If you eat fish and other sea products, make sure you get it from sustainable sources. A certificate of origin and checking how the fish got into the shop is an excellent place to start. It may not be cheaper, but it will ensure you live more sustainably. Otherwise, soon we won’t have any fish – cheap or expensive.
Here are some resources to help you:
Sustainable Seafood Guides by country (mostly Europe, parts of South East Asia, Russia, South Africa, and Hong Kong) by WWF