How much do baleen whales eat in a day?


A recently published study v Nature found that baleen whales, the largest animals on the planet, eat three or even more food than previously thought.

The study, conducted mainly in the Southern Ocean, observed several individuals of seven species of baleen whales – humpback, fin, blue, minke whales, right whales, bowhead whales, and Bride’s whales – during their daily activities.

Baleen whales are so named because they have bristles in their mouths (baleen) in which their prey (krill) gets stuck. Their close relatives from the order of cetaceans are toothed whales, which have teeth instead of bristles.

The whales were tagged with sensors that tracked their movements, and acoustics were used to determine where their prey was concentrated.

The methods used are remarkable in that this is the first time that the movement and diet of whales has been empirically controlled. Earlier studies used the study of the stomach contents of killed whales or the use of mathematical models based on the metabolic rate of baleen whales. Both of these methods had serious drawbacks.

Direct measurements of stomach contents were often taken at certain times of the year, which, however, gave a “biased” picture. Some even tried to fill the stomach with water or gas, but after death the elasticity of the stomach lining is greatly reduced. In terms of mathematical models, metabolic rates involved were often “assumed” or taken from some of the toothed whales or dolphins caught.

A new study has shown that baleen whales can consume up to 16 tons of food per day, which is up to 30% of their total body weight.

The researchers argue that previous studies grossly underestimated the gigantic appetites of the largest aquatic mammals, with “even their highest guesses … underestimating reality.”

The authors argue that generalized whales such as fin and humpback whales, as opposed to specialized whales (blue, right and bowhead whales), may be better protected from the impacts of climate change on marine life.

Different feeding strategies

The researchers also highlight differences in the feeding strategies of these species. Right whales and bowhead whales hunt crustaceans, passing through a school of crustaceans with their mouths open. This strategy is called “ramming” or “continuous” feeding.

Another strategy, called lunge feeding, involves discrete jumps (lunges) on prey colonies. Lying feeding is demonstrated by blue, fin and humpback whales. A single whale using a lunge strategy can filter up to 17,000 cubic meters of water per day, while feeding a ram typically processes four times as much.

Whales and the iron cycle

These results are especially important because whales are the main predators in the food webs they control and therefore provide important ecosystem services and functions.

The most important of these is the marine iron cycle. Most of the iron in the ocean is found in biomass. One of the largest reservoirs of iron in the ocean is krill. Krill populations account for almost 24% of the total iron in surface waters, an earlier study found. When eating krill, whales defecate in iron-rich feces. They are then eaten by the planktonic community, which in turn is eaten by krill. And the cycle continues.

A 2010 study estimated that the amount of iron in whale feces could be “ten million times more than in Antarctic seawater,” while the current study claims that whales can recycle between 7,000 and 15,000 tons of iron each year.

The authors also highlight how whales play the role of ecosystem engineers, mixing iron with water by virtue of their natural movement.

This also explains the “krill paradox” where it was observed that krill populations actually declined during the whaling years (1910–70), whereas prey populations usually explode in the absence of a predator.

Even the number of competing predator species, which was expected to increase due to whaling, either decreased or remained the same (mainly because their food source, krill, was declining).
“Encouraging the restoration of cetacean populations can restore ecosystem functions lost in the 20th century and lead to increased productivity of the oceans,” the authors hope, although they admit that whaling in the 20th century reduced the population of baleen whales by more than two-thirds.

– The author is a freelance scientific communicator. (Mail[at]ritvik[dot]com)


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