Many types of fish--including perch, bluegill and crappie--are adept at nibbling a worm until they are able to remove it from the hook while avoiding capture. You can turn the table on worm thieves by making a few adjustments.
Hook night crawlers multiple times; as the number of places the worm has a hook in it increases, so does the difficulty a fish will have trying to pull it off the hook.
Impale the worm through its end and thread it, lengthwise, on the hook from the point to the shaft.
Substitute small pieces of a worm for the entire worm. Fish such as white perch and crappie travel in schools. Once they begin biting, it is no longer necessary to have a big worm on the hook. These fish will go after even the smallest pieces of worm.
Place the worm close to the shaft of the hook. This forces the fish to get its mouth is close to the point of the hook, making it more likely for you to hook it when it bites.
Set the hook when you feel even the slightest tug on the line, reducing the opportunity for the fish to steal the bait. When using fishing floats, watch the bobber for ripples that mean a fish is trying to grab the worm.
Check the worm frequently. Never cast and wait without periodically reeling in the hook to check the status of the worm. The action of casting a worm sometimes loosens it from the hook, as does the current.
Change to a smaller hook if you consistently lose worms to fish. Many bait-stealing species have small mouths, so a large hook allows them to nibble away.
Change to a narrower, lighter line if you consistently lose worms to fish without feeling a tug on the line or seeing any movement on the line or bobber. Thicker and heavier line is not as sensitive to bites, allowing fish more time to steal the bait.