Ken Grewe plunges his hand into the bottom of a small, black plastic bag - as if he were searching for remaining treats in a sack of candy.

What Grewe is seeking is anything but sweet. Like he has for the last three decades, the 74-year-old Grewe is hunting for leeches.

Yes, leeches - the wormlike creature with suckers on each end. Thousands of leeches are sold in the United States to hospitals and clinics and used for medicinal purposes.

But Grewe sells his leeches to a bait shop he once owned on the west side of Otter Tail Lake in Minnesota - where the walleye fishing season opens Saturday. You can bet many of the millions of anglers hitting the lakes will be putting leeches on their hooks.

"It's a very popular bait," said Grewe, who began harvesting leeches in 1979 when he purchased his bait shop that is still called Ken's Tackle. "If I could get my own bait, it would save me a ton of money. Good bait is about the most important thing you can get for a tackle shop."

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It was trial and error when Grewe started harvesting leeches. During those 31 years, Grewe has perfected a system that allows him to confidently say, "Now, if there are leeches in the slough, I will catch them."

Here's how Grewe catches the slimy leech that can range in size from a half-inch to 10 inches long:

- The traps are the small bags Grewe makes himself out of black plastic. "That black plastic bag is a security blanket for them ... it keeps it dark," said Grewe, who once used cans as traps. "As soon as it gets bright light, they would swim out of those cans. We graduated to better stuff."

- Grewe discovered beef kidneys are the best bait to lure leeches into the bags, which have slits cut on each side for easy entry. "The beef kidneys don't deteriorate as fast, especially in warmer weather," Grewe said. "Leeches do not like anything that is stale. Everything has to be fresh."

- Grewe estimates he has used about a dozen sloughs to harvest leeches. He now pays rent to a private landowner to use two sloughs - one an 8-acre body of water where Grewe has 90 traps set around the perimeter and the other a

3-acre pond that has more than 60 traps.

On a drizzly, cool morning last week, Grewe was rowing his fishing boat around the smaller slough. The Styrofoam buoys he connected to his bags with twine dotted the perimeter of the slough.

It usually takes Grewe about an hour to check on the 60-plus traps.

"This is more than I expected," Grewe said, as he pulls a handful of leeches out of the bag and tosses them into a bucket of water. "You never know from one day to the next how many you will get."

On good days, Grewe said he will get as many as a half-pound of leeches in one bag. With 60 bags in this slough, a productive harvest could generate nearly 30 pounds of leeches.

"I am small compared to others who harvest leeches," Grewe said. "Some do it big time ... really, really big with 10 young guys running around trapping them."

Grewe, who stores the leeches in water tanks, sells the majority of them to his old bait shop, now owned by Gary Peterson, who sells the leeches for $3 per dozen or $12 per half pound.

"Fish in general like those leeches," Peterson said. "They are not native to the lake ... very seldom will you see a leech swimming in a lake. They come out of ponds."

Like the pond Grewe was rowing his boat around last week. He knows, from experience, that the warmer is gets, the fewer leeches he will trap. Harvest usually ends by June 1.

"They go into the mud, and they stay there until the next spring," said Grewe, whose weathered hands pull another bag from the frigid water. "My hands are getting tougher, and the arthritis is getting worse. One of these days, I'll quit."

But he still enjoys the peacefulness and isolation of the sloughs, where he sets the traps in the evenings and checks by the next sunrise. Besides working 12 hours a week at a nearby lumber yard, the "retired" Grewe still likes to hunt leeches.

"Oh, it's beautiful out here most of the time," said Grewe, who every spring reacquaints himself with a pair of loons on the larger slough. "They kind of get used to you. When the sun is shining, it's just beautiful. I like that."

Readers can reach Forum sports editor Kevin Schnepf at (701) 241-5549