Then follow up by stating that you care about sustainable seafood. If they carry it, thank them. If they don't, suggest that they do.
Overfishing is the greatest threat to our oceans today. The world's marine life is quickly being depleted. An estimated 90% of all large, predatory fish are already gone from the world's oceans. A recent scientific study predicted a world-wide fisheries collapse by 2048. The only solution is to turn back from the brink, and to begin consuming seafood in a sustainable manner.
By letting restaurants and stores know that we care, we encourage them to care. And that can lead to a better future for everyone.