McCartney is one of Dave Lister's robot goldfish, the other one being Lennon. They are kept in a fish tank in his sleeping quarters, and he apparently bought them on Titan since there is a "souvenir of Titan" sticker on the fish tank, like much of Lister's "junk" (although the episode "Ouroboros" would suggest they were bought on Mimas; however, this did occur in Kochanski's dimension. In the novel Infinity Welcomes Careful Drivers, Lister bought them on Miranda).