As Juliet Eilperin reports in her dismaying new book, “Demon Fish,” more than 73 million sharks are killed each year by fishermen who hack off their fins to sell as a coveted ingredient for soup. As many as 90 percent of sharks in the world’s open oceans have disappeared. 

Sharks reveal much about the ocean, how it functions and why it is now in peril. As so-called top predators (like lions, tigers and polar bears), they help keep the marine ecosystem in balance through efficient hunting and killing. 

So when they are eliminated, cascades of ecological interactions are disrupted. Food webs (who eats whom) unravel. Diseases emerge. Jellyfish populations explode. 

By slaughtering sharks, we degrade the sea itself. 

Ms. Eilperin, an environmental reporter for The Washington Post, spent two years traveling the globe looking for sharks and people who worship, loathe, hunt, exploit or study them. She takes us to Papua New Guinea, where “shark callers” lure the animals from the depths and catch them by hand, using snares. She interviews shark fin dealers in Hong Kong and commercial fishermen in Florida who entice customers with the question “Are you man enough to catch a shark?” 

She swims in shark-infested reefs, once lowering herself into the water inside a shark cage, and visits a South African beach where spotters blow sirens when great white sharks come close to shore. Over all, she explains why sharks, so often reviled and feared, deserve our attention and, as she argues convincingly, our affection. 

“Demon Fish” reveals the close relationship between humans and sharks through the interplay of history and culture, with China as protagonist. 

The power of shark fin soup to convey status cannot be overstated. The dish is central to middle-class weddings and banquets, where it is a symbol of a family’s good reputation. 

But shark fin soup is one of the greatest scams of all time, Ms. Eilperin writes — “all symbol, no substance.” Its essential ingredient, stringy, tasteless cartilage, adds nothing to the value of the end product, yet the appetite for it continues to soar, along with Chinese affluence. 

The problem, she points out, is that we humans fail to comprehend our collective effect the planet. As we pursue activities like fishing, we don’t realize how many of us there are. 

Americans’ relationship with sharks, meanwhile, is based not on status but on fear. The dread began in the summer of 1916, when one or more sharks attacked and killed four people off the Jersey Shore, a bloody episode that helped inspire Peter Benchley’s 1974 horror classic, “Jaws.” In the Steven Spielberg film based on the novel, ominous music announces the approach of a huge white shark that performs the role of mass murderer. 

In real life, however, shark attacks are rare, claiming the lives of four or five people worldwide each year. 

“You are more likely to die from lightning, a bee sting or an elephant attack than from a shark bite,” Ms. Eilperin writes. “On average more than 40 times as many Americans seek hospital treatment for accidents involving Christmas tree ornaments than incidents involving sharks.” 

When fear is set aside, sharks emerge as likable, fascinating creatures. They are fish with skeletons made from cartilage, whose ancestors swam the oceans before the continents took their current shape. Found the world over, in saltwater and fresh water, they range from six inches long to the size of a bus. Some large sharks must swim at all times, mouth agape, to take in oxygen — which is why a large octopus can sneak up, grab and immobilize a shark and drown it. 

Shark skin is armored with tiny teeth called denticles that reduce friction by forcing water to flow in channels. Speedo, the swimsuit company, has tried to imitate shark skin, while engineers are looking for ways to coat ships’ bottoms with the material to keep off barnacles. 

Meanwhile, scientists are tracking shark movements via satellite tags and accelerometers, like ones in the Nintendo Wii, that capture how the fish roll and pivot underwater. 

As new findings emerge, a new breed of marine biologists has taken up the challenge of protecting sharks from those who exploit them. They are behind international movements to ban the fin trade and establish marine sanctuaries. Diving destinations in Palau, the Maldives, Fiji, Mexico and other shark hot spots now realize they can make more money by protecting sharks than by butchering them for soup. 

By the end of “Demon Fish,” readers may be tempted to don a wet suit and go hug a shark. Given that only 6 percent of shark species pose a threat to humans, it might not be such a dangerous thing to do.