How Do Whales Survive By Eating Krill?

The name ‘whale’ almost immediately conjures up the image of a mammoth sea creature splashing in the waters of the ocean. To other people, who are more like me, the same image would be subtitled “whale: the largest mammal on Earth.”

Indeed, whales are some of the largest creatures on the planet. Blue whales, in fact, are the largest mammals on Earth. It can grow to be longer than 100 feet and weigh upwards of 200 tons! It’s heart can be as big as an automobile and its tongue alone can weigh as much as an elephant. Think about that for a moment!

One might wonder what in the world must such huge creatures eat every day to sustain themselves! One might logically guess that blue whales must eat big fish and other large sea creatures; it does make sense, after all, to think that only big aquatic creatures could adequately quench the appetite of the biggest mammal on Earth.

If you’ve ever considered this subject, but don’t know the facts, you may be surprised to know that most whales primarily feed on krill – a family of extremely tiny shrimp-like crustaceans! So, despite how big whales themselves are, their diet consists of incredibly small aquatic creatures. The question is, how does that work? How do whales, some of the biggest creatures on the planet, survive by feeding on krill, some of the smallest?


Krill are found in every ocean of the world and are small crustaceans that exist near the bottom of the food chain. They belong to the large arthropod subphylum, Crustacea. The most popular group of crustaceans (the class Malacostraca) includes the superorder Eucarida, which further comprises three orders: Decapoda (shrimp, lobsters, crabs, prawns etc.), the planktonic Amphionidacea and Euphausiacea (krill).


Krill always appear in massive swarms. (Photo Credit : Øystein Paulsen / Wikimedia Commons)

How do whales survive by eating krill?

First off, while it’s true that whales eat krill, that’s not the only thing they eat. Furthermore, whales actually gorge on tons of krill every day in order to satisfy their hunger and cater to their nutritional requirements. Allow me to elaborate on that second point…

You see, practically all the food in the ocean originally comes from plants; while plants only need water and sunlight to survive, animals have to actually eat stuff. Thus, plants grow using water and sunlight, and then certain animals eat those plants. Subsequently, certain other animals eat the animals that eat plants, and then certain animals eat those animals. In this way, the cycle goes on and on.

At the bottom of every food chain is some kind of plant. (Photo Credit : Flickr)

Note that not all food that is eaten gets turned into new food that another animal can eat. Some part of the eaten food is used up while providing energy to move around and some part is just excreted as waste. Therefore, if an animal eats, say, 100 grams of food, it may only produce 10 grams of food that another animal could eat. What this means is that if you’re a big animal, then you have to eat a LOT of food. It also means that you have more food to eat if you eat plants, rather than surviving on a diet of other animals.

Finally, it means that you have more food if you eat an animal that eats plants, rather than an animal that eats other animals.

Now, let’s see how this reasoning applies to the whale-and-krill example:

Krill eat plants, specifically algae (although algae are not highly differentiated in the way that plants are, so technically, they are not exactly ‘plants’), and the thing about algae is that it grows unbelievably fast. Therefore, krill have it pretty easy – they eat as much as they want and make tons of babies, as there is no dearth of food. This is also quite favorable for whales, because whales often find incredibly huge numbers of krill huddled up in swarms.

As big as whales are, when they see a giant swarm of food floating in the water, all they have to do is swim through the swarm with their mouths open and get as many krill into their mouth as possible in a single pass.

Shark eating krills

A whale can eat a couple tons of krill just by swimming through their swarms! (Photo Credit : KAZ2.0 / Flickr)

This is how whales can easily eat a few tons of krill every day. In a nutshell, whales, some of the largest creatures on the planet, can survive by eating krill because they eat so many of them. To put this in perspective, consider a single grain of rice, which is positively puny in comparison to a human. Still, the latter can survive by eating thousands of rice grains when they’re served in a bowl!


  1. Stanford University
  2. Wheelock College
  3. University of Hawaii
  4. Australian Broadcasting Corporation
  5. Saint Louis University
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About the Author:

Ashish is a Science graduate (Bachelor of Science) from Punjabi University (India). He spends a lot of time watching movies, and an awful lot more time discussing them. He likes Harry Potter and the Avengers, and obsesses over how thoroughly Science dictates every aspect of life… in this universe, at least.

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