According to Pam Johnson-Bennett of Cat Behavior Associates (cited below):
“Tuna as a treat now and then is harmless but as a steady diet can lead to malnutrition. Tuna doesn’t have adequate amounts of vitamin E and this may lead to a condition called steatitis (also known as yellow fat disease). Tuna has a very strong taste and smell so many cats can become addicted to it. Veterinarians refer to them as “tuna junkies.” The problem can be serious because these tuna junkies may refuse to eat anything else. The other problem with a steady diet of tuna is that there’s a risk of mercury poisoning. If you incorporate tuna into your cat’s diet, do it in a controlled way.”
Source: “Ten common mistakes when feeding cats” — Cat Behavior Associates