Food Standards Agency says on their website.....
Most of us should be eating more fish. It's an excellent source of protein and it contains essential vitamins and minerals, such as selenium and iodine.
White fish such as Cod, haddock, Plaice and Whiting are very low in fat. Oily fish is rich in omega 3 fatty acids and a good source of vitamins A and D. White fish contain some omega 3, but at much lower levels than oily fish.
Fish such as Whitebait, canned Sardines, Pilchards and Salmon - where you also eat the bones - are also good sources of calcium and phosphorous, which help make our bones stronger.
Shellfish contain similar nutrients to white fish and similar amounts of omega 3, though some types of shellfish contain more omega 3 than others. For example, Crab and Mussels are quite good sources of omega 3. Oily fish are the best sources of omega 3. Shellfish are good sources of selenium, zinc, iodine and copper.