Why would a can of tuna taste extremely bitter?!
Answers: The expiration date says it's good through July 2008.
It could happen if the coating on the inside of the can was damaged; it's also always possible that something besides tuna made its way into the can. Obviously, you shouldn't eat it. I'd consider reporting it to the manufacturer; you might save somebody's life.
I don't care what the expiration date is.
If it tastes bad, throw it out.
Then contact the company that cans it.
May be a bad batch.
botulism, it went bad and now there are micro-organisms living in it--do not eat it!