Why is fresh tuna counted as an oily fish when tinned tuna isn't?

Fresh tuna is considered an oily fish because it has a higher fat content, particularly omega-3 fatty acids, which are beneficial for heart health. In contrast, tinned tuna often undergoes processes such as cooking and canning that reduce its oil content, resulting in a leaner product. Additionally, the type of tuna and its preparation can influence the fat levels, making fresh tuna richer in oils compared to its tinned counterpart.