Which is worse for the oceans - eating meat or eating fish?

It's a commonly-encountered sentiment amongst people who care about oceans: many marine ecosystems are in trouble; fisheries are a primary cause for much of this trouble; ergo, to help the ocean a person should stop eating fish.

This seems like a fairly straightforward argument. However, it gets murky very quickly if the person replaces seafood with meat from terrestrial production systems. Consider this: the UN FAO estimates that 18% of global greenhouse gas (GHG) emissions are attributable to livestock production. Read that again - 18% of all anthropogenic GHG emissions are associated with livestock production. This is more than the share attributable to transportation. (Read the FAO brief here).