It’s a good question. Why don’t vegans and vegetarians eat fish?
First, there’s the fact that fish feel pain.
The book Eating Animals describes how every fish suffers in order to become food for humans. Every one.
We can imagine that possibly some cows or chickens could be raised relatively humanely and slaughtered so quickly that they never suffer. That is possible (though unlikely) for land animals.
But fish always always always suffer. Farmed fish suffer in filthy water cramped in tight spaces before death. Then they suffer when they’re killed. Caught wild fish suffer in traps, nets, and on hooks. Not a single fish died peacefully. Not one.
Second, there’s the fact that fish-eating as currently practiced is not sustainable.
This reason is described in the film End Of The Line. At the rate of fish consumption we’re currently at, our children and grandchildren will live (if they survive) in a world without fish. We are literally draining the oceans of sea creatures at an alarming rate. It’s unsustainable and could result in ecological disaster.
Early in Eating Animals, author Jonathan Safran Foer explains what the term “bycatch” means:
bycatch refers to sea creatures caught by accident — except not really “by accident,” since bycatch has been consciously built into contemporary fishing methods. Modern fishing tends to involve much technology and few fishers. This combination leads to massive catches with massive amounts of bycatch. Take shrimp, for example. The average shrimptrawling operation throws 80 to 90 percent of the sea animals it captures overboard, dead or dying, as bycatch. (Endangered species amount to much of this bycatch.) Shrimp account for only 2 percent of global seafood by weight, but shrimp trawling accounts for 33 percent of global bycatch. We tend not to think about this because we tend not to know about it. What if there were labeling on our food letting us know how many animals were killed to bring our desired animal to our plate? So, with trawled shrimp from Indonesia, for example, the label might read: 26 pounds of other sea animals were killed and tossed back into the ocean for every 1 pound of this shrimp.
Or take tuna. Among the other 145 species regularly killed — gratuitously — while killing tuna are: manta ray, devil ray, spotted skate, bignose shark, copper shark, Galapagos shark, sandbar shark, night shark, sand tiger shark, (great) white shark, hammerhead shark, spurdog fish, Cuban dogfish, bigeye thresher, mako, blue shark, wahoo, sailfish, bonito, king mackerel, Spanish mackerel, longbill spearfi sh, white marlin, swordfish, lancet fish, grey triggerfish, needlefish, pomfret, blue runner,black ruff, dolphin fish, bigeye cigarfish, porcupine fish, rainbow runner, anchovy, grouper, flying fish, cod, common sea horse, Bermuda chub, opah, escolar, leerfish, tripletail, goosefish, monkfish, sunfish, Murray eel, pilotfish, black gemfish, stone bass, bluefish, cassava fish, red drum, greater amberjack, yellowtail, common sea bream, barracuda, puffer fish, loggerhead turtle, green turtle, leatherback turtle, hawksbill turtle, Kemp’s ridley turtle, Atlantic yellow-nosed albatross, Audouin’s gull, Balearic shearwater, black-browed albatross, great black-backed gull, great shearwater, great-winged petrel, grey petrel, herring gull, laughing gull, northern royal albatross, shy albatross, sooty shearwater, southern fulmar, Yelkouan shearwater, yellow-legged gull, minke whale, sei whale, fin whale, common dolphin, northern right whale, pilot whale, humpback whale, beaked whale, killer whale, harbor porpoise, sperm whale, striped dolphin, Atlantic spotted dolphin, spinner dolphin, bottlenose dolphin, and goose-beaked whale.
Imagine being served a plate of sushi. But this plate also holds all of the animals that were killed for your serving of sushi. The plate might have to be five feet across. (source)