From page 217 of The Book of Fish and Fishing by Louis Rhead, 1917:
For big trout, lying low in deep pools, more particularly the brown trout, the worm should be sunk to the bottom; it is sure to be taken quick, if the worm is actively alive. Of course, suckers, eels, and other vermin are liable to take it, if left in one position for any length of time. To prevent such annoyance, keep it moving, a yard or so every few seconds, not so violently as to scare the fish.
Later (p. 300) he accuses suckers and other disreputable fish of interfering with the stocking of trout:
Fry, unless bred in enormous quantities, are very little use in rivers which already contain feeders on fish, like eels, catfish, suckers, carp, pickerel, and perch.