The blue shark is one of the most beautiful and graceful species in our oceans. Sadly, their global population has declined dramatically over recent years, with the U.S.
National Marine Fisheries Service (NMFS) finding that “the North Atlantic and North Pacific population is decreasing by more than 5% each year.”
This highly migratory species is difficult to protect as it roams up to 6,000 miles a year, passing through numerous jurisdictions as it travels.
Scientists believe that a better understanding of the specie’s habitat requirements and nutritional needs is crucial if we are “to improve conservation of this near-threatened species.”
So let’s learn a bit more about blue sharks and what’s being done to secure them a future.
What Do Blue Sharks Look Like?
Blue sharks are eye-catching and distinct from all other species, being the only ones with a bright blue coloration. Their dorsal sides or backs are dark blue, appearing almost indigo.
This intense coloration fades on the flanks to produce a lighter yet just as vibrant blue. Like most other sharks, the blue shark has a light-colored belly that appears almost white.
Blue sharks are slender and streamlined with a long caudal fin or tail. They also have long pectoral fins that they extend while swimming. The tip of the blue shark’s dorsal fin is often visible above the surface.
The blue shark’s long, conical head adds to its aerodynamic appearance and yet lacks the spiracles commonly found between the eyes and gill slits of most sharks.
Blue sharks have large eyes with a layer of reflective cells called the tapetum lucidum behind the retina. This feature enables them to see better in low-light conditions.
Like most sharks, the mouth of the blue shark is lined with several rows of sharp, triangular teeth, each of which is “replaced every 8 to 15 days.”
While some sharks have sandpaper-like skin, the blue shark’s is smooth to the touch due to the dermal denticles being “overlapping and small.”
Blue Shark’s Taxonomy
With their cartilaginous skeletons, blue sharks join all other sharks in the Chondrichthyes.
Within that class, they belong to the subclass Elasmobranchii, even though they lack the spiracles that help to categorize most other elasmobranchs.
That means the blue shark has to keep swimming to force water into its gills in a process known as ram ventilation.
Blue sharks belong to the extensive Carcharhinidae family, along with all other types of requiem sharks.
There are around 50 different species in the Carcharhinidae family, including tiger sharks, blacktip reef sharks, and bull sharks.
The blue shark was given its scientific name, Prionace glauca, back in 1758. The word “glauca” is derived from the Latin word for a bluish-gray or green color, while Prionace comes from the Greek words for saw (‘prion’) and point (‘akis’), possibly in reference to the blue shark’s distinctive and strongly serrated upper teeth.
Blue Shark Characteristics
Blue sharks are amongst the fastest in the world, reaching top speeds of around 40kph. They still lag far behind the Mako shark, but for good reason.
The Mako has an endothermic heating system, which means it can warm its muscles before an attack.
A blue shark can’t do this, instead relying on “behavioral thermoregulation,” moving into shallower, warmer waters to re-heat up after a deep dive.
Blue sharks use their long pectoral fins and tails to propel themselves through the water, sometimes even leaping up to 6 feet into the air.
These leaps are still not fully understood but could provide the blue shark some relief from the parasites that literally get under their skin. Studies indicate that a single blue shark “can carry up to 3,000 individual parasites… at any given time!”
Most of the time, blue sharks cruise around the ocean using their large pectoral fins to ride the currents so they can conserve energy during their long-distance migrations.
Blue sharks are largely solitary but can sometimes be found in small groups. These groups are segregated according to age and gender, with the males and females meeting only briefly to reproduce.
Blue Shark’s Life Cycle
Mating can be a painful and violent experience for sharks, and blue sharks are no different.
Internal fertilization isn’t that easy underwater, so the males hold themselves in place by biting the female “between her first and second dorsal fins.”
To accommodate this aggressive mating behavior, a mature female blue shark has skin “more than twice as thick as that of the male.”
There’s no love lost between the male and female blue sharks, and they separate as soon as they’ve mated successfully.
Once fertilized, the eggs hatch inside the blue shark’s uterus, where she feeds them via her placenta for 9 to 12 months.
Pregnant blue sharks migrate north to give birth, producing an average of 25 to 50 pups, although there are reports of litters containing as many as 135 individuals.
Blue shark pups measure between 15 and 20 inches long at birth and are usually born between March and July. Once born they immediately leave their mother’s side to fend for themselves.
As one of the fastest-growing sharks in our oceans, blue sharks reach sexual maturity much younger than most other species.
Males mature at the age of around 4 to 6 years old when they reach approximately 187 cm long. Females take a little longer, maturing at 5 to 7 years and a length of 220 cm.
Researchers are unsure if the blue shark mates annually or can store sperm for later use, as the bonnethead shark does.
Where do Blue Sharks Live?
This shark species is one of the most abundant and widespread in the world, occurring in all the world’s oceans except the Arctic.
Highly migratory, the blue shark travels clockwise around the world’s oceans, covering up to 6,000 miles (nearly 10,000 km) a year!
This wide-ranging pelagic species occurs near the surface “where water depths are greater than 200 m” (approximately 650 feet).
