What are oily fish?
Oily fish include salmon, trout, mackerel, sardines, pilchards, herring, kipper, eel and whitebait. These fish count as oily fish when they're canned, fresh or frozen.
Fresh tuna is an oily fish but canned tuna doesn’t count as oily. This is because when it's canned these fats are reduced to levels similar to white fish. So, canned tuna is a healthy choice for most people, but it doesn't have the same benefits as eating oily fish.