NEWSLETTER 12/2013 17.12.2013

2-headed bull shark fetus (Photo: Journal of Fish Biology)

2-headed bull shark fetus (Photo: Journal of Fish Biology)

2-headed bull shark fetus (Photo: Journal of Fish Biology)

Despite absences as long as 17 years, the pregnant sharks returned to the exact spot in the Bahamas where they were born when they were ready to become mothers, according to scientists who have been tracking the creatures since Bill Clinton was in the White House.

Researchers have long suspected that sharks returned to their birthplaces to give birth themselves – a phenomenon known as natal philopatry. Salmon do it. Species that take longer to mature, such as sea turtles, return to the general area of their birthplaces but may be off by hundreds or thousands of kilometers. (Researchers think this may be because animals use the geomagnetic field to home to their birthplace, and that field can change over many years.)

Sharks are considered late-maturing animals, but there have been signs that they practice natal philopatry. For instance, researchers have noticed that sister blacktip reef sharks use the same nursery sites in French Polynesia. Also, analysis of mitochondrial DNA – which is passed directly from mothers to their children – has shown that sharks in different families give birth in different places.

PHOTOS: Tracking lemon sharks for nearly 20 years

But for hard proof, scientists would have to track an individual shark from birth to motherhood. That would be a huge undertaking, but a group of researchers from the U.S., Canada, the Bahamas and Saudi Arabia rose to the challenge. Their initial findings were published online Thursday in the journal Molecular Ecology.

The international team focused on lemon sharks in the Bimini islands of the Bahamas because they are known to spend their first three years in very confined nursery areas before gradually swimming further and further away. Tagged females have been observed giving birth in the same place multiple times, though the birthplaces of those mothers wasn’t known.

So the scientists began tagging baby and toddler lemon sharks in 1995, including a few that were 1 or 2 years old at the time. Each shark was fitted with a transponder, and a tissue sample was removed from a fin for genetic analysis. The researchers returned to the same site for the next 17 years to tag sharks in subsequent birth cohorts. (You can see photos of their work here.)

By 2008, the scientists figured the oldest sharks in their study would be ready to give birth, and they kept their eyes peeled for pregnant sharks returning to the nursery. They used nets and ropes to capture these mothers-to-be and took DNA samples to see whether they were study participants. (Some sharks still had their transponders, but others had lost them over the years.)

After much effort, the researchers captured two sharks. One of them had been enrolled in the study in 1995, when she was estimated to be 2 years old. A few months later, they captured a baby shark that DNA showed to be her offspring. The baby was captured less than 2.5 miles from where the mother had been tagged 13 years earlier.

Study lead author Kevin Feldheim of the Field Museum of Natural History in Chicago said he was “excited and anxious” when the team set out to make their first catches.

“We knew that starting in '08/'09 we should start seeing some females return (if in fact we were correct about our hunch that this was a relatively common phenomenon),” he said in an email. “When we realized we found the first female come back to give birth, we were all relieved.”

The second shark still had the transponder she got in 1997, when she was a newborn. The scientists didn’t find any offspring of hers in 2008, but four of her children were identified in 2012.

Capturing pregnant sharks turned out to be “extremely labor intensive,” so after that first year they simplified things by checking the DNA of baby sharks to see whether their mothers had been tagged in the early days of the study. This method turned up four more cases of mothers returning to their former nursery to give birth.

PHOTOS: Weird sea creatures

In 2012, the most recent year included in the study, the team counted 15 mother sharks using the Bimini nursery. Nine had used the nursery before and were judged to be too old to have been tagged in the study’s early days. Among the six first-time mothers, three had been tagged as babies.

That would seem to nail things down, but the researchers added one more observation. Over the entire course of the study, 59 sharks gave birth in the waters off North Bimini island and six gave birth off South Bimini island – but none of the sharks used both. “Without exception, females were very faithful to one nursery site or the other,” they wrote (italics are theirs).

The evidence led the researchers to conclude that at least some female lemon sharks practice natal philopatry in the Bahamas. And that has “important implications for long-term sustainability of local nursery areas,” they wrote. “It is becoming increasingly clear that these imperiled predators have a complex population structure, and some species can benefit from investments in local conservation measures nested within broader international efforts.”

Feldheim said he felt "very fortunate" to have been part of such a long-term research project, which involved hundreds of student volunteers over the years.

"A study like this comes around once in a great while," he said. "As long as we can continue funding the field and lab work, I hope to keep going for another 20 years."

In addition to Feldheim, the research team included members from the Rosenstiel School of Marine and Atmospheric Science in Miami, the King Abdullah University of Science and Technology in Saudi Arabia, the Great Lakes Institute for Environmental Research in Windsor, Ontario, McGill University in Montreal, Stony Brook University in New York and theUniversity of Illinois at Chicago.