Over the last few decades, shark populations around the world have started to show signs of trouble, and NATURE’s Shark Mountain reveals one of the causes. In one scene, a hook and line trails from a shark’s mouth like a slender warning flag — a remnant from a close call with a longline fishing boat.

Luckily, this shark escaped, but researchers say many other sharks have not been as lucky. Longliners — which set lines that can be up to 15 miles long and are laden with hundreds or thousands of baited hooks — pose an increasingly potent threat to sharks, which are often hunted for their fins and meat. Other marine animals not intended to be caught by longliners, such as sea turtles and seabirds, can also take the bait and drown.

Sharks living in the waters around Cocos Island have some protection from the threat: The area is a marine reserve, and the Costa Rican government has barred certain shark-fishing techniques in its territorial waters. But sharks don’t always stay in protected waters; they can move hundreds and even thousands of miles in search of food and mates.

The water around Cocos Island is a marine reserve, offering sharks some protection from the threat of longliners.

There’s plenty of danger along the way. For instance, long-line fishers leave lines drifting in the ocean for hours, sometimes days, then come back to collect the catch and throw away the dead animals they can’t sell.

Researchers believe such practices have contributed to a 60 percent decline in shark numbers in Costa Rican waters over the last decade. “The biggest problem is the overwhelming impact of international longline fishing fleets,” says Randall Arauz, President of Pretoma, a Costa Rican environmental group. “They fish incessantly in all corners of the world, setting billions of hooks, depleting sharks and anything that takes the bait.”

“Sharks are highly migratory species,” adds Allan Bolanos, Lead Fishery Observer for Pretoma. “They know no boundaries. They spend time in our national waters and migrate through international waters.”

Arauz believes that the best way to restore shark populations in Costa Rican waters is to significantly reduce fishing efforts in the region. “It’s a three step process: close our ports to foreign longline vessels; convince our neighbor countries to do the same; and get the U.N. to ban long-line fishing in international waters of the Eastern Pacific.”

Many fishing groups say such drastic steps aren’t necessary, and that shark populations can take the pressure. They also note that new innovations, such as special hooks, can reduce unwanted “by-catch” on long lines.

But biologists are skeptical, noting that sharks often take a long time to reach sexual maturity, and only produce a few young at a time. As a result, it can take a long time to rebuild threatened populations.

These ruins are all that is left of the legendary Cabo Blanco Fishing Club, an exclusive resort for wealthy deep-sea fishermen who flocked there over half a century ago. Like Rick Rosenthal in Superfish, these men were on the hunt for “granders” — black marlin weighing in at over 1,000 pounds. Just a few miles offshore, the waters were so plentiful that sports fishermen didn’t bother trolling for their catch — they would simply throw their lines in the direction of the giant billfish they spotted from their boats. They called this place “Marlin Boulevard.” Back at the club, they celebrated their record hauls by lining the pathway with marlin tails and tossing back a drink at the bar.

Standing on this hillside, looking out at the water, one can still see the ocean currents that brought this bountiful sea life to Cabo Blanco. Up and down the coastline, dark blue waves meet a bulwark of lighter blue water. Here, the icy waters of the Humboldt Current collide with warmer water from the Pacific Equatorial Current. Where they meet, upwellings of cold, nutrient-rich waters bring millions of plankton to the surface. Anchovies feed on the plankton. In turn, the anchovies support a breathtaking array of undersea creatures, including squid, sailfish, marlin, swordfish, and shark. But at no point in time were the waters here as rich with life as they were during the heyday of “Marlin Boulevard” in the 1950s and 60s.

On a sunny day in August 1953, one of the Fishing Club’s founders, Texas oil magnate Alfred C. Glassell, Jr., was fishing eight miles offshore. Suddenly, there was a violent tug on his line — something had grabbed onto his five-pound mackerel bait. For nearly an hour and forty-five minutes, Glassell wrangled his catch. When it finally surfaced he saw just how enormous the creature was. Again and again the massive black marlin leapt from the water, trying to free itself from the hook. But it was no match for Glassell. At 1,560 pounds and over 14-and-a-half feet long, it was — and still is — the largest bony fish ever caught by rod and reel.

In the years that followed, game fishermen who heard of Glassell’s giant catch swarmed to Cabo Blanco to try their luck. Among the celebrities said to have visited are Jimmy Stewart, John Wayne, and baseball player Ted Williams. Ernest Hemingway spent several weeks there during filming for the movie adaptation of his book The Old Man and the Sea. He later wrote, “We fished 32 days, from early morning until it was too rough to photograph and the seas ran like onrushing hills with snow blowing off the tops.” According to the locals, Hemingway himself hauled in ten marlins. Five decades later, however, it’s sometimes impossible to tell fact from legend.

The Cabo Blanco Fishing Club has been closed for decades.

The Cabo Blanco Fishing Club has been closed for decades. Like these distant memories, the granders have also faded away from Cabo Blanco. In the years that followed Glassell’s record-breaking catch, a dramatic increase in the commercial fishing of anchovies, which are often used for fishmeal or bait, led to a significant decline in this important billfish food source. According to some, a particularly severe El Niño event in the Pacific likely compounded their scarcity. In 1970, the Cabo Blanco Fishing Club finally closed its doors, due to the military rule of General Juan Velasco Alvarado and the hostile environment toward North Americans his policies engendered. The giant billfish were gone, and so were the tourists.

Today, Peru’s commercial fisheries continue to harvest the sea. It is an endless process, driven in part by the poverty of seaside villages. Nearly one-fifth of the world’s total fishing catch comes from the Humboldt Current ecosystem, yet local fishermen make only pennies on the pound. Rick Rosenthal thinks our insatiable appetite for seafood is having dangerous consequences. In Superfish, Rick watches the fishermen unload their bounty on the docks. “It’s taken away by truck and put on a plane, and most of it going to the United States,” he explains.

In recent years, however, the Peruvian government has taken steps to prevent overfishing. In 1999, Peru’s maritime research institute, Imarpe, joined with the fisheries ministry to institute a satellite tracking system to help monitor the country’s entire fleet of fishing vessels. Boat owners foot the cost of renting the mandated equipment, which relays data about water temperature and salinity (to help identify the occurrence of upwellings) as well as information about the types and quantities of fish that are caught. All of the data is tracked by a central system, allowing the government to better manage temporary fishing bans and other sustainability measures. On April 2, 2008, Peruvian president Allan Garcia signed a Presidential Order that, effective immediately, will ban the commercial harvest of billfish in the country’s waters. Developed in cooperation with The Billfish Foundation, a Florida-based non-profit dedicated to conserving billfish populations, the plan also includes other conservation measures and promotes a sustainable, catch-and-release sportsfishing industry in Peru.

Many conservationists see sportsfishing as animal exploitation and thus oppose the practice outright. However, tourism generated by the sportsfishing industry in Peru could provide an economic boon that may help some in poverty-stricken coastal communities shake their dependence on commercial fishing, which many fear is irreversibly depleting the ocean’s ecosystems.

Time will tell if the government’s measures will help spur an ecological revival here. For now, however, the village of Cabo Blanco can only reminisce about its glory days long past.