The blue shark (Prionace glauca) is the most widely distributed pelagic shark species, inhabiting the temperate and tropical waters of every ocean basin. Its slender body is built for long-distance travel across the open sea, where it operates primarily as a solitary predator. The blue shark’s feeding strategy is highly opportunistic and generalist, meaning its diet is determined by what edible prey is immediately available in the vast, calorie-scarce environment of the epipelagic zone. This adaptability allows the species to thrive in diverse ecological settings, from surface waters down to depths of several hundred meters.
The Staples: Cephalopods and Schooling Fish
The bulk of the blue shark’s diet is composed of two main categories: cephalopods and small schooling bony fish. Squid are the most important food source, particularly for adult sharks, and are consumed in large quantities. Stomach content analyses frequently reveal numerous squid species, including fast-moving pelagic squid and slower-moving, gelatinous deep-sea species like those from the family Cranchiidae.
The consumption of bioluminescent deep-sea squid and octopuses, such as Histioteuthis species, suggests that the sharks utilize vertical migration patterns to intercept prey as it moves upward toward the surface at night. Pelagic octopuses, like Ocythoe tuberculata and Tremoctopus violaceus, are also regular components of the diet. This reliance on cephalopods is due to their abundance and relatively defenseless nature compared to other prey.
Schooling teleost fish constitute the other primary dietary staple, providing a dense source of calories. Common prey fish include species such as herring, sardines, mackerel, and hake. Blue sharks also consume specialized open-ocean fish like snake mackerel (Gempylus serpens) and long-snouted lancetfish (Alepisaurus ferox). The high volume of these schooling fish found in shark stomachs indicates that the sharks exploit moments of high prey density to maximize their intake.
Opportunistic and Varied Diet Items
The blue shark is an indiscriminate feeder whose menu includes a wide array of secondary items beyond the staples. Crustaceans are regularly consumed, with certain populations feeding heavily on pelagic red crabs (Pleuroncodes planipes) when they bloom. They also possess specialized gill rakers that allow them to strain minute prey like krill, a feeding method more often associated with filter feeders.
The opportunistic nature of their feeding extends to scavenging and preying on surface-dwelling animals. Blue sharks consume the carrion of marine mammals, such as whale and porpoise blubber. Small seabirds, including petrels and shearwaters, are also occasionally consumed, typically when they are resting on the water’s surface.
This indiscriminate feeding behavior results in the documented ingestion of non-food items. Blue sharks have been found with debris, plastic, and garbage in their stomachs, which they likely encounter and attempt to consume in the open ocean. This reflects their generalist approach, where they will ingest nearly any item they can overpower or scavenge.
Hunting Methods and Migratory Feeding
The blue shark’s hunting success is linked to its migratory patterns and physical adaptations for oceanic life. These sharks undertake immense, transoceanic migrations, spanning thousands of kilometers, following seasonal temperature shifts and the distribution of prey. They track the movements of squid and schooling fish to ensure continuous access to feeding grounds, such as spawning aggregations.
Daily vertical movements are a significant hunting strategy, particularly in the mesopelagic zone. Blue sharks perform deep dives during the day, sometimes exceeding 400 meters, to follow the deep scattering layer of organisms that retreat from sunlight. They then ascend to the surface at night to feed on this prey, including deep-sea squid, as it moves closer to the surface.
While not built for explosive bursts like mako sharks, their streamlined bodies allow for sustained speed and agility to pursue fast-moving prey. The sharks often rely on cooperative or social hunting, aggregating to herd dense schools of fish or squid into a tight ball before attacking. The blue shark uses its sharp, serrated teeth to grasp and incapacitate its prey quickly.