Buy sustainable seafood
Unsustainable fishing practices are destroying habitats and endangering many different species in our oceans. While eating less seafood can help - don't worry, you can get some omega 3s from plants - where you do buy fish, look out for labels that prove it was sourced sustainably. The MSC (Marine Stewardship Council) "ecolabel" is a good indicator as are terms such as "line-caught" or "sustainably harvested. In restaurants, make a habit of asking whether the fish served is ethically sourced.