Blue sharks have a propensity for romance, getting caught, and never crossing the equator.
Blue sharks swim in all the world’s oceans, and a new study reveals surprising stories about their migrations and behaviour. For his doctoral research at the University of Auckland, Riley Elliott carefully attached satellite tags to 15 blue sharks—11 males and four females—in the waters off northeastern New Zealand between 2012 and 2015. With limited funding available for the expensive tags, Elliott turned to community groups and individuals, the sponsors following the animals’ movements online.
In some cases, the tag stayed attached for at least a year. One shark travelled more than 14,000 kilometres from New Zealand to the Pacific Islands and Indonesia and back. Another dived to more than 1364 metres below the surface—a record at the time for blue sharks—and a third swam all the way to the equator. “We were all kind of cheering for him,” says Elliott. “The scientific theory is they don’t cross the equator.”
One kilometre from the line, as though the shark had sensed it, he turned and swam south. “Unfortunately, he went through a real hotspot of tuna fishing, and we stopped hearing from him.”
These long-distance travellers were all males. The tagged females remained in and around New Zealand waters all year round—a surprise, given fisheries data had suggested there were few mature females here.
In a shark fairy tale, Elliott tagged a male and a female off Aotea/Great Barrier Island in 2014. The female had fresh mating scars (shark romance is not gentle), and there were young pups close by. In late summer, the male swam north to the tropics, while the female stayed around Northland. The following spring, they reunited—returning to the same spot near Aotea at the same time.
Blue sharks are declining almost everywhere, as they’re caught more often than any other shark in longline fisheries. In 2019, surface longline fishers in New Zealand caught almost as many blue sharks as they did southern bluefin, the species they were actually targeting. About three-quarters of the sharks caught were killed and processed for their fins or meat.