The music icon's new Peta ad.

PAUL McCARTNEY isn't stripping off his clothes for PETA like other Hollywood stars, but the veteran rocker makes an impact by stating why he decided to go vegetarian.

In the ad, Sir Paul says that "many years ago, I was fishing, and as I was reeling in the poor fish, I realized, 'I am killing him -- all for the passing pleasure it brings me.'"

For Paul, "something inside me clicked. I realized as I watched him fight for breath, that his life was as important to him as mine is to me."