From the NOAA's Question of the Month:

Q: Why does NOAA protect sharks

A: With all of the negative stories in the news about sharks, it is easy to wonder why NOAA would want to protect these "ferocious" sea creatures. However, most shark "attacks" are a case of mistaken identity - arms and legs hanging off surfboards and shiny jewelry resemble the shark's natural prey of fish and marine mammals. In fact, sharks hardly ever attack humans unprovoked, and when they do attack it is usually a "hit-and-run" with the shark quickly leaving after it realizes that what it has attacked is not a fish.

Sharks are awesome creatures whose biology has remained virtually unchanged for millions of years. Considered to be at the top of the marine food web, sharks play a role in the ocean similar to that of the top predators on land. There are not many known species that would be able to replace the shark at the top of the food web. The only known predators of sharks, other than humans, are killer whales and other sharks. Because of this, sharks play a very important ecological role affecting almost every other marine organism.

NOAA is mandated by Congress under the Magnuson-Stevens Fishery Conservation and Management Act to conduct stock assessments, monitor the species' abundance of sharks and implement fishery regulations that maximize the benefits of sharks as a resource for humans, while also ensuring that shark populations are not depleted. Due to overfishing, NOAA began managing shark fisheries by limiting harvests in 1993. No data is available yet about the impact of this protection because sharks are long-lived, take many years to mature and only have a few young at a time. Experts estimate that it will take 30 or 40 years before NOAA's conservation efforts fully pay off with healthier shark populations.

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See also the NOAA FISHERIES: Shark Web Site, especially "It's The Ocean, Not A Swimming Pool"