Find a reliable fishmonger.
Cultivate a relationship with a reputable neighborhood seafood store. If you don't have one, select your grocery store carefully. Look for evidence that the store cares:
Is the fish properly labeled with country of origin and whether it's fresh or farmed? Does the store offer products with the Marine Stewardship Council's logo? Is the staff knowledgeable?
Ask the fishmonger questions.
Three major factors affecting sustainability are: biology (short-lived, quick-maturing fish with vigorous reproductive cycles are highly sustainable); catch methods (hook-and-line and traps are often favored because they have low environmental impact and don't snag many unwanted species); and fishery management (whether good policies are in place for preserving the habitat and the species). Ask your purveyor: