A dusky shark. Photo: Seaworld via Wikimedia Commons.

Two years after they were first sighted near the northern Israeli coastal city of Hadera, two sharks nicknamed “Judy” and “Silvy” by researchers have returned to the same Mediterranean Sea spot.

Judy and Silvy are dusky sharks, a migratory species that gravitates toward warm waters. They are considered endangered or vulnerable by several organizations due to overfishing and their slow rate of reproduction.

Dr. Aviad Sheinin of Haifa University told the Hebrew news site Mako that Judy and Silvy were “3.07 and 2.89 meters long respectively, and they survived almost two years in the Mediterranean.”

“The Mediterranean is known as very hostile to sharks,” he explained, “and yet they returned to the area of Hadera.”

Judy and Silvy’s return is a boon to researchers, said Dr. Sheinin, and “can give us additional understanding of the reproductive cycle of female sharks, their long-term survival, growth rate, and an assessment of the population size of female sharks in the eastern Mediterranean.”

To keep track of sharks in the area, researchers attach an acoustic tag that emits radio signals individual to each shark. “Until now, we saw that the tagged female sharks spend most of the winter in warm water, and leave in the spring,” said Sheinin, making Judy and Silvy’s appearance unusual.

“We assessed that the sharks’ arrival in Hadera during the winter is connected to their two-year reproductive cycle,” explained Sheinin. “This year we will conduct ultrasound and blood tests in order to research this unique phenomenon.”