A video of 15 sharks circling near paddleboarders in California hides a bigger problem

The video opens with a close-up shot of a great white shark swimming in a turquoise ocean. But when it zooms out, the camera reveals the bustling coast of a Southern California beach.

"You are paddleboarding next to approximately 15 great white sharks," says a voice. "The sharks are as close as the surf line."

The voice belongs to Deputy Brian Stockbridge of the Orange County Sherriff's Department, who on Wednesday advised a group of paddleboarders at Capistrano Beach to "exit the water in a calm manner," according to The Orange County Register.

The sharks were seen just south of California's San Onofre State Beach, where a woman was bitten by a shark last month. The area was put under shark advisory, according to an announcement from Marine Corps Base Camp Pendleton.

But while it might seem as though sharks are becoming a bigger threat to people, experts say the opposite scenario is closer to the truth.

"We may never know exactly how many sharks are out there, or exactly how many are killed each year," said David Shiffman, a marine biologist. "What we do know, from a variety of different types of analysis, is that many species of sharks are decreasing in population at alarming rates."

The causes? Hunting sharks for their meat and fins, and irresponsible fishing practices.

According to a recent report by the nonprofit conservation group Oceana, thousands of sharks are caught and trapped in fishing nets and other fishing gear every year. Some estimates say this unintended catch, or "bycatch," is 40% of the world's total catch, or about 63 billion pounds a year.

One frequently cited survey analyzed data gathered from fisheries between 1986 and 2000 and found that hammerhead populations had declined by an average of 89%, great whites by 79%, tiger sharks by 65%, thresher sharks by 80%, blue sharks by 60%, and mako sharks by 70%.

In sharp contrast to the way they're typically portrayed, sharks also possess several characteristics that make them vulnerable to exploitation, such as maturing later and having fewer young.

"We all know sharks are in trouble," Jennifer Schmidt, a geneticist and the director of science and research for the nonprofit Shark Research Institute, wrote in a recent blog post for the institute.

Shiffman told Business Insider that although some shark populations, including great whites, were recovering, declining populations spell trouble for other marine life since the animals play a role in oceans' health.

Sharks are apex predators, which means they keep dozens of other ocean populations in check, according to the nonprofit organization WildAid. Sharks keep food webs in balance, stabilize other fish populations, and prevent the fish they prey on from taking over vital seagrass bed habitats.

"Recent victories in restricting shark fishing and regulating the fin trade are essential to prevent extinction of many shark species," Schmidt said, "but it will take a long time for these actions to impact such depleted populations."