It's an interesting question. I ended up somewhere near "salmon-or-smelts-a-week mostly-vegan" from a health perspective (good source of omega 3's, low mercury, and available from extremely sustainable sources). But a lot of shellfish has pretty similar advantages, and I do sub in mussels or shrimp as our weekly fish now and then. None has quite the omega 3 load, but there are other sources of that.