Oily fish
Fish whose flesh contains more than 5% fat (salmon, mackerel, sardines, herring, anchovies), the main dietary source of the long-chain omega-3s EPA and DHA.
Oily fish are defined by a fat content above about 5% of the flesh: salmon, mackerel, sardines, herring, anchovies, trout. This fat is precisely what makes them valuable, as it carries the long-chain omega-3s.
They are the main dietary source of EPA and DHA, the two omega-3s the body can use directly. This is an essential distinction: plant sources (flax, walnuts, rapeseed) provide ALA, which the body converts to EPA and DHA at a very low rate, in the order of a few per cent. The two are therefore not interchangeable.
They also supply vitamin D (one of the few significant dietary sources), vitamin B12, iodine and selenium. French guidelines set consumption at two portions of fish a week, one of them oily fish.
The size of the fish matters more than the species. Heavy metals, mercury above all, concentrate as you move up the food chain: large, long-lived predators (tuna, swordfish, shark) accumulate far more than small oily fish. Sardines, mackerel and anchovies therefore offer the best ratio of omega-3 to contaminants, and are also the most affordable and the least pressured in terms of resources.