Tuna are large, powerful, and highly migratory pelagic fish belonging to the Scombridae family, inhabiting the open ocean. These torpedo-shaped marine animals are warm-blooded, allowing them to maintain a higher body temperature than the surrounding water. This adaptation enables them to remain active and fast even in cold, deep waters. As apex predators, tuna have a high metabolic rate that necessitates a continuous and energy-rich diet.
Primary Prey Categories: The General Tuna Menu
The diet of tuna is broadly categorized into three major groups: small pelagic fish, cephalopods, and crustaceans. Teleost fish constitute the largest portion of the diet by weight, often making up 64% to 88% of the total food mass consumed by adult tuna. This category includes highly energetic baitfish such as mackerel, herring, anchovies, and sardines, which are typically consumed in large schools near the surface. Tuna also target flying fish, lancetfish, and even juveniles of other tuna species.
Cephalopods, including squid and occasionally small octopuses, are the second most important prey group, accounting for 6% to 25% of the diet by weight. These invertebrates are a high-protein food source found at various depths, valuable for building muscle mass. Specific squid species like Stenoteuthis oualaniensis and Onychoteuthis banksii are commonly identified in the stomach contents of several tuna species.
The third category, crustaceans, makes up the smallest proportion by weight, ranging from 0.2% to 9%, but is significant for smaller individuals. This group includes larval fish, various small shrimp, and planktonic crustaceans like megalopa larvae. While individually small, the sheer quantity of these high-nutrient organisms provides necessary fats and micronutrients, especially for juvenile tuna.
Dietary Shifts Based on Species and Life Stage
The tuna menu is highly flexible and changes significantly based on the tuna’s species, life stage, and geographical location. The diet is not static; it reflects a functional response to what is most available and energetically worthwhile in the immediate environment. This opportunistic behavior leads to clear differences between species and size classes.
A major factor influencing diet is the tuna’s size, which dictates an ontogenetic shift in prey selection as the fish grows. Juvenile tuna rely almost exclusively on tiny invertebrates and planktonic organisms found in the shallow mixed layer. Once a tuna reaches a fork length of around 45 centimeters, it develops sufficient endothermic capability to access deeper, colder waters.
This physiological change enables larger tuna to target mesopelagic (mid-water) and bathypelagic (deep-water) prey, which are larger and more energy-dense. Species variation also dictates prey preference. Skipjack tuna are highly piscivorous and remain primarily in epipelagic (surface) waters, consuming mostly small schooling fish. In contrast, Bigeye and Albacore tuna have a higher percentage of deep-sea prey, with Bigeye known for diving hundreds of meters to feed on deep-dwelling squid and fish.
Yellowfin tuna demonstrate a highly diverse diet that changes significantly depending on the region, making them a generalist feeder. Their diet can be dominated by crustaceans in one area and by fish and squid in another, directly correlating to local prey availability. This flexibility is also seen in Atlantic Bluefin tuna, whose young have shifted their diet from historically favored sardines and anchovies to horse mackerel in some regions.
Feeding Mechanics: How Tuna Hunt Their Prey
Tuna are built for speed and endurance, which are the primary tools used to capture fast-moving prey. Their streamlined, fusiform bodies and specialized circulatory system help maintain muscle temperature, allowing them to execute high-speed pursuits. This physical structure is necessary for sustained hunting, as their prey are often capable of rapid escape maneuvers.
Tuna frequently utilize schooling behavior as a collective hunting strategy to maximize efficiency. When a school of baitfish is detected, the tuna work cooperatively to herd, corner, or confuse the prey. This coordinated attack makes it easier for individuals to capture targets and increases the feeding success rate compared to solitary hunting.
Tuna also exploit the predictable movement of prey through Diel Vertical Migration (DVM). Many mesopelagic organisms, such as lanternfishes and certain squid, ascend to shallower waters at night to feed and descend during the day for protection. Tuna follow this migration, diving to depths where the prey aggregates. This strategy allows larger tuna, particularly Bigeye and Yellowfin, to access a vast, high-biomass food source.