A University of Nevada professor and the host of National Geographic's ‘Monster Fish’ has just published a new book.

"This book was an opportunity to tell the story of these big fish all around the world... the threats they face and what we can do to protect them,” says Zeb Hogan, Ph.D.

The book ‘Chasing Giants: In Search of the World's Largest Freshwater Fish’ details the search for giant fish around the world.

In 2005 the world's largest fish was a mekong giant catfish.

That record was broken last year when Cambodian fishermen caught on 661-pound giant freshwater stingray.

There are 30 species of freshwater fish that can get to "monster fish" status which is over 6 feet long or more than 200 pounds and you can learn about them at The Discovery museum's exhibit, also called ‘Monster Fish.’

He says Nevada is home to some big ones itself including a minnow that can reach six feet long!

“These big fish, they're indicators of river health. They're the most vulnerable fish in the system so if we can keep these fish in our rivers, it's a sign we're doing a good job keeping the rivers and the waters healthy."

Hogan tells us to help keep fish populations healthy -- it's important to keep rivers clean, follow fishing regulations and just pay attention to the health of our rivers and lakes.

The book is currently on sale.