiʻi. Not every whale sampled in Hawaiʻi is from the same feeding grounds, so the hope is to compare body conditions across feeding grounds.

Due to expansiveness of the ocean, only five to ten mom-calf pairs out of 700 were sampled within the same year at both the feeding grounds and breeding grounds. Can this be improved upon?

Can we look at various environmental factors to explain any variation found in body condition? Factors of importance include biotic metrics (such as chlorophyll-a, as a proxy for productivity; phytoplankton; fisheries catch data), abiotic data (such as sea surface temperature anomalies, salinity, upwelling) and climate data (El Niῆo, La Niῆa, Pacific Decadal Oscillation, and North Pacific Gyre Oscillation).

• Martin van Aswegen is a PhD student at the Hawaiʻi Institute of Marine Biology, Marine Mammal Research Program.

• Adam Pack is with the University of Hawaiʻi at Hilo, Departments of Psychology and Biology, and the Dolphin Institute in Hilo.

• Jean Souza serves as the on-site manager of Kaua‘i Ocean Discovery at Kukui Grove Center and is a Program Specialist with HIHWNMS. She can be contacted at Jean.Souza@noaa.gov