Oily fish are species whose flesh contains more than about 6 percent fat by weight, with the fat rich in long-chain omega-3 fatty acids EPA and DHA. Salmon, mackerel, sardines, herring, anchovies, trout, and pilchards are the main examples. Dietary guidance in many countries recommends one to two portions a week because the omega-3s support heart and brain health, and oily fish also supply vitamin D, iodine, selenium, and high-quality protein. The fat that makes them nutritious also makes them perishable: unsaturated oils oxidise quickly, so freshness, chilled storage, and prompt cooking matter, and smoking or canning are traditional preservation methods. Sustainability and contaminant advice complicate the picture, with guidance balancing omega-3 benefits against methylmercury in large predators and the pressures of overfishing on stocks like mackerel.