The answer to this question is actually quite simple.
Despite being the largest living mammal in existence the blue whales primary diet consists of krill, a small oceanic creature that generally measure in at a measly 1-2 centimeters, although a few species can grow close to 6 inches in size.
Blue whales in comparison can measure in at more than 100 ft. long (60 – 80 ft. on average) and weigh over 150 tons.
They can also consume as many as 40 million krill per day which ends up weighing close to 8,000 pounds of food on a daily basis!
When hunting for food the blue whale will dive underneath the water and swim towards their prey using a technique known as filter feeding (a common hunting method used amongst baleen whales) in which they swim towards large schools of krill with their mouth open engulfing both their prey and the water surrounding their prey.
They then push the water out of their mouth with their tongue while keeping the krill trapped inside their baleen bristles, which are said to resemble the teeth found on a comb.
In some cases blue whales may coincidentally swallow fish, crustaceans and squid that happen to be swimming in or near the school of krill it is hunting.
Dives for blue whales generally last around 10 minutes although some blue whales can stay submerged for up to half an hour.
Krill (their primary food source) generally migrates toward the surface during the night and dives down into the ocean during the day.
Since their prey migrates toward deeper waters during the day time most hunting takes place at least 300 ft. underneath the water.
It is only during the night that blue whales will feed on krill near the surface of the water.
Due to the fact that blue whales need to consume so much food they are almost always found swimming where large abundances of krill reside.