Reasons that fish isn't considered meat for the purpose of fasting include environmental differences and the intention behind fasting. There are differing opinions based on varying interpretations of religious texts.
Although some vegetarians allow fish in their diet, the concept of allowing fish has religious roots. During Lent, Catholics are required to abstain from eating meat on Fridays, but many modern Catholic churches will hold fish fry events during Lent.
One possible reason for allowing Catholics to eat fish is because of a distinction that is made between land-locked animals, including birds, and water-dwelling creatures such as fish. Another possible reason, mentioned in the writings of Saint Thomas Aquinas, is that red meat was forbidden because it was "a great incentive to lust," a characteristic not possessed by fish. Finally, the idea that red meat came from animals that were farmed meant that abstaining from red meat involved making a sacrifice of one's property.