Oily fish, which means fish rich in oil that is derived from the fishes’ diets, include: herring, trout, mackerel, sardines, fresh tuna and salmon.

Canned tuna is not rich in fish oil because most of the fish oil is removed during the canning process. Canned Salmon, sardines, mackerel, trout all count as oily fish when they're canned as the canning process doesn't significantly reduce the fat content of the fish. 
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Oily fish provides a rich source of omega 3 fatty acids and is also a good source of vitamins A and D.

Omega 3 fatty acids can also be found in the livers of non-fatty fish such as cod. Cod liver oil can be used a dietary supplement both in liquid form or in capsules to provide omega 3 fatty acids and vitamins A and D. However, as with oily fish, there are concerns about levels of toxic contaminants in fish oil.