Humpback whales lose around 36 percent of their energy reserves during their long annual migrations, according to a new study published in the journal Marine Mammal Science.
Using drones, researchers monitored 103 humpback whales (Megaptera novaeangliae) to see how their body conditions – how much muscle and blubber it has – changed during their migration between their Antarctic feeding grounds and their breeding grounds in Colombia.
The whales’ body condition changed considerably between early autumn (March to May), when they were at their fattest after feasting on krill, and late spring (August to December) when they had lost significant amounts of weight.
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“An average size adult humpback whale loses about 36 percent of its body condition during migration – equivalent to 11,000 kilograms of blubber, which is as much as the weight of standard single-decker city bus or two adult African elephants,” says lead author Alexandre Bernier-Graveline, PhD candidate from Griffith University’s Southern Ocean Persistent Organic Pollutants Programme.
To gain this amount of energy for their remarkable journeys, the whales would need to eat 57,000 kilos of Antarctic krill. At around 196 million kilojoules of energy, it would take the average adult human more than 62 years to consume this much.
Bernier-Graveline was surprised when he calculated the figures. He was used to working with four-metre, 1,000-kilo beluga whales, but humpbacks can grow to 13 metres and tip the scales at around 35,000 kilos – so everything is supersized. Their sheer size “makes any estimates quite impressive and difficult to grasp.”
Bernier-Graveline first became interested in marine mammals’ energy reserves while studying beluga whales in the St. Lawrence River, Quebec, Canada.
He decided to study how declining reserves could offer insights into the health of marine mammal populations. “Understanding early warning signal of population collapse, such as changes in behaviour, morphology, and life-history traits is crucial for anticipating population declines and implementing effective conservation strategies,” he says.
Data-collecting drones
The researchers used drones to monitor the whales’ body conditions. Although this is an effective and non-invasive method of collecting data, the approach is technically demanding and lots of things can go wrong.
“The method relies on converting pixel-based measurements from overhead images into accurate real-world dimensions, which requires precise data and good image quality,” Bernier-Graveline says. “Working in marine environments presents several logistical challenges. Weather conditions can be unpredictable and often limit flight opportunities.”
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Any issues with the drone can cause problems and, even if they do capture what they need, processing the images is a long and complex process. “It involves managing various sources of variability, such as whale posture, sea state, image quality and light conditions, all of which can affect measurement accuracy,” he says.
There are also many benefits to using drones. Compared with other research methods, like biopsies, they are less invasive and help the scientists to keep track of more whales.
Importance of krill
The findings demonstrate just how important it is for humpbacks in the Southern Hemisphere to binge on krill during the Antarctic feeding season. This important energy source will sustain them for months as they swim thousands of miles to their tropical breeding grounds.
But krill are under threat. “The Antarctic sea-ice ecosystem is changing rapidly, significantly impacting krill populations,” Bernier-Graveline says.
The loss of krill could cause huge problems for humpback populations. “With less food comes less energy, directly affecting their health, body condition, and reproductive success, factors closely linked to krill abundance and sea ice extent,” he adds.
Without enough to eat, whales could be in serious trouble. “Modelling studies suggest that while humpback whales may fully recover from historical whaling by 2050, their numbers could decline sharply by 2100 due to ocean warming and reduced prey availability,” says Bernier-Graveline – and this will have a ripple effect on ecosystems across the ocean.
The idea is worrying but there is hope. Many species of whales were nearly wiped out by industrial whaling but, since a global ban was put in place, they have rebounded.
Bernier-Graveline says: “History has shown that bold conservation actions can reverse negative trends.”
Read the full study: Drone-Based Photogrammetry Provides Estimates of the Energetic Cost of Migration for Humpback Whales Between Antarctica and Colombia.
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Top image credit: Griffith University
