Don’t be shy or embarrassed!
A good fishmonger or conscientious restaurateur will be happy to discuss the subject with you. Don’t hesitate to write to the manager of the supermarket where you shop regularly or leave a message at the restaurant where you have just eaten, whether you were happy or unhappy with their commitment to sustainability.
You can find some helpful ideas on these websites:
Monterey Bay Aquarium (USA)
Environmental Justice Foundation