Judy and Sylvie had such a great time on their last Israeli Mediterranean vacation that they’ve returned again this winter. And it looks like they brought a bunch of friends along – fellow sharks, that is.

In a small, shallow area off the northern city of Hadera, 30 female dusky sharks – including Judy and Sylvie — and nine male sandbar sharks have been tagged over the past four winter seasons by marine biologists from the Morris Kahn Marine Research Station of the Charney School of Marine Sciences at the University of Haifa.

“The sharks are so accessible to us and that is such a unique phenomenon,” says PhD student Eyal Bigal, manager of the research station’s Top Predator Laboratory.

The rare confluence of marine biologists, marine engineers and an aggregation of sharks in shallow water, all in one place, enables the lab to monitor these sea predators without resorting to the usual method of fishing them out.

Not much is known about Mediterranean dusky sharks, and sandbar sharks are an endangered species.

“Any information we can get is helpful in preserving them,” Bigal says. “We know they are threatened and commercial fishing is only getting more intense, so this is quite important.”

The research is planned and supervised by Prof. Dan Tchernov, head of the Kahn Marine Research Station, and Top Predator Lab Director Aviad Sheinin. Local partners include the university’s Marine Imaging Lab and Acoustics and Navigation Laboratory.

The rare phenomenon is attracting interest from the worldwide media and from prominent marine biologists near and far.

Tagging provides clues

Many mysteries swim around the sharks.

Why are there no male dusky sharks or female sandbar sharks in this particular aggregation spot at the junction of the Hadera stream and the city’s Orot Rabin power plant? Perhaps the creatures are attracted to the warm water coming from the plant’s discharge pipes?

“There have been reports of sharks in other marine environments aggregating around coastal power stations, but only here does it still occur, and it’s unique to this region that they’re aggregating in such a small area around a power plant,” Bigal tells ISRAEL21c.

The scientists also don’t know exactly why the sharks stay only in the cold months or whether the same ones return year after year.

So far, only Judy and Sylvie have been positively identified as returnees, thanks to the lab’s sophisticated tagging system.

The tagging process begins by catching the shark and tying it to the side of the boat – without taking it out of the water – and measuring it every which way in order to learn about the animals’ growth rate in this part of the world.

“The second step is marking them, which allows us to determine the percentage of recaptures we have over the years, and can tell us about the size of the population,” says Bigal.

The scientists use several different methods. A long, thin “spaghetti tag” on the fin is easily visible so the researchers avoid recapturing the same shark twice in one season. A subcutaneous electronic chip, like pets have, is added so that if the spaghetti tag is lost or obscured it’s possible to scan the shark to see if it’s been tagged before.

“We also flip them on their backs and surgically implant an acoustic tag the size of a battery,” Bigal explains. “It transmits signals that are picked up by a set of receivers we’ve deployed along the coast of Israel. We upload the data from these receivers, so we know which shark was in proximity to the receivers and when.”

In the case of Judy and Sylvie, “We know they left a couple of months after tagging and didn’t come back to Hadera for two years.”

Finally, Scheinin’s crew has started using satellite tagging to keep better tabs on where the sharks go after they leave the Hadera coast.

After tagging, the scientists take blood and genetic samples from the sharks for analysis to understand what they feed on and other aspects of shark life that couldn’t be studied before.

“We also develop new technology for aerial surveys using drones and UAVs as well as underwater acoustics so we can detect sharks hundreds of meters below the sea,” says Bigal.

Babies, food, or something else

The Top Predator Lab is testing various theories for the seemingly intentional visits.

“We are now doing ultrasounds to find out if the sharks come while they’re pregnant for a certain part of their gestation period because of the warmer water. Or they might come for food,” says Bigal.

Lower-level species drawn to the water temperature could be appetizing to the sharks. Or the sharks may remember that the Hadera Stream used to be a dumping site for yummy agricultural waste.

“It’s like a puzzle and each piece we get helps us understand why they return. The ultimate idea is to be able to survey these animals at population scale, not only where they aggregate but in the entire Mediterranean and in other areas of the world,” says Bigal.

The large number of people, including divers, coming to see the wintering sharks is an opportunity and a challenge.

“This is an amazing opportunity to educate people, using sharks as an umbrella species to protect other species that maybe aren’t so charismatic, and to connect people with the Mediterranean Sea and the importance of conservation,” says Bigal. “But it’s also a potential threat to the sharks.”

Therefore, an observation platform is being built where many people can watch and learn about the sharks without disturbing them. Efforts are being made to educate people not to litter the beach and not to feed the sharks. Safe diving protocols are being formulated to keep both divers and sharks safe.

This project is a joint effort of the Israel National Parks Authority, the Hadera municipality, the Nahalim Development Company, the Society for the Protection of Nature in Israel, the Aquaculture Association, the Morris Kahn Marine Research Station, EcoOcean and the Israel Diving Association.

Bigal is often asked if the sharks are dangerous. “We are not the food they are looking for,” he replies.

“Nevertheless it’s a large wild animal,” he adds. “Whether or not they are dangerous may depend on the situation. Are they feeding? Are they feeling threatened? Do they maybe have their own individual personalities? I believe so. Some sharks, for example, look more curious or comfortable around us than others.”