I've pit tagged fish, and they didn't seem to have issues. But fishies generally don't live to be 80. Same tags are used in pets - which helped bring down the cost for us fisheries biologists, we were down to one company that made the chips and scanners. Got to love the laws of supply and demand.
That said, I don't want to be micro-chipped. I think that tracking ability is too George Wellian for me.
Edit - we did put a chip in someone's butt tho - involved being bored in camp, bottom of the Grand Canyon, many moons ago. I believe alcohol was also involved. I think if you ran the tag he'd come up as a Humpbacked Chub (native fish of Arizona).

