The blue shark (Prionace glauca) is a highly migratory, sleek predator distributed globally across temperate and tropical waters. Based on scientific study of their growth structures, the typical lifespan of a blue shark in the wild is estimated to be between 15 and 20 years. While most aged specimens fall within this range, some research suggests a maximum age that may approach 25 years.
Methods for Calculating Blue Shark Age
Determining the precise age of a blue shark is challenging because, unlike bony fish, sharks lack ear bones (otoliths). The primary method involves analyzing the shark’s vertebrae, which are composed of calcified cartilage. Researchers prepare a cross-section of the vertebral centrum and examine it under a microscope.
This process reveals a series of alternating opaque and translucent bands, or annuli, similar to the growth rings found in trees. The assumption is that one pair of these bands is laid down each year, reflecting seasonal changes in growth rate and calcification. To make these bands more visible and easier to count, scientists often stain the vertebral sections with silver nitrate or other chemicals.
Validating this annual deposition rate is crucial for accurate age estimation. Direct validation involves tagging a live shark and injecting it with a chemical marker, such as oxytetracycline (OTC), which leaves a fluorescent mark on the calcified tissue. If the shark is recaptured, the number of growth bands laid down after the OTC mark confirms the annual nature of the band formation. Studies using this technique have confirmed the annual deposition of growth bands, at least in smaller, younger individuals.
Growth Rates and Sexual Maturity
The growth rate of the blue shark is relatively fast compared to many other large shark species, contributing to its life history timeline. Blue shark pups are born live—a reproductive strategy known as viviparity—at a length of approximately 35 to 50 centimeters. For the first few years of life, they exhibit a rapid growth rate, sometimes increasing in length by up to 30 centimeters annually.
This swift early growth allows the sharks to quickly reach a size where they are less vulnerable to predation. Sexual maturity, however, is not reached until several years into their life. Males typically mature between four and six years of age, reaching a fork length of around 180 to 200 centimeters.
Females mature slightly later, usually between five and seven years old, at a greater length of approximately 220 centimeters. This late age of maturity, combined with a gestation period of nine to twelve months, results in a long generation time for the species. The embryos are nourished by a yolk-sac placenta inside the mother.
Ecological Role and Conservation Impact
As an oceanic species, the blue shark is one of the most widely distributed and abundant pelagic sharks globally, inhabiting deep temperate and tropical waters. They are opportunistic mid-level predators, feeding primarily on small schooling fish and cephalopods, particularly squid. Their sheer abundance and predatory nature mean they play an important role in regulating populations of smaller pelagic organisms throughout the world’s oceans.
The blue shark’s life history traits—specifically its relatively slow growth to maturity and long lifespan—make it particularly sensitive to human pressures. Because they take several years to reproduce, populations are slow to recover when subjected to high mortality rates. The primary threat to the species is bycatch in high-seas longline and driftnet fisheries, which target tuna and swordfish.
Blue sharks are the most frequently caught large shark in commercial fisheries worldwide. They are often retained for their fins, which are highly valued in the global shark fin trade, and their meat, which is consumed in various countries. The International Union for Conservation of Nature (IUCN) lists the blue shark as Near Threatened globally, with a decreasing population trend. However, in specific regions like the Mediterranean Sea, where fishing pressure is higher, the species is listed as Critically Endangered.