The nightcrawler is the bait fishing version of "tie one on and start fishing." Almost every freshwater predator will eat one — trout, bass, smallmouth, walleye, panfish, catfish, even the occasional pike. They're sold live at every bait shop in worm country, kept refrigerated in tubs of bedding.
Pick worms that are firm and active when you tap the container. A limp worm is half dead and won't wiggle in the current. Keep your remaining stock cool and out of direct sunlight on the bank — a wet sponge in the worm cup helps in summer.
For drifting streams, hook them lightly through the collar (the thickened band) so they keep some wiggle. For stillwater, thread one onto a worm hook so it hangs straight. Replace when the worm goes soft.