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You don't ask a trout to open up and say "aahh".... I simply hold the little fellers down the back of my waders for a bit after a lunch of chili and baked beans, and then draw 'em back out and hold an empty cup under their mouth... They usually give up their stomach contents freely...

Or... you could use a stomach pump... ;)

Actually, find some old plastic window screen (or buy a bit at your local Home Depot, Lowe's, or other) and staple a foot square or so piece between two wooden dowels... Hold that rig under water for a couple of minutes and sein up what's floatin' down the stream that the trout want to eat...
Tight lines, ya'll!!

Terry
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NorthEast Arkansas Fly Fishers
"If'n I'd knowed ya's comin', I'da knowed what ta do. I'da rose both hands and wove atcha!" - Earnest T. Bass, c. 1962