Zeb Hogan was already familiar with the legendary Mekong giant catfish. After all, he’d been studying the beasts, which grow to hundreds and hundreds of pounds, for years. But when a colleague in Thailand phoned him up in 2005 to say that fishermen had hauled a 646-pounder ashore, it seemed…unprecedented. So Hogan, a biologist at the University of Nevada, Reno, did some poking around. He found some records that showed that it was not only the biggest Mekong giant catfish, but the biggest recorded freshwater fish ever caught.

And it all got him thinking: Could there be even bigger freshwater fish out there? National Geographic apparently thought it was a good enough question to fund him, so cash in hand he set out to find the answer. On over 50 expeditions across six continents so far, Hogan’s been wading through river after river and hooking giant fish after giant fish—building a better picture of Earth’s little-understood freshwater monsters in the process. So far that 646-pound catfish stands as the world’s biggest, but in his quest Hogan has found that the picture he’s built ain’t pretty.

“After working on this for about 10 years,” he says, “it turns out that there are about 30 species of freshwater fish that can get over 6 feet long or weigh more than 200 pounds, and they occur in large rivers and lakes all over the world and on every continent except Antarctica. And about 70 percent of them are threatened with extinction.” All manner of humanity’s shenanigans, from pollution to dam-building to overfishing, are imperiling these giants, to the point where some may vanish before science even gets to really know them.

But not if Hogan has anything to say about it.

Living Large

So, why do fish get so big in the first place? Most species grow throughout their lives, and as these aquatic monsters get bigger and bigger, at some point they can take on pretty much any prey in their habitat. After all, it’s clearly an advantage to be able to eat whatever comes your way. And they tend to swim in highly productive rivers, such as the Mekong, so there’s plenty of food about. As an added bonus, some fish eventually get too big to worry about being eaten themselves, Hogan says, “so most of these fish—giant catfish, sturgeon, giant stingrays—once they get to a very large size they don’t have predators.”

Up until fairly recently, these fish were dominating lakes and rivers across the planet, including the US—and they were going almost totally unnoticed. Even Arizona, not a place known much for its water, has its own variety of giant fish. Cutting through the state is one of the mightiest waterways on Earth, the Colorado River, and Hogan knows its power intimately: While on a survey of one of the Colorado’s tributaries as an undergraduate, a flash flood swept through, trapping his party in a canyon for two days. Swimming in these waters is a little-known giant: the Colorado pikeminnow. Minnows may be typecast as the runts of the fish world, but this pikeminnow can grow to 6 feet long—back in the early 20th century, when the fish wasn’t yet endangered, anglers were using rabbits as bait to snag it.

The central irony of the world’s biggest freshwater fishes is this: For all their enormity, they sure are hard to find. I’m willing to bet, for instance, that while you’ve probably heard of the sawfish, with its long snout studded with teeth, you’ve probably never heard of its cousin, the largetooth sawfish, which grows over 20 feet long. And what about the 16-foot stingray of Southeast Asia, whose toxic, serrated barb grows to 15 inches long—a creature that’s been known to drag fishing boats up and down rivers for hours, with men working in shifts to reel it in?

Many of these critters have only been officially described recently. The enormous whipray of Australia, for example, can grow to 10 feet across, yet wasn’t described until 2008. “And the short-tailed river ray down in Argentina in the Parana River, I’ve never seen any study of it at all except for reports that came from fishermen, reports that were in newspapers,” Hogan says. There’s so little information on these creatures, it’s difficult to tell whether they’re endangered or just hard to find.

It’s this information that Hogan is after. In the meantime, scientists can say one thing with relative confidence: These giant fish are living for a very, very long time, perhaps as long as a century. As a general rule in nature, the bigger you are, the longer you tend to live. A blue whale, for instance, will far outlive your typical fruit fly (though there are exceptions like the hydra, a tiny invertebrate that may actually be immortal). Living for so long also means that giant freshwater fish take a very long time to reach sexual maturity. And that’s a huge problem when you’re living in a world ruled by the human race.

