Meat, such as beef, tends to be associated with livestock while poultry is a term used to refer to the meat from warm-blooded animals with wings. Both 4 legged livestock and birds are warm-blooded while fish are cold-blooded and this seems to be the loophole members of the Church found when the Catholic Church began requiring its members to abstain from eating meat on Fridays and during Lent. Fish along with meat from animals and birds have always been a part of the human diet as it is a source of protein. While people don't need to consume protein everyday people in the habit of consuming meat every day probably found fish to be a good substitute for meat on Fridays. Plants are a less expensive source of protein so the Church's requirement to abstain from meat on Fridays probably affected the wealthier classes more than the poorer masses.