A recently published study in Nature has established that baleen whales, the largest animals on the planet, eat thrice or even more the amount of food than previously thought.
The study, conducted primarily in the Southern Ocean, monitored multiple individuals from seven baleen whale species – humpback, fin, blue, minke, right and bowhead and the Bryde’s whale – as they went about their daily business.
I am delighted to share our paper, published today in @nature, on baleen whale prey consumption and what it means for marine ecosystems. https://t.co/Fh5c4ZVMnZ A thread 1/n pic.twitter.com/OCKzJCpyc8
— Matthew Savoca (@DJShearwater) November 3, 2021
Baleen whales are so named because they have bristles (baleens) inside their mouth in which their prey (krill) gets stuck. Their close relatives, from the order Cetacea, are toothed whales that have teeth instead of bristles.
Whales were tagged with sensors that tracked their movements, and acoustics were used to identify places where their prey was concentrated.
The methods used are notable as it is the first time that whale movement and diet could be empirically monitored. Earlier studies employed examining the contents of the stomach of killed whales or by employing mathematical models based on metabolic rates of baleen whales. Both these methods suffered veritable disadvantages.
Direct measurements of stomach contents were often done during specific times of the year, which, however, gave a “biased” picture. Some even tried to fill the stomach up with water or gas, but the elasticity of the stomach membrane decreases significantly upon death. As for mathematical models, the metabolism rates involved were often ‘assumed,’ or taken from some captured toothed whales or dolphins.
The new study has observed that baleen whales can consume as much as 16 tonnes of food per day, which constitutes as much as 30% of their total body mass.
Prior studies, researchers maintain, have grossly underestimated the gargantuan appetites of the largest aquatic mammals, wherein “even their highest assumptions…underestimates reality”.
Generalist whales like fin and humpback, as opposed to specialist ones (the blue, right and bowhead whales), may be better buffered against the effects of climate change on marine life, the authors argue.
Different feeding strategies
The researchers highlight differences in the feeding strategies of these species as well. The right and bowhead whales prey on crustaceans by moving through a swarm of crustaceans with an open mouth, a strategy called ‘ram’ or ‘continuous’ feeding.
Another strategy, called ‘lunge’ feeding, involves discrete jumps (lunges) at prey colonies. Lunge feeding is exhibited by the blue, fin and humpback whales. A single whale adopting a lunge strategy can filter up to 17000 cubic metres of water a day, while a ram feeding tends to process four times as much.
Whales and iron cycle
These findings are particularly important as whales are apex predators in food chains they operate and therefore render important ecosystem services and functions.
The foremost among these is the marine iron cycle. Most iron in the ocean exists in biomass. One of the largest reservoirs of iron in the ocean is krill. Krill populations constitute nearly 24% of the total iron in surface waters, an earlier study has established. Upon devouring krill, whales defecate iron-rich faeces. These are then eaten by the planktonic community, which are then, in turn, eaten by krill. And the cycle goes on.
A 2010 study estimated that the amount of iron in whale faeces can be “ten million times that of Antarctic seawater”, while the present study asserts that whales could recycle 7000 to 15000 tonnes of iron each year.
The authors also highlight how whales play the role of ecosystem engineers by mixing iron in the water by virtue of their sheer movement.
This also explains the ‘krill paradox’ whereby it was observed that krill populations actually declined during the whaling years (1910-70), whereas the prey population usually explodes in the absence of a predator.
Even the numbers of competing predator species, which were expected to increase with whaling, have either declined or remained the same (essentially because their food source, krill, was declining).
“Encouraging cetacean populations to recover may restore ecosystem function lost in the 20th century and lead to enhanced oceanic productivity,” authors hope, even as they acknowledge that twentieth-century whaling reduced baleen whales populations by more than two-thirds.
– The author is a freelance science communicator. (mail[at]ritvikc[dot]com)