The answer to this question is actually quite simple.
Despite being the largest living mammal in the world the blue whales primary diet consists almost exclusively of krill, a small oceanic creature that generally measure in at a measly 1-2 centimeters; although a few species of krill can grow close to 6 inches in size.
In comparison to the small size of krill the blue whale can grow to be more than 100 ft. long (60 – 80 ft. on average) and weigh over 180 tons (100 – 150 tons on average)!
Even though these marine mammals are larger than most whales their bodies are quite slim and streamlined for their size allowing them to reach for speeds of up to 25 miles per hour or more when threatened or startled; however the average traveling speed of these marine mammals remains closer to 5 mph.
When it comes to eating food the blue whale can consume as many as 40 million krill per day, which ends up weighing close to 8,000 pounds of food on a daily basis!
Note: Instead of krill the baby blue whale consumes milk during its first 6 – 18 months of birth and can drink as much as 150 gallons of milk per day during its first year. This feeding will continue until the young whale is able to hunt for food and survive on its own.
When these marine mammals hunt for food the blue whale dives underneath the water and swims towards its prey using a technique known as filter feeding (a common hunting method used among baleen whales) in which they swim towards large schools of krill with their mouth open and engulf both their prey along with the surrounding water.
They then push the water out of their mouth with their tongue while keeping the krill trapped inside their baleen bristles, which researchers and marine biologists state resemble the teeth found on a comb.
The baleen plates and bristles of these whales acts like a filter (hence the name) by allowing water to pass in and out of the bristles while keeping their prey trapped inside.
Baleen bristles can be thought of as a fence that allows air to move through but keeps small animals from escaping.
In regards to diving for food the amount of time a blue whale dives for generally last around 10 minutes, although these whales have been recorded staying submerged for up to half an hour.
It is typically only during the night that blue whales will feed on krill near the surface of the water because krill generally migrates toward the surface during the night and dives back down into the ocean during the day.
The reason krill do this is to minimize potential threats from predators while they feed near the surface.
Since the blue whales prey migrates toward deeper waters during the day time most hunting takes place around 300 ft. underneath 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, which is typically in cold waters around the northern and southern polar hemispheres.