1231. Faithful as a Mother Shark

By Editorial Board, The New York Times, December 13, 2013

Lemon shark and a new born

The popular view of sharks as wandering predators, roaming the seas like outcast swordsmen in an old samurai movie, will have to be adjusted, what with this month’s big news about lemon sharks.

A study published Dec. 5 in the journal Molecular Ecology reports that female lemon sharks are homebodies, returning year after year, decade after decade, to have their babies in the sheltered nurseries where they themselves were born — the mangrove-lined shallows of Bimini, the Bahamas.

Researchers say these sharks — so named for their yellowish skin — return not just to regional breeding grounds to give birth, but to their specific birthplaces, like salmon. As the mothers do, so do their offspring, for generation upon generation. This homing behavior, called “natal philopatry,” is also seen in seals and some sea turtles, but this is the first time it has been shown in sharks.

The research that proves this took a long time, because sharks are very slow to mature. Sharks in this study were born between 1993 and 1997 and gave birth 14 to 17 years later. Lemon sharks give birth to litters of live pups every two years. The researchers, using microchip tags and genetic analysis to identify sharks and chart their family ties, found that the homing behavior was amazingly reliable: “Without exception, females were faithful to one nursery site or the other across multiple returns to Bimini.” They say there’s every reason to believe that other shark species do this, too.

Sharks are threatened across the globe by overfishing, especially because of the appalling Asian practice of cutting off their fins for soup. Lemon sharks, which have two prominent dorsal fins, are especially vulnerable. Conservationists have tried to protect sharks with fishing limits, but this study suggests that protecting breeding grounds is also essential.

Sharks are important predators in healthy marine ecosystems, yet they are feared, even though the damage they do to humans is vanishingly small. While there is still a lot we don’t know, now we know that some do go home again. If a fitting phrase — “faithful as a lemon shark” — ever takes hold, maybe all sharks will have a better chance of surviving.