The real question is what you want from a stock. If you just want flavor, the best bet with seafood is a gentle simmer several times. That is, you make stock, strain, chill, then use this as the liquid to make more stock. Don't boil hard or long. This will keep your flavors delicate but crank the strength. If you want thickness as well, examine the final product fully chilled and add gelatine to stiffen.