What we see on the plate of sushi isn’t a static object called bluefin tuna. Bluefin is an animal, an active one whose biology are deeply affected by its life cycle. From tiny larvae to giant adults they do more than just grow. It’s a process that reveals ability of fish to endure rough conditions and it alters your perspective about this species. It’s a biological machine designed for endurance and speed.
This puts the life events in perspective across three decades of life. There’s also an infographic that map them all. Initially the growth curve is steep, leveling off as you age. A bluefin will grow approximately ½ meter long during its first year. That rate is fast due to a high metabolism in the warmer spawning grounds. This help them transition from eating plankton to hunting for herring, anchovies, and other small fish. If they don’t catch this timing, they seldom survives into winter months.
The Life Stages of Bluefin Tuna
As the fish get bigger, so does its diet: zooplankton becomes replaced by larger food, including squid and mackerel. Once the fish gets to be three years old, it’s a whole new ballgame about growth patterns. It no longer grows in length as rapid; instead it begins to bulk up. This is when their unique anatomy become the real story.
Unlike other fish, bluefins have an internal system known as rete mirabile which maintains their internal muscle temperature above ambient seawater temperatures. As such, they don’t have to remain in tropical waters. They are able to make long migrations or dive deep for food sources unavailable to other predators. That’s how we get an Atlantic bluefin, but not one from the south.
The infographic say they reach upwards of seven hundred kilograms in weight and more than thirty years old. Their physiology explains this: their flexibility also allows them to grow big. According to the infographic, a bluefin is a conditioned athlete (not a mere hunk of blubber). And it has probably swum across the Atlantic many times by the time it reaches age fifteen.
It moves based off changes in water temperature and other magnetic cues. The long trips have been confirmed with tagging, which shows that older fish travel farther and dive deeper then do young fish.
“Fish in aquaculture pens are reared very differently than how they grow out in the wild. They don’t move around and expend energy chasing food so they’ll put on weight more rapid. But you pay a price for that rapid growth with less natural conditioning.” Natural conditioning is developed by wild fish due to their constant swimming, resulting in more dense muscle fibers which is much wanted for preferred texture. “Farmed fish can become large quite fast, though sometimes at the expense of the biological rigor present in the wild counterparts.”
As a bluefin ages, you can see how it learns to balance its energy consumption against what the environment demands. When they are juveniles, nothing is more important than reaching a size where starvation isn’t a constant threat. As young adults, it’s all about becoming strong enough to travel the world. As they get older, it’s all about packing on experience and mass until they become lone predator.
At each age, it’s always about efficiency. They’re made to endure an ocean that will try to freeze them. From larval stages drifting in the Gulf current to massive adults in the North Atlantic, they’re endurance machines. To us, they’re food.