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Sharks play a crucial role in keeping aquatic wildlife in balance. Scientists now understand that the ocean ecosystem has been evolving over hundreds of millions of years as an integrated whole--a biological machine in which each component has a vital function. For most sharks, that function is to serve as what biologists call an apex predator, the ocean equivalent of a lion or tiger or bear. Not only do they keep prey populations in check, but they also tend to eat the slowest, weakest and least wily individuals. In so doing, they improve the target species' gene pool, leaving the smarter, stronger individuals to reproduce.
When an apex predator is removed from the food chain, this carefully balanced machine tends to go haywire. Without wolves and mountain lions to keep them in check, for example, some deer populations in the U.S. have skyrocketed. And in just the same way, experts believe, overfishing of sharks off Australia and Tasmania years ago led to an explosion in the octopus population and a subsequent decline of the spiny-lobster fishery. Declining numbers of hammerhead sharks off the Florida panhandle may have allowed stingrays to reach record numbers there. "It's impossible to predict the implications of removing sharks from the food chain, but it could be disastrous," says shark specialist Sonja Fordham of the Center for Marine Conservation in Washington.
Fordham can't be more specific because biologists are only beginning to understand how sharks live and behave and what their relationship to other sea creatures really is. That's hardly a surprise, since sharks spend most of their time out of sight of human observers. Thanks largely to increasingly sophisticated electronics, though, scientists are finally opening a window on the life history of the shark.
To make reasonable but vague assertions like Fordham's more rigorous, for example, marine biologist Chris Lowe, a colleague of Meyer's at the University of Hawaii, has developed an ingenious way to measure the role of one shark, the hammerhead, in a well-defined environment. Every year thousands of hammerhead pups are born in Kaneohe Bay, on the east shore of Oahu. (About 40% of shark species lay eggs; the rest bear live young, and some of these carry their young just as mammals do, with an umbilical cord connecting the fetus to the uterus.) For the next 12 months or so, the baby hammerheads are an integral part of the region's ecosystem.
As it happens, the university's shark-research lab is located on Coconut Island, right in the middle of the bay, so Lowe can study them easily. In order to understand how much impact a hammerhead has on the bay ecosystem, Lowe is trying to learn how much energy it expends and how much food that takes. He has designed a miniature sensor that attaches to the baby shark's back and registers every beat of the tail as the shark swims along. By feeding the babies a precise amount of fish, then putting them in a tank with constantly flowing water--a sort of shark treadmill--he can determine just how many calories they burn in swimming a given distance.