A human diver is easily dwarfed by adult billfish. The largest species can reach 15 feet in length.

Black Marlin

The record-breaking black marlin that Alfred C. Glassell, Jr. reeled in off the shore of Peru in 1953 was 14.5 feet long and weighed 1,560 pounds. However, it is believed that the longest black marlin in the sea could reach 15 feet. Black marlin are dark blue on the back, and their bellies are a silvery white. All “granders,” or marlin weighing over 1,000 pounds, are female.

Sailfish

Slimmer and shorter than marlin, sailfish have a long, rounded spear extending from the snout. The largest known sailfish reached 11.4 feet in length. They are distinguishable by their dark backs, silvery bellies, and elegant dorsal sail.

Swordfish

Swordfish are slender and scaleless. Unlike sailfish, the “sword” extending from the swordfish’s snout is flat — not rounded. Swordfish are bluish on the back, sometimes with a purple sheen, and silvery below. While the very largest among them may reach 15 feet in length, a 7-foot-long swordfish is considered very large.

MISSION: SUPERFISH

Marine biologist and award-winning filmmaker Rick Rosenthal set out to capture on film the biggest, fastest, most dangerous gamefish in the sea — the ancient creatures known as billfish.

The largest of all billfish is the marlin. They top speeds of 60 miles an hour on migrations that can span 9,000 miles. The largest, always female, weigh in at over 1,000 pounds, and are known as “granders.” Ernest Hemingway immortalized the grander in The Old Man and the Sea, the story of an elderly fisherman locked in a life and death struggle with this apex predator. To Hemingway’s great disappointment, he himself never landed a grander, although his novella captures the impact of this huge and graceful creature.

Rosenthal’s pursuit of these exotic fish unfolds in NATURE’s Superfish. Two years in the making, Rosenthal’s documentary travels to the Pacific and Atlantic Oceans as well as the Caribbean Sea in pursuit of these giants of the deep. In one extraordinary scene, off Australia’s Cape York Peninsula, Rosenthal is shown swimming with a stunning giant female marlin and two courting males.

Throughout his journey, Rosenthal encounters commercial fisherman, anglers, and researchers, who are also out looking for billfish. The search has become increasingly difficult for everyone due to over-fishing of billfish and their prey, as well as the environmental degradation of the oceans, which has resulted in a dramatic decline in their populations. The trajectory of population decline in all large fish has been precipitously steep, estimated at 90 percent within the last half century.

In the 1950’s, sports fishermen — like those who flocked to Cabo Blanco, Peru in the heyday of “Marlin Boulevard” — were known for testing themselves against marlin with rods and reels. Today, some fisherman are defending billfish against commercial fishing fleets that, while going after the seafood we eat, kill these increasingly rare sport fish in their nets as so called accidental by-catch. In major part because of the absence of regulations that limit such by-catch, marlin and other billfish populations continue to decline. In the case of marlin, some anglers, frustrated with governmental inaction, have pressed for local bans on the sale of such fish, and have sought commitments from local restaurants not to serve marlin.

Rosenthal’s difficult quest to encounter a giant marlin in the wild, rewarded by his exceptional footage of this fabulous creature, not only offers a rare aesthetic and learning experience for viewers but may serve to awaken people to the need to preserve these remarkable billfish in their native habitats. Buy the DVD. This film premiered May 2008.

Seafood lovers take note: there’s good news and bad news.

First, there’s the bad news for those who enjoy tuna on rye.

High concentrations of mercury, a neurotoxin that can damage developing brains in fetuses, are found in some kinds of popular fish such as albacore tuna. Swordfish and shark, king mackerel, marlin, orange roughy and tilefish also contain dangerous levels of mercury.

Women of reproductive age and young children are advised to avoid these types of fish and limit overall consumption of all fish to no more than 12 ounces per week, according to the Food and Drug Administration, as it takes months for the body to rid itself of mercury.

The danger from mercury is not just to developing brains. There is evidence to suggest an association between mercury exposure and heart disease, making it dangerous for everyone, but especially those who are already at risk.

The American Heart Association, however, recommends eating fatty fish at least twice a week because it is high in omega-3 fatty acids which are believed to help lower rates of heart disease, reduce hypertension, relieve some arthritis symptoms and prevent cancer. Fatty varieties that are low in mercury include herring, sardines, and wild salmon. Some popular fish that are also good choices include sole, tilapia, clams and oysters.

“It all depends on your diet — you can’t eat a lot of big, wild fish,” said Tim Fitzgerald, a marine scientist for Environmental Defense Fund, who provides health consumption information to Monterey Bay Aquarium’s Seafood Watch.

The problem with large, predatory species like marlin and swordfish is that they contain much higher levels of mercury than small fish, such as anchovies and sardines, because of the way mercury moves up the food chain. “Sharks, marlin, polar bears and people at the end of the food chain have the highest concentration of mercury,” Fitzgerald said.

It’s sometimes difficult for consumers to make seafood choices that are good for their health — and the environment. According to Tim Fitzgerald, “Billions of pounds of imported fish come into the United States annually, and less than one percent is tested for environmental toxins by the FDA.” Because marlin is not a popular dining choice in the U.S., many people are not aware of this. And while the FDA is the regulator body that creates consumer advisories about mercury for pregnant women, they actually do very little testing for this neurotoxin.

Another problem with the advisories is that they are not terribly specific and there’s a lot of room for interpretation, according to Fitzgerald. It’s also difficult for consumers to make the best seafood choices because sometimes what’s best for the environment is not always best for their health, and vice versa. For example, blue marlin and striped marlin from Hawaii are fairly resilient to fishing pressure and are listed as “good” alternatives for the environment on Seafood Watch. But, Seafood Watch also lists a health advisory for these fish, due to high levels of mercury. Monterey Bay’s other regional pocket guides provide further guidance for consumers and note that imported blue marlin and striped marlin should be “avoided.”

So, for U.S. consumers, the situation is “buyer beware — eat with caution,” but certainly not to give up on all fish. Consumers may just need some help from Monterey Bay’s Seafood Watch, which maintains a list of “which seafood to buy and why,” including a comprehensive seafood search, regional seafood guides — and printable pocket-sized guides for your wallet.

And, if you are a tech-savvy-seafood-lover, a “fish phone” may be more of what you’re looking for. Environmental Defense Fund’s Seafood Selector to-go allows mobile web users to look up their seafood guide on a blackberry or iPhone and download the information.

Ultimately, it’s ideal to exercise moderation and caution when eating seafood by taking into account both environmental and health concerns. Fortunately you don’t have to wonder whether the seafood menu at your favorite restaurant is environmentally friendly, the answers to your questions may just be a text-message away.