Should Cats Eat Tuna?

Catnip or tuna? Given a choice many cats would choose the tuna.  It's just one of those things cats go crazy for.  But, it isn't a good idea to feed a cat tuna, other than a very small amount as a rare treat. For one thing tuna, like most fish is contaminated with mercury.
The Consumers Union recently analyzed 42 samples of packaged tuna, and found that about half a can of white (albacore) tuna (which tends to have more mercury than light tuna) contains more mercury than the EPA's recommended daily limit for women of childbearing age.  That's for a woman, a cat is a whole lot smaller.
Another issue is BPA, a toxic chemical found in most can linings. 
In analyzing canned goods, Consumer Reports has found measurable levels of the chemical additive bisphenol A in canned tuna, including tuna labeled as “BPA-free.”

If you still choose to give your cat tuna, remember that tuna does not contain significant amounts of vitamin E, for example, so too much of the fish can lead to vitamin E deficiency, resulting in yellow fat disease, or steatitis.  Felines who are fed too much tuna can develop other nutrient deficiencies, too, because most de-boned fish are lacking in calcium, sodium, iron, copper and several other vitamins.