There are plenty of environmental reasons to switch from lead shot when weighting your trout rig for nymph fishing. We like tungsten putty for a few reasons:

• It’s non toxic. Biting lead shot to crimp them on the line isn’t a great idea. Tungsten putty just presses on the line.
• It’s non abrasive. Splitshot abrade your leader, tungsten putty doesn’t. Who likes losing fish to ratty line?
• It’s easy to remove and reuse. Mash shot with pliers and it’s pretty much over. Just pull off tungsten putty and mash it back into the container. Repeat.
• You’re never stuck with the weight you don’t need, or without the weight you do. Just add a little or remove a little until you find your sweet spot.