It depends of course what else is in their diet. The most likely deficiencies would be in �polyunsaturated fatty acids�, the so-called �omega-3-oils�. They are being more and more recognised as essential to health, and are contained in oily fish such as salmon, herring, mackerel, and tuna. The current recommendation is to have a portion of oily fish at least once per week�.or of course to take a fish-oil supplement as capsules.
In general fish (and again especially oily fish) is a good source of the fat-soluble vitamins A, D and E, and all fish contains useful quantities of the water soluble vitamins B1, B2 and B6.
Calcium, magnesium, phosphorus, and iron are the main trace-minerals supplied by fish. The vitamins and minerals in fish can also be supplied by a normal balanced diet.
I hope that this helps.