dunno about mercury build up, but I'm pretty sure it only becomes a real risk in the larger models.
I'f you don't want it release it. These bullies play an important part in the eco system and food chain. Don't just kil them for no reason other than to kill them. Couldn't imagine them being any good for crab bait
Be aware that any and all sharks must be released if over the 1.5m max length.
Suppose it depends on how big and green the shark is, if you've worn it out bring it aboard and give it a good whack (if you want to keep it that is), if not best to just cut the line and let them go, the hook will come out eventually and you don't put your hands anywhere near the dangerous end