Some types of fish and shellfish have seasons. Shellfish such as mussels and oysters are taditionally only eaten when there is an r in the month, like game. But farming of both fish and shellfish means that you don't really have to worry about this anymore. Fish farming got a bad name in recent years due to the intensive nature of, say shrimp farms in south asia or salmon farming in Europe and Scandinavia but rope grown mussels, native oysters and more recent additions to the industry like cod are generally regarded as sustainable and even beneficial to the environment. If you want a nice, user friendly guide to which fish are in season when, Hugh Fearnely whittingstall's river cottage year book (Hodder, 2003) has charts of what's good and when.