Rosenstiel School professor emeritus Samuel Gruber is co-author of a study that may prove useful in conservation efforts for over-fished sharks.

A long-term field and DNA study by the Institute for Ocean Conservation Science at Stony Brook University, the University of Miami, the Field Museum of Chicago, and others has shown that young lemon sharks born in the waters off the Bimini Islands, Bahamas, tend to stay near their coastal birthplace for many years.

While shark research and conservation typically focus on baby sharks in shallow habitats or ocean-roaming adults, less is known about intermediate-aged animals, which are the breeders of tomorrow and are roughly similar in development to human “tweens” and teenagers. Tropical island nations that sacrifice their nursery habitats to coastal development are therefore likely to lose not only babies but also much older sharks from their local areas, with potentially dire effects on the surrounding ecosystem.

The study, conducted over a 14-year period at the Bimini Biological Field Station, is the cover article in the August issue of Molecular Ecology, a leading international scientific journal.

“It takes some sharks more than a decade to reach reproductive age, so we set out to better understand the phase of their development from when they are a couple of years old until they are on the verge of sexual maturity,” said lead author Demian Chapman, shark scientist and assistant professor with the Institute for Ocean Conservation Science at Stony Brook University. “We were surprised to document that lemon sharks lingered for years around the island where they were born, often more than half of their development to adulthood.”

Fear of deep water and the bigger predators that live there, combined with abundant prey in the mangroves around Bimini, probably keeps these island-born sharks in safer waters near home for several years after their birth. “This means that using marine reserves and other local conservation measures may help protect sharks born around tropical islands for much longer than we thought,” Chapman explained.

Love them or not, sharks are essential to healthy oceans. Removing these top-level ocean predators disrupts the local food web and causes negative consequences for other species and the ecosystem at large. Moreover, many tropical islands generate substantial revenue from shark-dive tourism, which this new research suggests is heavily reliant on sharks born in local nursery areas.

During the course of the Bimini study, conducted from 1995 to 2007, more than 1,700 immature lemon sharks were caught, tagged, and released. The implanted tags, plus subsequent recaptures and DNA analysis, showed that more than half of the 3- to 7-year-old sharks caught off Bimini were born locally and had lingered near their birthplace for years.

“In general, the survival of these intermediate-aged sharks is critical for sustaining shark populations,” said study co-author Samuel Gruber, professor emeritus at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science and director of the Bimini Biological Field Station, who has led the lemon shark research program at Bimini since 1978. “Our study suggests that local conservation efforts can help many lemon sharks born at islands like Bimini to survive through roughly half of their development to adulthood. Broader scale, sometimes international, management is needed to protect them after they’ve left their birthplace as adolescents and adults.”

Detailed information on how sharks disperse from their birthplace could be very useful for conservation efforts throughout the tropics, given that many tropical shark species are threatened by overexploitation to meet the demand for shark-fin soup, which is especially high in Asia. Between 22 million and 73 million sharks are killed each year to supply the fin trade, and international management agencies are scrambling for solutions to stem severe shark population declines.

The research team is now extending its study to answer one of the great mysteries of shark biology: Do sharks return to their birthplaces as adults? “This research showed that most of the young sharks left the island by the time they were mature,” said study co-author Kevin Feldheim of the Field Museum in Chicago, who led the genetics part of the study. “Now we want to find out if they end up coming back to the place where they were born to breed, much like salmon and sea turtles do.”