The spot is meant to demonstrate the "frightening" death for "more than 1 trillion fish every year," the ad says.
Phoenix was inspired to become vegan by a fishing outing when he was 3 years old, The Hollywood Reporter reported Friday.
"To this day it is a vivid memory," he said in an interview in 1996. "My family and I were on a boat catching fish. As one fish was caught, he was writhing. Then he was thrown against the side of the boat. You couldn't disguise what it was. This was what we did to animals to eat them. The animal went from a living, vibrant creature fighting for life to a violent death."