Scalloped hammerhead sharks, Sphyrna lewini, are known to form great schools. But scientists are trying to solve mysteries about them, such as where do they breed and migrate?
Hammerhead sharks, with their unique, wide-eyed appearance, are among the most iconic species in the Galapagos. But as the sharks become increasingly endangered, scientists are searching for ways to help their population rebound.
One way they're doing it is by searching for where hammerhead sharks have their babies (called pups). Past studies have documented where and why some of the region's hammerhead populations migrate, but scientists have been struggling with one mystery—where the females that live around the Galapagos's Darwin and Wolf Islands go to have their pups. Darwin and Wolf Islands have some of the world's highest concentrations of sharks, meaning the pregnant females that live there contribute to the population of the species overall.
Now, thanks to tracking data from tags attached to some of the sharks, scientists have some of the first evidence showing pregnant hammerheads from those Galapagos islands may be migrating to mainland South American countries like Ecuador to give birth. If they can further confirm these findings, it may help governments better protect the species before it slides closer toward extinction.
Collecting the data
Hammerhead sharks, despite their intimidating names, are notoriously shy.
“Even the sound of bubbles scares them away,” says marine ecologist and National Geographic explorer Pelayo Salinas, who studies the animals in the Galapagos.
To outfit a pregnant female with a tag, Salinas and his research team free dove as much as 30 to 50 feet underwater with spear-like tools that attach the tags to the sharks. The tags are programmed to pop off after a set amount of time. Once free from the shark, the tags transmit light and temperature data back to the researchers, who determine where and at what time the tags surfaced.
From 2016 to 2018, 11 sharks were successfully deployed with tags.
“That tracking data shows at least three of the sharks leaving for the mainland,” says Mahmood Shivji, a marine scientists from Nova Southeastern University who is working with Salinas on these studies. The researchers say they knew the sharks are migrating to the mainland, but the new tag data could help determine if they're doing so specifically to give birth.
While the tags help bring to light new information, they also have a substantial margin of error. Loose tags can be as much as 100 to 200 kilometers off, says Shivji.
“That's where the DNA comes in,” he adds.
Pacific angelshark. Santa Catalina Island, California
For next steps, the team will conduct paternity tests between adult sharks in the Galapagos and pups found in nurseries around the Galapagos and coastal mainland regions.
“Recreating tracks from geo locations is a very crude way of defining movement,” says Steven Kessel, the director of marine research at the Shedd Aquarium in Chicago, who was not involved in this work. “The genetics is going to answer a lot of those questions more conclusively.”
Why do the migration routes matter?
According to the International Union for the Conservation of Nature, the scalloped hammerhead species in and around the Galapagos is endangered. The species falls prey to the lucrative shark fin trade, and many end up killed as unwanted bycatch by fishermen.
Based on some of the light and temperature data from the tags he deployed, Salinas suspects some of the sharks he identified fell prey to stray fishing lines.
Sharks help keep marine prey species in check, thus creating a more balanced ecosystem. Studies often cited by conservation groups also show how much they can benefit coastal economies. From activities like tourism and diving, a shark in Palau may be worth more than $1 million throughout its lifetime; sharks in the Bahamas generated $114 million in 2014; and sharks generated $221 million for Florida's economy in 2016.
“They're a big deal,” says Shivji. “If they're going to the mainland to give birth and those mainland areas are being fished, which we know they are, then what you're doing is impacting the recruitment of hammerheads in the Galapagos.”
In 10 months to a year, he says his research team should have a better idea of whether the hammerheads are in fact migrating to mainland areas to have pups.
The find should change how protections for the species are put in place, says Salinas. Rather than relying solely on regional MPAs, he says the migrating species would benefit from international marine protection agreements.