Tuna fish probably seems like a tasty treat to feed your cat.  An occasional small amount like a bite or two probably won’t do any harm. Be careful not to feed her too much fish.  First, there is the risk of mercury which could build up in your cats system over time. A cat that eats too much tuna may also develop nutritional deficiencies.  Tuna alone can’t provide all of the vitamins and nutrients that your cat needs.  Tuna fish is especially lacking in vitamin E, an important anti oxidant. Without vitamin E, your cat can develop a disease called yellow fat disease.  Make your cat stick to a regular, balanced diet and only indulge his taste for tuna every once in awhile. Stick to cat treats instead.