It never made sense to me why fish was ok for Catholics to eat during lent. Why is a piece of salmon ok put not a piece of steak? Well, I consulted the google machine, and here was the best answer I found:
"The law of abstinence requires a Catholic 14 years of age until death to abstain from eating meat on Fridays in honor of the Passion of Jesus on Good Friday. Meat is considered to be the flesh and organs of mammals and fowl. Moral theologians have traditionally considered this also to forbid soups or gravies made from them. Salt and freshwater species of fish, amphibians, reptiles and shellfish are permitted, as are animal derived products such as margarine and gelatin which do not have any meat taste."
So there you go. Other animals eat fish, so it's ok for us too as well. But not pigs. Those are off limits.