Fish and shellfish are both classified as seafood. A well-balanced diet generally includes a variety of different seafood, including both fish and shellfish.
Although all fish that live in the sea are seafood, some of the fish at your local grocer or fish market may be freshwater fish. These fish live in rivers, lakes and streams, and are not technically seafood. Freshwater fish and saltwater can differ dramatically in the levels of omega-3 fatty acids that they contain. For example, trout, a freshwater fish, contains higher concentrations of omega-3 fatty acids than Atlantic cod, a saltwater fish. Still, some saltwater fish can have high levels of heart-healthy omega-3s, including king salmon and white tuna.
When buying fresh fish, select only fish that smells mild, not sour or fishy, and that is displayed on a bed of thick ice, if not in a refrigerator. Select fillets and whole fish that feel firm, and look for fish with bright red gills and shiny flesh that springs back when it's pressed. If buying fish fillets, be sure that the fillets exhibit no signs of spoiling — such as dryness around the edges of the fillet — or darkening or discoloration of the flesh.