The simple answer is no, a leader is not necessary for trout fishing.  The use of a leader will be determined by the rig, which will usually be determined by the geography, conditions, and target zone. In places where you’re fishing heavy cover, a leader can be advisable. However, unlike saltwater fishing, we’re often using a lighter leader than our mainline for trout. If you’re lure fishing using mono or fluoro, a leader is not necessary. If you’re using braid, then yes, you will need a fluoro or mono leader.