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Episode 306: Behemoths of the Sea, Blue Whales

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Image from the BBC

The Blue Whale is the largest animal alive today, with records confirming that some whales have reached lengths of up to 29.9 metres, weighing up to 199 tonnes, with their hearts alone reaching the size of a small car! Blue whales are baleen whales, meaning they have huge keratinous plates in their mouths that they use to sieve out their krill prey from vast mouthfuls of seawater. They belong to a group of whales called rorqual whales, the largest group of baleen whales that hosts the biggest whale species, like the northern Minke whale and the fin whale. There are four subspecies of blue whale classified today, including:

  • Balaenoptera musculus musculus: The Northern subspecies, mainly reported from New England along Eastern Canada to Greenland, although they have been reported grouping in Iceland.
  • Balaenoptera musculus indica: The Northern Indian Ocean subspecies found year round in the Norhwestern Indian Ocean.
  • Balaenoptera musculus brevicauda: the pygmy blue whale, consisting of a Madagascar, Australian, Indonesian and New Zealand population.
  • Balaenoptera musculus intermedia: the Antarctic subspecies, found around the Antarctic but travelling as far as the tropical Pacific.

Blue whales diverged away from the rest of the Balaenopteridae around 10.9 million years ago. Due to their increasing size throughout their evolution, they underwent a series of morphological changes very quickly, developing the baleen plates necessary to catch enough food to maintain their enormous size. The first fossil evidence of a modern blue whale is a partial skull in Italy, with the estimated length of this individual being 26 metres and carbon dated to the Miocene period, 23 million years ago. No animal ever found is as big as the modern Blue whale. The largest dinosaur is thought to have been Argentinosaurus. While these dinosaurs measured slightly longer at 122 feet (37 meters), they weighed much less at 100 tons, or half that of a Blue whale. The persistent existence of Blue Whales in the fossil record indicates just how important this species is to marine ecosystems.

The fossilised skull of an ancient baleen whale species, Parabalaenoptera baulinensis (image from https://coastalpaleo.blogspot.com/)

Big appetites: ecology and behaviour of the Blue Whale

Blue Whales are migratory, moving to feeding grounds at the Poles in the summer, then heading to their breeding grounds in the Tropics during the winter. 10 years of satellite tracking reveals that blue whales exhibit long term memory, with the movements of blue whales following the spring phytoplankton blooms in order to feed off the krill that eat them. The migration patterns are not well understood, although almost every population studied exhibits some kind of migration, moving to winter and summer grounds.

The migratory routes of the Blue Whale (image from www.offthemap.travel)

They can also dive to incredible depths, with tagged whales revealing they can go down as far as 315 metres. Their heart rate drops to as low as 2 beats per minute when they get down to these depths. Their diet consists almost entirely of krill, which they capture through a method known as lunge feeding. This involves the whale swimming at high speeds towards a swarm of krill, opening their mouths wide. They apply pressure to the baleen plates with their pouch and tongue, forcing the water to sieve through, straining out the krill. Being the biggest animals in the world, the blue whale needs to eat some pretty significant meals, ingesting up to 3.6 metrics tonnes of krill per day. When lunge feeding, blue whales are known to seek out the densest patch of krill, providing them with the most energy for migration and reproduction.

A rare image of a Blue Whale engaging in lunge feeding behaviour for krill (image from www.earthtouchnews.com)

A big hole in the ecosystem: the threats facing blue whales

The biggest historical threat to Blue Whales was the whaling industry, which reached unsustainable levels thanks to the industrialization of whaling fleets with gas powered harpoons. Prior to this, it was virtually impossible to hunt Blue Whales due to their size and speed. They began to be hunted in significant numbers in 1868 in the North Atlantic, and their numbers were thought to be depleted down to the low hundreds in 1966, when the International Whaling Commission banned their hunting, although the Soviet Union continued illegally into the 1970s. After these decades of hunting, blue whales are classified as endangered by the IUCN red list, although their numbers are steadily increasing year on year. You can check out more about the history of whaling by reading the writeup for Episode 257, the Harmony of Humpback Whales.

