A very serious issue which isn't dealt with often enough. I've wondered a few times about how much contamination is occurring in all bodies of water where fishing lead is used. But now that you mention the issue of radioactive isotopes(assuming that there is some amount clinging to the lead in some way), it makes me wonder what could be happening to some of our fish. To take it even further, I question the safety of eating chinooks out of the hanford reach. The nuclear waste is known to be leaking and reaching groundwater and entering the columbia.
One question I haven't an answer for is how much lead does it take to significantly affect a fish's normal bodily functions? When you drift fish your lead comes out looking all scratched up. It's fairly safe to conclude that some of it got scraped off from banging on rocks. If the radioactive isotopes that we're talking about which we think stick to the lead are heavier than both the lead and the water, then logically it should sink. But aha! If the isotopes are bonded to the lead and the lead is but a mere flake, how long does it take to sink?
On an aside, what about those fly fishermen who still crimp lead shot on their line with their teeth?! Based on what chemistry I know, calcium and lead both have a 2+ charge. In plants, there is no differentiation between lead and calcium since they both have the same charge. So in soils where there is lead "contamination"(any level above the background level), plants take up more lead than calcium and thus you have a problem. I don't know if this is true for other living things like fish, but if it works the same way then I'm sorta scared about what this could mean.