Tuna (fresh or tinned) is no longer classified as an oily fish.
'As of 25th September 2018, the UK's official advice on oily fish will change, in respect that fresh tuna will no longer count as an oily fish.
This is because current data shows that levels of long chain n3 poly-unsaturated fatty acids in fresh tuna are comparable to that found in most white fish. As the evidence no longer supports fresh tuna being classified as an oily fish, PHE are changing government advice to reflect this. Advice on canned tuna will remain the same; it does not count as an oily fish. The advice on NHS Choices and other gov.uk websites will be altered from the 25th September 2018'. Association of Nutrition