Juvenile blue sharks utilize off-shore nursery areas for the first two years of their lives, even though these oceanic nurseries offer little protection against predators.
As they mature, the males and females separate, with the large juvenile females heading into higher latitudes than the males.
Studies suggest that the female’s thicker skin gives her “greater tolerance to relatively cold waters,” allowing her to extend her territory further than the males.
Both males and females return regularly to the nursery area, presumably to “mate and deliver young.”
Blue sharks rarely stray close to shore and largely avoid “shallower, continental shelf waters,” Although frequently seen near the surface, blue sharks have been known to dive as far as 1,148 feet.
They appear to descend deeper in warmer waters and generally prefer cooler waters of between 53.6 and 68℉.
Blue Shark Behavior
Blue sharks exhibit a confusing combination of solitary and group behavior. While they frequently hunt alone, they will also form loose coalitions to target specific species, such as schooling fish and carrion.
Although their mating rituals can be violent, blue sharks are generally non-aggressive around humans, which has led to them becoming an important species for marine tourism.
Blue shark attacks on humans are extremely rare, with the International Shark Attack File listing just 13 incidents, of which four were fatal.
In addition to their long-distance migrations, blue sharks also make regular “vertical excursions between the surface and depths of several hundred meters.”
Scientists believe these vertical migrations could be used as a hunting tactic or a means of “behavioral thermoregulation.”
Blue sharks remain segregated into groups determined by size and sex for most of the year, possibly due to the aggressive nature of the males and their indiscriminate hunting techniques.
Not only are blue sharks one of the few species to jump from the water, but they also swim at the surface with their dorsal fins exposed more than most other species.
This behavior, known as knifing, is particularly evident at dusk and dawn when, researchers believe, it maximizes the shark’s “foraging opportunities.”
The light conditions at these times make surface-dwelling prey more visible, increasing the shark’s chance of a successful hunt.
What do Blue Sharks Eat?
Blue sharks are most active at night but will feed at any time during a 24-hour period. These opportunistic hunters will eat almost anything that comes their way, including bony fish, other sharks, turtles, and seabirds.
Nevertheless, studies suggest that “cephalopods are probably the most important in the diet.” If the opportunity arises, blue sharks will gorge themselves on young squid until they’re so full that they’re forced to regurgitate their meal before they can continue feeding.
Blue sharks are unique in that they’re the only non-filter-feeding species known to eat krill. They have developed a special technique for hunting these tiny crustaceans.
By allowing smaller predators like anchovies to herd the krill into tight balls, they then swim through the tightly-packed crustaceans, “straining them from the water in a manner reminiscent of feeding baleen whales.”
Although they are tenacious hunters, blue sharks will happily scavenge off the carcasses of whales and other large animals, although they always give larger sharks space to feed before moving in themselves.
The blue shark’s teeth enable it to grasp small, slippery prey species just as effectively as it can tear chunks out of a larger food source.
This gives it greater dietary flexibility, which, when combined with its ability to strain krill from the water, gives it one of the most varied diets of any shark species.
What Hunts Blue Sharks?
Blue sharks are apex predators within their habitat, but that doesn’t stop larger predators from targeting them.
Great white and mako sharks are both known to hunt blue sharks, and there’s even a possibility that large male blue sharks predate smaller juveniles in a stark act of cannibalism.
Few marine species can escape the unwelcome attention of the ocean’s top predator, the orca, and the blue shark is no exception.
Aside from the usual suspects, blue sharks have another less likely predator – the Cape fur seal. Off the coast of South Africa, researchers saw Cape fur seals hunting and killing juvenile blue sharks on several different occasions between 2004 and 2015.
Interestingly, the seals ate only the blue shark’s internal organs, suggesting they were targeting “the most energy-dense parts” of their prey as orcas do.
Humans also prey on blue sharks, targeting them for their fins, skin, and meat and accidentally entangling them in commercial fishing equipment.
Approximately 20 million blue sharks are caught by the shark fin industry every year, and millions more are discarded as bycatch.
Conclusion
Blue sharks are one of the most abundant and widespread species in the world, but the shark fin industry and commercial fishing operations are taking their toll on the global population.
Fortunately, recent developments offer hope for the blue shark, with the fishing industry introducing a “small battery-powered device known as SharkGuard.” So far, SharkGuard has “reduced the numbers of blue sharks accidentally caught by commercial fishing gear in a French longline tuna fishery in the Mediterranean by 91%.”
Another positive note for the blue shark is the recent decision at the Convention on International Trade in Endangered Species of Wild Fauna and Flora “to limit or regulate the commercial trade in 54 shark species of the requiem family, including tiger, bull and blue sharks.”
Let’s hope this graceful predator can bounce back from the devastation caused by human activity and enjoy many more decades speeding through our oceans and engaging in their strangely violent mating rituals.
Nicky is a British adventurer and animal lover who spends her time exploring the natural world and writing about her experiences. Whether on horseback, underwater, running, hiking or just standing with a fishing rod in hand, she embraces everything her adopted home of South Africa has to offer.