Ventral surface of liver striated. Bluefin tuna do not reproduce in Canadian waters. Spawning occurs during April, May, and June in subsurface waters. Females of 270-300 kg may produce upwards of 60 million eggs in a spawning season. Fertilized eggs are small, usually 0.9-1.1 mm in diameter, buoyant, spherical, transparent, and have a single oil globule.
Early growth is rapid.
Recent work on aging larvae using otolith microstructure has enabled biologists to estimate daily growth increments. In general, males tend to grow larger and are therefore heavier than females, while females appear to live longer than males. Males and females of the same length weigh about the same. This rapid increase in weight-length relationships is great. Bluefins may reach a maximum age of 38 yr.