There are different sorts of half-assed vegetarians and the type I chose to be some years ago was pescetarian, meaning that the only animals I ate were fishes. The general theory of this, put forward by, among others, Peter Singer, was that fish were not exploited when they are alive. They got to have a nice life swimming about the sea before they became my dinner. They were not kept in pens like chickens or habitually mistreated like cows. The Omega 3 fatty acids were just a bonus.
That’s all changed now. 90% of the larger fish species such as tuna have been killed off, and 70% of fisheries have been over-exploited. We are fishing whole species to the point of extinction.
So the answer people came up with to solve this difficulty is the fish farm. About half of the fish consumed in the world today come from fish farms. The living conditions of fish in such enclosures are cramped and rather horrible. I will not attempt to describe them in detail here.
Almost all fish contain mercury.
Today, if I must eat animals, I am going to eat chickens.
Most chickens are also treated horribly while alive, but their living conditions are getting somewhat better, and in many places, free-range chicken is available. Chickens are not faced with extinction. Organic chicken is available that is not polluted with hormones and antibiotics.
When being vegan is just not practical, being pollotarian is superior to being pescetarian.