Although whaling is almost entirely a thing of the past, there are still modern-day concerns that affect Blue Whales. Ship strikes are one of the biggest mortality factors, with most deaths occurring in the waters off the US West coast and Sri Lanka, where their preferred routes cross some of the busiest shipping lanes in the world. The waters off Sri Lanka are calving grounds, which makes the whales here particularly vulnerable to accidents.

Blue Whales foraging close to the busiest shipping lanes in the world (image from IIangakoon, 2012)

Like so many marine mammals, blue whales are impacted by the scourge of marine plastic, ingesting vast amounts of microparticles during their lunge feeding, and at times becoming entangled in discarded fishing gear. They may drag these nets and other equipment for long distances, becoming fatigued and compromising their ability to feed or reproduce successfully. Studies of Blue whales killed in ship strikes has revealed that their tissues contain pollutants like pesticides and mercury and of particular concern is the contaminants being transferred from mother to baby in maternal milk, with evidence that levels of persistent organic pollutants, sometimes known as ‘forever chemicals’ in blue whale bodies have been transferred from the mother during lactation. It is estimated that thanks to their indiscriminate feeding, blue whales are ingesting up to 10 million pieces of microplastic each day, as they can’t filter out the tiny pieces which are no larger than their krill food.

The oceans are getting noisier, and this is impacting blue whales, who communicate using vocalisations across the vastness of the ocean. Commercial shipping and seismic surveys for new oil and gas discovery can causes whales to vocalise less in response to the din, with a study off the coast of California revealing that whales were less likely to produce calls when sonar was present. This noise has also been shown to interrupt deep dive feeding, although feeding in shallower depths doesn’t seem to be affected. It hasn’t been directly proven yet, but it is likely that this cacophony in the waters is affecting blue whales foraging behaviours, as well as possibly their location of mates.

Conservation optimism

Although whaling is not entirely consigned to the history books, the vast majority of countries have banned whaling, and today Blue Whales are at far more risk of plastic pollution and ship strikes than overexploitation.

Research is being conducted into the incidences of ship strikes, and there is evidence being considered that simply moving shipping lanes in Sri Lanka a little further offshore could considerably reduce mortality in this vital blue whale habitat.

Whale and Dolphin conservation is a British Charity working for 30 years to conserve whales and dolphins across the world. Their main aims are to stop the captivity of cetaceans, end whaling, stop deaths from nets and create healthy seas by working alongside partners to clean up pollution from the ocean, as well as prevent it from entering the ocean in the first place. They also carry out a number of vital research projects across the globe. You can check out their work at https://uk.whales.org/. We can all protect whales and their ocean habitat with one simple action: PICK UP YOUR TRASH! Cut back on your plastic use where you can, but where you can’t, take your rubbish with you, dispose of it correctly and keep the seas as pristine as the Whales want them to be. You can even join a local beach clean if you fancy some hands on conservation action that can make a big positive difference.

Awesome videos!


Abrahms, B. Hazen, E.L. Aikens, E.O. Savoca, M.S. Goldbogen, J.A. Bograd, S.J. Jacox, M.G. Irvine, L.M. Palacios, D.M. and Mate, B.R. (2019) ‘Memory and resource tracking drive blue whale migrations.’ Proceedings of the National Academy of Sciences of the United States of America

Arnason, U. Kumar, V. Nilsson, M.A. and Janke, A. (2018) ‘Whole genome sequencing of the blue whale and other rorquals finds signatures for introgressive gene flow.’ Science Advances

Bianucci, G. Marx, F.G. Collareta, A. Stefano, A.D. Landini, W. Morigi, C. and Varola, A. (2019) ‘Rise of the titnas: baleen whales became giants earlier than thought.’ Biology Letters

Blue Whale | Species | WWF (worldwildlife.org)

Blue Whale | NOAA Fisheries

Iiangakoon, A.D. (2012) ‘Exploring anthropogenic activities that threaten endangered blue whales off Sri Lanka.’ Journal of Marine Animals and Their Ecology


November 16, 2022
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