Dolphin vs. Whale Feeding
Frigid, arctic waters support some of the largest whale species, such as the humpback and the blue whale. Baleen whales feed only in arctic waters, eating mostly krill, plankton, and other small marine animals. The humpback whales often hunt by forming a ring of bubbles around their prey which then obstructs escape. See also this video: Humpback Whale: Hunting Technique
Certain species of dolphins come closer to shore for feeding, often chasing fish into shallow water to catch them easily. Another method they adopt is driving prey onto mud banks for easy capture. See also this video: Atlantic Bottle Nose Dolphins mud-ring feeding
Echolocation
Whales and dolphins hunt for food with the help of echolocation, i.e., they direct clicking sounds into the water and listen to the strength of the rebounded echoes, which allows them to know how far away they are from prey. The clicks and whistles used are emitted from their blowhole(s).
Cetaceans have developed sensory enhancements for this process. The melon-like shape of a dolphin's head serves the same function as that of the large sinus cavities in whales' skulls: these adaptations create echo resonance to help pinpoint distances, speeds, and angles with greater precision. They also enhance the capacity for communication.