Perhaps you mean, "Even if you're a vegan and you don't feel like being one anymore you should consider oysters."
---Wikipedia
Other discussion: http://ideas.blogs.nytimes.com/2010/04/08/why-shouldnt-vegan...
The mercury argument is not correct. I believe all the credible research studies done have suggested only pregnant women and children are susceptible to the "high" mercury levels in canned tuna.
Also for environmental sustainability if we examine the Seafood Watch's website, we notice both farmed Oysters and Pacific Sardines on the Super Green List in the Best of the Best category http://www.montereybayaquarium.org/cr/cr_seafoodwatch/sfw_he... [ Further examination: On sardine page, only choice is "best choice". For tuna, everything is generally "avoid" unless it is "trole or pole" caught. ]
I object to the notions sardines taste fundamentally bad, I think Alton Brown's Avocado-Sardine sandwich was one of his few works of genius: http://www.seriouseats.com/2010/01/alton-browns-sardine-avoc...
Even if you're a vegan you should consider Oysters. They are the most concentrated natural source of Zinc, do not have a central nervous system, and oyster farming actually helps clean the environment. Diets that are mainly vegetarian have too high of a copper to zinc ratio, so Oysters are a good way to offset this.
Also sardines are a really good, cheap source of Omega3 fatty acids, Calcium, Vitamin D, and B12, which also all might be low in such a diet. Also unlike much larger fish such as tuna, they contain much lower levels of Mercury and other heavy metals and are much more likely to be sustainably fished.