PHOTOS: On the Hunt for the World's Monster Fish

July 19, 2010— -- Some fish are so big and bizarre that they defy a fisherman's exaggeration.

Take, for example, the alligator gar, which outlived the dinosaurs and boasts a mouth full of razor-sharp teeth. Or the Mekong giant catfish, which grows to a massive 10 feet in length. Or even the invasive Asian carp, which can jump 10 feet above the surface of the water.

Reel in one of these fine freshwater specimens and the truth will be impressive enough.

This week, National Geographic premieres "Monster Fish," an entertaining expedition around the world to uncover the biggest and most mind-blowing fish on the planet.

Hosted by Zeb Hogan, an aquatic ecologist, the show highlights the massive creatures, as well as the work of conservationists to protect them.

Below are a few of the "elusive giants" they find in the depths of the world's waterways.

Asian Carp

Like a "mutant swarm in a B-movie," Asian carp are invading America's river systems, Hogan says in "Monster Fish."

The country's waterways were free of the invasive species until the early 1970s, when a fish farmer imported bighead and silver carp from China to see if they would eat algae and help clear the lagoons.

Since then, the species' population has grown by leaps and bounds. Now there are about 2 million invasive carp on the Illinois River.

And, Hogan said, those carp are particularly skittish. As boaters power through the river, carp have been known to "fly" 10 feet out of the water, sometimes injuring fisherman along the way.

But they're not just harmful to people above the water, they're endangering native fish species.

"If the carp grow too numerous, they could squeeze out native fish, including two ancient giants, the Mississippi paddlefish and the lake sturgeon," Hogan says.

Scientists say the carp are multiplying so quickly, that they are the greatest threat to the Great Lakes.

Alligator Gar

With its jaws full of prickly pearly whites, the alligator gar looks more like its namesake than a fish.

But Hogan said this monster is one of the most unusual fish in North America and one of the world's largest freshwater fish. The alligator gar, which dates back to the age of the dinosaurs, can grow up to 10 feet long and live up to 50 years.

"The alligator gar has outlived the dinosaurs in part because it has some exceptional survival tools," he said. "Unlike most fish, Alligator Gar can breathe air from the surface, allowing them to live and hunt in swamps and marshes where oxygen levels are low."

But though the fish are known as survivors, they're facing new threats in the modern world. Their population is healthy in Louisiana, but the fish is considered rare from Tennessee to Ohio.

Wels Catfish

If you come across this creature on a fishing trip, you'd better be careful. According to legend, the Wels catfish grow to more than 8 feet long, feasting on birds, cats and, some even claim, humans. One of the world's largest freshwater fish, the Wels catfish has rows of tiny teeth.

The freshwater monsters flourish mostly in Europe and Asia, including the Danube River, and grow to about 10 feet and 450 pounds.

Lake Sturgeon

These giants are the second-largest freshwater fish in North America. But overfishing and dam construction has depleted their population.

In the U.S., they're considered threatened or endangered in 19 of the 20 states they inhabit.

Lake Sturgeon can sometimes grow to more than 7 feet and 300 lbs. The average male lives 55 years and females can live for 80 to 150 years.

Arapaima

If you want to peer back into the Jurassic age, take a look at the arapaima.

The South American tropical fish grow to more than 9 feet. And the river predators can eat nearly anything, sometimes even birds and monkeys that find their way into an arapaima's path.

Though the fish's habitat is mostly unchanged, over-fishing has made the arapaima rare around the world, National Geographic says. But conservation projects in South America that limit fishing are trying to restore the creature's numbers.

National Geographic Channel's "Monster Fish" premiered Sunday, July 18, 2010 at 10 p.m. ET/PT and continues Mondays at 10 p.m. ET/PT. For more information, click here.