David Jakubiak (r.) and Ryan Williams hold a carp Williams caught off the Chicago Riverwalk on July 2. View Full Caption Ryan Williams

DOWNTOWN — The secret to catching a giant carp in the Chicago River: McDonald's French fries.

That's the bait Ryan Williams used to land a 20-pound carp on July 2 with lunchtime fishing buddy and co-worker David Jakubiak. They've hooked four other carp since then, wearing dress clothes on their fishing trips to the Chicago Riverwalk, just paces from the Environmental Law & Policy Center office Downtown where they work.

"Carp are bottom feeders, and they’ll eat anything, so we shot over to the 7-Eleven in our building and tried hot dogs, Combos, cheese puffs, gummy worms and a bunch of other stuff that didn’t work, either,” said Williams, a development officer with the law and policy center. “As we talked to other people fishing they told us McDonald’s fries were the only way to fly. The first day out with fries, I landed my first monster carp, and we’ve hooked four more with the fries! I guess Chicago River carp just love McDonald's.”

Another successful day #fishing on the @chicagoriver with @DavidELPC. 10lb carp for me & 18in channel cat for him. pic.twitter.com/bPqrTzQtNX — Ryan Williams (@ryanlwilliams) July 23, 2015

Williams and Jakubiak, a media relations manager, have fished the Chicago Riverwalk two to three times a week for the last month. In addition to carp, they've also caught catfish and have had small bass follow their bait — in this case, plastic lures.

Whenever they fish, Williams and Jakubiak said they draw big crowds of folks hoping to see them land a lunker.

Justin Breen says fishing on the river is completely legal:

"People get really excited about seeing a huge carp come up out of the water," Jakubiak said. “They’re less enthused about native species like catfish, but we’re working on that.”

Added Williams: "Once people see you fighting a big fish, they start to gather around and pull out their cellphones for photos. A lot of people are surprised there are fish in the river at all. Some want to know what we’re catching, and a few are worried that we might actually try to eat it."

And, no, they don't eat the fish. They throw 'em back.

Fishing on the Chicago Riverwalk — and anywhere on the Chicago River, for that matter — is perfectly legal, according to Margaret Frisbie, executive director of Friends of the Chicago River.

"It is absolutely legal to fish in the whole Chicago River system, including the Riverwalk, as long as you are on the water or public land, or land where you have permission," she said.

The Chicago River holds all kinds of fish, including crappie, rock bass and largemouth and smallmouth bass. In June 2014, 10,000 channel catfish were released at 333 Lower Wacker Drive, and officials with the Illinois Department of Natural Resources hope those fish and others will be safe to eat in the near future. For now, the IDNR warns folks from eating fish from the river.

Jakubiak said he loves catching fish, but he also wants to show people that the river provides fun outdoor options.

"Part of our job is the work towards a cleaner Chicago River, and the more people there are who see the river as a resource, the more people there will be to advocate for its protection,” Jakubiak said.

Added Williams: “With the number of people who stop to talk to us, I wouldn’t be surprised if we see more urban anglers on lunch in their suits and ties with a fishing pole on the Riverwalk. The more people who are on the river having fun, the more support river cleanup efforts will have, and if we can help make that happen, and have a little fun, that’s what we’re gonna do.”

For more neighborhood news, listen to DNAinfo Radio here: