Most of the world's vegetarians eat small amounts of fish, seafood or poultry depending on the culture. The healthiest vegetarian diet in the world belongs to the Okinawans and theirs is a pesco-vegetarian diet. They eat fish, seafood but the major portion of their diet is plant/fruit based.
Regardless of what the people say on here ... those cultures that have been vegetarians for centuries upon centuries rarely exclude fish, seafood, or poultry from their diets. The people on here have their own little planet that doesn't know anything about the rest of the world.
Omega 3 complex from fish is more readily absorbable and useable than flaxseed or other veggie head substitutes and ounce for ounce fish (esp. oily fish ... tuna, mackeral, salmon) has more Omega -3.
You might want to check out the modern equivalent of the vegetarians you find in other cultures ... Google "Flexitarian Diets" or "flexitarians".