What do onion sacks have to do with raising brook trout? Well, nothing and everything! The famous onion sack project is simply a way for students to learn about in-stream entomology. Trout feed upon numerous species of insects, most of which spend almost their entire lives underwater.
In this project, TU volunteers fill empty onion sacks with leaves from the forest floor, tie up the bags, and anchor them to the bottom of a local trout stream. After two or three weeks, the sacks are removed and placed into a container with enough water to cover each sack and brought into the classroom. There, the students remove the contents and identify the various mayfly, caddis and stonefly nymphs as well as other squirmy creatures such as midge larva and maybe even a crayfish. The students are always amazed at both the diversity and amount of insect life that is found with such a project.
It is best to do this activity prior to the opening of fishing season when the rivers are full of anglers who might remove the sacks, thinking they are trash or trample on them without realizing they are in the stream.