A current the color of chocolate milk slips past my boat on a comfortable summer night, making an occasional slurping sound. The calls of barred owls join the chirps of crickets here on the Minnesota River. A half-hour ago, I anchored the boat near a flat riverbank ledge that drops into a deep pool.

Baiting a hook the size of my palm with a plump black bullhead, I lob the bait, hook, and a 4-ounce sinker into the pool. Leaning back with legs kicked up, I'm ready to wait. In the fading dusk, daydreaming of trips gone by, I come to immediate attention as I feel my bait's wiggle—transmitted through line to rod—become more frantic. Wiggle. Wiggle. Wiggle. Thud! The thumping noise sounds like a spring-loaded doorstopper being struck. The clicker on the reel ticks as line slowly peels out. I give slack to encourage the fish to keep the bait, then I rear back on the huge rod to set the hook when I am sure the fish has my bullhead. The hook set stands me straight up. This fish is big!

Ten minutes later, after guiding the fish away from my anchor rope and fighting the fish and the current, I summon the strength to pull the creature into a giant landing net in my other hand. I have the behemoth aboard. In the beam of my headlamp, I am staring at an olive-green and mottled-brown fish that weighs more than my preschool daughter. I have caught a flathead catfish, the unquestioned king fish of the Minnesota River.

Like a droopy belt across the southern third of the state, the Minnesota River stretches 318 miles entirely within Minnesota's borders. In the west where lake-fishing opportunities are scarce, this river is the local fishing draw for good reason. Here you can catch a fish of mammoth proportions. Here, on the extreme northern edge of the species' range, you can find flathead (Pylodictis olivaris), the largest catfish native to Minnesota.

Apex Predator.

Flathead catfish thrive in turbid, slow-moving, fertile streams. They often can be found near deep, dense cover such as logjams, fallen trees, and boulders. "The Minnesota River fits the mold perfectly," says Tony Sindt, Minnesota River specialist for the Department of Natural Resources. "It is full of deep holes, littered with fallen trees and woody debris, has turbid water for ambushing prey and an abundance of suitable prey—common carp, freshwater drum, sucker species, and gizzard shad. Almost every stretch of the Minnesota River from the Granite Falls Dam to the confluence with the Mississippi River holds an abundance of flathead catfish."

Sindt spends his field season assessing the health of this often-overlooked fishery. Because flatheads here are long-lived, up to 30 years, and experience relatively low mortality, Sindt says the Minnesota River has "a tremendous population of large flathead catfish that would rival any other fishery in the country." He adds, "Since most flathead catfish anglers practice catch and release, I suspect we will continue to see larger and larger flathead catfish being caught from the Minnesota River."

While channel catfish are best categorized as bottom-feeding scavengers, flatheads are quite the opposite. To consistently catch flathead catfish, you need live or fresh cut bait, such as bullheads, creek chubs, or suckers. Flatheads are typically apex predators in major river systems.

Managing flathead catfish on the Minnesota River is a challenge because of the difficulty in surveying the fishery. The anglers are largely nocturnal because that's when flatheads are most active. And the habitat in a river system is always changing.

Complex Management.

The river's size and habitat complexity, varied depths, and dense cover complicate the task of sampling its flathead population. Biologists have found that large hoop nets or baited trotlines left overnight can be effective methods. Four-foot-diameter hoop nets sit in the river, stretched taut by current and large anchors. Fish enter the mouth of the net and funnel into the upstream tapered end, from which they cannot escape. The net creates a current break that attracts flatheads, much like a hollow log or a snag pile does. Other fish incidentally caught in the net may attract flatheads moving upstream in the dark, looking for prey.

DNR fisheries managers have long wrestled with how to actively manage flathead catfish in the Minnesota River. The DNR's management on the river and elsewhere has been largely monitoring relative abundance, estimating population density per river mile, and indexing size structure; continued research on the post-spawn home range and baseline population data; and some regulation changes. Prior to the 2003 fishing season, an angler could keep a total of five catfish of any size or species. After taking public input, the DNR changed the catfish bag limit to five, of which only one could be larger than 24 inches and only two could be flatheads.

"The regulation was put in place to protect this beautiful, vulnerable fish from overfishing," said retired DNR regional fisheries manager Huon Newburg. "There had been reports of anglers targeting large flatheads and taking home limits of these large fish. The concerns came from DNR Fisheries, DNR Enforcement, and anglers."

Flatheads, in contrast to channel catfish, are found in winter months in river holes stacked together in a state of low metabolic torpor, a sort of pseudo-hibernation that makes the fish vulnerable to snagging and potential overfishing. Starting in 2015, the winter season, from December through March, will be closed for flatheads.

High-Quality Fishery.

After several summers on the river sampling flatheads and several winters aging the fish from pectoral fin spines, DNR fisheries scientist Steve Shroyer says he likes what he's seen. "The Minnesota River is one of the highest-quality flathead catfish fisheries in the country," he says. "We have a very high proportion of big fish. Thirty- to 40-pound fish are common. A lot of fisheries have overall abundance that may be similar but don't have the numbers of big fish."

Shroyer notes that growth of the Minnesota River's flathead population is similar to that of catfish populations in other parts of the country. But Minnesota River fish live to older ages.

"If they didn't live so long, it would just be a run-of-the-mill, small- to medium-sized flathead catfish size structure. Even though their growth slows way down when they reach maturity, if they live for 30 years, they can exceed 40 inches," he says. Shroyer says maximum size is pretty comparable to anywhere in the United States, even southern states.

Big fish with big fighting ability have experienced tremendous popularity gains in Minnesota in recent years. The state is a fishing destination for muskellunge and lake sturgeon. Flathead catfish, brought to the national consciousness through reality television shows like River Monsters and Hillbilly Handfishin', may be the next species to take off.

Flathead catfish are a part of the identity of Minnesota River towns. Franklin Catfish Derby Days attracts anglers at the end of July. The Belle Plaine River Fishing Contest will reach its 40th anniversary this August. Grand-prize winners regularly reel in flathead catfish over 40 and sometimes even 50 pounds. Could the Minnesota River someday produce a state-record flathead catfish? Exceeding the current 70-pound record is conceivable, according to DNR fisheries biologists.

On a warm August Friday night, the public water access parking lot on the Minnesota River just outside of Jordan is nearly full. On one bend upstream, a flotilla of boats is beached and tethered to a sandbar. The boat captains are on shore leave, kibitzing near a bonfire, their fishing rods set in PVC rod holders with light sticks. This informal gathering was organized by Darren Troseth, catfish convert and operator of King of the Cats, an annual fishing contest initiated in 2007 as a for-fun event (with no money involved) that promotes catch and release.

"I think flathead catfish captivate myself and other anglers not only because of their physical size and the battle they put up, but also the mystique around catching them," Troseth says. "A river changes from day to day, and every day on the river is an adventure. Flathead fishing gives me a good reason to be on the river, but when you strip everything away, I think it's really more about being on the water and just taking it all in than it is about the fishing."

The King of the Cats fishing contests have expanded across Minnesota and the upper Midwest. On this night, everyone is trying to catch a flathead catfish and earn end-of-season bragging rights. But the night ends with just one small flathead, a reminder of the challenge these fish present. Troseth cautions, "Flathead fishing is not for everyone. And I will say this: If you decide to take it up, be prepared to spend a lot more time fishing than you do catching."