On its sustainable fish website, Greenpeace has formulated a model policy for fishmongers. The website also contains interesting information about fish labeling and certification.
There are many guides to sustainable supply policies online:
You can also get useful information from INCOFISH, a European Commission-funded project which brings together 200 experts from 35 research institutes in 22 countries, 15 of which are developing countries. The database is one of the few to supply detailed information about the adult size and reproductive seasons of fish species, along with clever and practical Fish Rulers which illustrate the minimum size of the most common fish in different seas.
SeaLifeBase is another detailed fish database.