Femme fatale ready to mate (Image: Design Pics Inc/Rex Features)

The femmes fatales of the ocean are now a little less enigmatic – thanks to long-life batteries.

For the first time, female great whites (Carcharodon carcharias) have been tracked over the whole of their two-year migratory cycle. Previous studies had been limited to a year by the lifespan of the tracking tag’s batteries. This was good enough to follow male sharks on their shorter, annual round-trip, but not the females.

Michael Domeier and Nicole Nasby-Lucas of the Marine Conservation Science Institute in California tagged four adult females at Guadalupe Island, 240 kilometres off Mexico’s Baja California peninsula. The females were tracked for thousands of kilometres, spending up to 16 months in the open ocean between the mainland and Hawaii. They eventually headed to one of two sites next to the mainland before returning to Guadalupe Island.


Mating site

The period when the females were near the mainland coincides with sightings of newborn sharks, suggesting that this is where they go to bear their young. Given that their gestation period is around 18 months, their migration route supports the idea that mating takes place at Guadalupe Island, where the males return every year.

It had been suggested that mating occurs while both males and females are out at sea. In a previous tracking study, Salvador Jorgensen of the Monterey Bay Aquarium in California speculated that male sharks were exhibiting what looked like a courtship ritual in a remote location known as the White Shark Café half way between the Baja peninsula and Hawaii. His idea was based upon electronic tag data that recorded males repeatedly diving up to 150 times a day.

But Domeier’s study found that the females only spent 4 per cent of their time offshore in the White Shark Café, providing very little opportunity for mating. This, coupled with the fact that the peak in the diving behaviour and therefore presumed mating season doesn’t fit with when the females return to the nursery sites, suggests that another explanation is needed for the diving behaviour.

“Males and females have very different habits,” says Domeier. “They are rarely in the same place at the same time”. Guadalupe Island is the exception, he says.

Vulnerable offspring

The study also found that the two sites where the females go to give birth are close to commercial fisheries, raising concerns that the newborns and juveniles could be injured or killed by fishing activities. In fact, one of the four tagged females stopped transmitting data when she was leaving the nursery, and was not subsequently identified at Guadalupe Island, leading the researchers to conclude that she had suffered a “fishery-related mortality”.

In 2012, shark fishing was banned in Mexican waters during the pupping season but enforcement is lax.

“There are few studies that have been able to track large shark migrations over multiple years,” says Neil Hammerschlag from the University of Miami, who was not involved in the study. He says the work identifies critical areas where conservation efforts can be focused.

Journal reference: Animal Biotelemetry, doi.org/k3z