During my masters course I had to do a report on the current state of a fishery, the laws and regulations associated with it, and how stocks were monitored. Let me tell you, reading about overfishing is depressing. There are many problems ranging from fishing techniques to the enforcement of regulations. Fish numbers are depleted while the laborious and time-consuming task of finding sustainable fishery solutions is underway.
Choosing sustainable seafood can be rather difficult. One approach is to look for the "certified sustainable seafood" label, endorsed by the Marine Stewardship Council. Their website has data on sustainable fisheries around the world. My strategy? I downloaded Australia's Sustainable Seafood Guide from the Australian Marine Conservation Society. I can just search by the name of the fish and results pop up on whether this fish is a "say no", "think twice" or "better choice". Details of why they have arrived at that conclusion are listed. It's all about being a better-informed consumer! Another easy thing to do is to shop at Aldi, who have an initiative to support sustainable fishing practices.