I think in the UK they are only allowed to use animals fit for human consumption - in the US, it is a different story though, their byproducts are a lot worse than ours. But, the fact it is fit for human consumption doesn't actually mean that much - we aren't obligate carnivores like cats. Never noticed that about the Tuna Imperial Mark - but then companies like HiLife use the dark tuna meat for their food, whereas we eat the light tuna meat, so maybe that is the difference.