Climate change could be responsible for leading humpback whales to exhaustion during their annual migration to warmer waters, a new study suggests.
Humpback whales (Megaptera novaeangliae) embark on an annual migration along the coast of Western Australia from their foraging grounds in the Antarctic and are likely to face increasing pressure from reduced food availability as a result of climate change.
For about three months every summer, humpbacks feed on krill in the waters of Antarctica, building up fatty blubber stores to fuel a nine month, 10,000‒18,000 km round trip to warm subtropical waters where they breed and calve. The whales do not actively feed during this migration journey and must rely entirely on their stored energy. In the event that their energy reserves become depleted (due to not feeding enough prior to their journey for example), the whales will become exhausted and likely become beached somewhere along the coast.
A recent study, lead by researcher Janelle Braithwaite of the University of Western Australia, set out to investigate the effects of declining krill populations (the humpback’s primary food source) in the Southern Ocean as a result of melting sea ice. Krill (Euphausia superba) are fundamentally important in the Southern Ocean ecosystem, forming a critical food web link between primary producers and top predators. The study explains how krill abundance fluctuates with changing oceanographic conditions, most notably with variations in winter sea ice. Sea ice is important for krill survival as it provides shelter during the winter months and microbial communities living in sea ice are a food source for juvenile krill. Hence, in some areas of Antarctica, more winter sea ice means more krill the following summer. With climate change set to instigate large scale melting of sea ice, the knock on effect of reducing krill populations could have a detrimental impact to the migrating humpback.
“If the ice declines in the area that these forage in, then that will reduce krill and that will reduce how much food they have,” Ms Braithwaite said in a press release.
“Whales live this feast and fast lifestyle. Over the summer they’re feasting up on krill down in the Southern Ocean but once they leave, they’re pretty much fasting during their migration journey. It’s a bit like a car, if there’s not enough petrol at the petrol station, then you’re setting off with three quarters of a tank and you might not be able to make it. If these whales run out of petrol before they get back to the Southern Ocean, then there’s no safety net, they will die from exhaustion.”
Due to the lack of concurrent long-term data regarding the effects of krill variability on baleen whales, the researchers used historical whaling data from Albany, Carnarvon and Point Cloates to examine the body condition of humpback’s caught in Western Australia waters between 1947 and 1963. This was achieved through using whale oil yields in whaling records as a proxy for whale body condition and found the whale’s fat content, or energy reserves, was highest when there was a lot of krill available. Annual estimates of krill abundance in the Southern Ocean where those whales foraged were correlated significantly with contemporary annual winter sea ice extent. Through predicting sea ice extent for the whaling period from reconstructed temperature data, the team found that whale body condition was significantly correlated with predicted winter sea ice extent, supporting the hypothesis that variations in body condition were likely mediated by associated krill fluctuations.
Ms Braithwaite explains this research is important in trying to assess climate change impact in the Southern Ocean in the future.
“If there’s less sea ice then there’s going to be less food for whales. If there’s less food for whales, then our results indicate that the whales aren’t going to be as fat and this is going to cause problems when it comes to trying to complete migration on their limited energy reserves as well as also reproducing.”
Article Header Photo:
Adult Humpback whale
Credit: Wilfried Niedermayr