Under Siege

Down in New Zealand there lives a creature called the longfin eel, which grows to 8 feet long. It has a rather epic life cycle: It’s born out at sea, migrates up rivers and into lakes, and spends up to 100 years there. Only near the end of its life does it head back down the rivers and out to sea to spawn. Waiting such an incredibly long time to spawn like other giant freshwater fish—relative to smaller fish that mature quicker and start getting busy earlier on—puts them at greater risk of getting picked off by humans before they can mate to propagate the species. Run out of sexually active adults, and you have a serious problem on your hands.

That is, if a longfin eel can get to its spawning grounds in the first place. New Zealand has come to rely on dams for electricity, irrigation, and maintaining water supplies. And these dams can make life a nightmare for migrating fish like the longfin eel. More and more dams are popping up on the lower Mekong as well. “You have Mekong giant catfish, sturgeon, longfin eel, the big catfish in the Amazon,” says Hogan. “These are all species that make long distance migrations to complete their life cycle.” And they may be running up against dams.

Overfishing is also an issue, particularly in the Mekong, where humans have traditionally relied on large species like the giant catfish for food. Think of it as a top-down approach: The catfish, with their plentiful meat, are typically targeted first, then the next biggest species, then the one after that. This puts huge strain on the giants’ populations. “What you see is that those larger fish are becoming very rare, and it’s shifted and the smaller species are the staple food,” says Hogan. “But that’s an indication that that resource is being hit very hard, it’s being overexploited potentially to a breaking point.”

Invasive species threaten the fish too. While the 6-foot pikeminnow isn’t threatened by overexploitation in the Colorado River, it is a victim of its own hunger. Catfish are invasive here, and they don’t compete for the pikeminnow’s food—they are the pikeminnow’s food. But they come equipped with defensive spines, which lodge the catfish in the pikeminnows’ throats, choking them. Add to this the fact that the Colorado river has been engineered to the point of absurdity (for a great history on this, check out Marc Reisner’s Cadillac Desert), and you’ve got one fish in serious trouble.

Saving the Giants

So what can be done? “From my perspective,” says Hogan, “really the first step in the process, or at least what I’ve been doing, is raising awareness of the fact that these fish even exist. I think things are changing now, but 10 years ago people didn’t know what a Mekong giant catfish was.” To that end, the National Geographic Museum in DC now has an exhibit, “Monster Fish: In Search of the Last River Giants,” running until October, to showcase these beauties (well, not in the flesh, but they do have five lovely life-sized models on display).

The second step, Hogan says, is gathering enough information on giant freshwater fish to start making rational decisions about their conservation. The Mekong River alone, for example, is home to the giant Mekong catfish, the giant freshwater stingray, and the 10-foot giant carp. “In the Mekong there’s very, very little information known about the life cycle of most of the fish,” he says. “And so as people start to make decisions about where to build dams, they’re doing so without any information about how these dams are likely to affect fish.”

With enough information, scientists and local peoples can further tackle problems like overfishing. Hogan has seen success with this in Mongolia, where the six-and-a-half-foot, 200-pound taimen, or Eurasian giant trout, swims. Local communities realized it was more profitable for them to support catch-and-release fishing tourism, a practice that proved so successful that the Mongolian government passed a regulation making all giant trout fishing catch-and-release. And in the same vein, Hogan has in the past been able to intercept fishermen who’ve snagged giant Mekong catfish, pay them market value for the beasts, then tag the catches and set them free.

Sadly, though, there was nothing Hogan could do for that record-breaking 646-pound catfish 10 years ago. By the time he’d arrived on the scene a month later, it’d of course been butchered and sold. But these days, fishermen are simply coming up empty—last year in northern Thailand, for instance, records show just a single giant catfish landed. And so it seems that for some of these species of giant freshwater fish, it may well be too late.