gill net

noun, a type of flat fishing net that hangs vertically from floats on the water, having a mesh through which fish of the desired size can swim only partially before becoming caught by the gills when trying to retreat (“gill net, n.” OED)

“Whitefish, we can get them in the river, but you have to gillnet them. The problem is you’re not supposed to fish gill nets in the river.”
“Oh, why?”
“Well, you catch everything with a gill net. If they gill themselves, nine times out of ten they die. So if you get a gill net full, what you can’t sell is garbage.”