Billions of fish are caught each year. All too often unwanted species are also caught and left to die without reason. So a fish eater is indirectly responsible for more deaths than the ones he sees on his table alone. Fish are sentient creatures and feel pain. They die by suffocation which is extremely painful and which can sometimes take many minutes.
Commercial fishing has decimated the aquatic environment. Shrimp nets kill countless sea turtles. Dragging trawlers kill all life including the plant life that fish thrive on, at the bottom of the ocean. Overfishing is causing the extinction of species that are fished, as well as those that depend on these fish for food.
Today, fish are also victims of the factory farming system. They are often farmed in floating cages or in artificial ponds. Because of the large numbers involved and the restricted movement, disease spreads rapidly, often also to wild specimens, and the water gets highly polluted.
Each vegetarian or vegan will save at least half a ton of fish in a lifetime.