Diet and feeding
The blue whale's diet consists almost exclusively of krill.[36] Blue whales capture krill through lunge feeding; they swim towards them at high speeds as they open their mouths up to 80°.[36][72] They may engulf 220 metric tons (220 long tons; 240 short tons) of water at one time.[76] They squeeze the water out through their baleen plates with pressure from the throat pouch and tongue, and swallow the remaining krill.[36][72] Blue whales have been recorded making 180° rolls during lunge-feeding, possibly allowing them to search the prey field and find the densest patches.[77]
While pursuing krill patches, blue whales maximize their calorie intake by increasing the number of lunges while selecting the thickest patches. This provides them enough energy for everyday activities while storing additional energy necessary for migration and reproduction. Due to their size, blue whales have larger energetic demands than most animals resulting in their need for this specific feeding habit.[78] Blue whales have to engulf densities greater than 100 krill/m3 to maintain the cost of lunge feeding.[72][79] They can consume 34,776–1,912,680 kilojoules (8,312–457,141 kcal) from one mouthful of krill, which can provide up to 240 times more energy than used in a single lunge.[72] It is estimated that an average-sized blue whale must consume 1,120 ± 359 kilograms (2,469 ± 791 lb) of krill a day.[80][81] On average, a blue whale eats 4 t (3.9 long tons; 4.4 short tons) each day.[60]
In the southern ocean, blue whales feed on Antarctic krill (Euphausia superba). In the South Australia, pygmy blue whales (B. m. brevicauda) feeds on Nyctiphanes australis.[82] In California, they feed mostly on Thysanoessa spinifera, but also less commonly on North pacific krill (Euphausia pacifica).[83] Research of the Eastern North Pacific population shows that when diving to feed on krill, the whales reach an average depth of 201 meters, with dives lasting 9.8 minutes on average.[82]
While most blue whales feed almost exclusively on krill, the Northern Indian Ocean subspecies (B. m. indica) instead feeds predominantly on sergestid shrimp. To do so, they dive deeper and for longer periods of time than blue whales in other regions of the world, with dives of 10.7 minutes on average, and a hypothesized dive depth of about 300 meters. Fecal analysis also found the presence of fish, krill, amphipods, cephalopods, and scyphozoan jellyfish in their diet.[82]
Blue whales appear to avoid directly competing with other baleen whales.[84][85][86] Different whale species select different feeding spaces and times as well as different prey species.[76][87][88] In the Southern Ocean, baleen whales appear to feed on Antarctic krill of different sizes, which may lessen competition between them.[89]
Blue whale feeding habits may differ due to situational disturbances, like environmental shifts or human interference. This can cause a change in diet due to stress response. Due to these changing situations, there was a study performed on blue whales measuring cortisol levels and comparing them with the levels of stressed individuals, it gave a closer look to the reasoning behind their diet and behavioral changes.[90]