You should start by questioning what health benefits people say you are getting. For example, you may not need omega-3 from fish if you reduce other sources of omega-6 in your diet. You should also question anyone that tells you to substitute beans for fish or any vegetable for an animal- the nutrient (and anti-nutrient) composition is going to be entirely different. Fish is generally a good source of vitamin D and iodine in addition to the nutrients of land animals. You can get iodine from seaweed. Eating more than small quantities of properly prepared tofu is making yourself a human experiment. Pasture raised meat is sustainable and healthy. Hopefully we can figure out some sources of sustainable seafood!