➜ Strike One: Low calcium levels.
Whole fish, even with bone, is far too low in calcium for a cat. Remember: if you're making homemade cat food, one of the most important things to get right is the ratio of calcium to phosphorus. You have some wiggle room here, but not much. A whole ground fish would be low in calcium. And while the high phosphorus is not good for any cat, elevated phosphorus levels are something you most definitely wouldn't want to feed a cat that is suffering from any kind of kidney problem.
➜ Strike Two: Thiamin destruction.
Raw fish contains high amounts of an enzyme called thiaminase--an enzyme that destroys Vitamin B-1 (thiamin). A thiamin-deficient diet can lead to neurological problems and seizures in cats. No good.
➜ Strike Three: Urinary tract problems.
Fish, with its high magnesium content, can contribute to a type of urinary tract problem in cats.
While I'm on urinary troubles, let me interject something here. Curiously, the discovery some time back of magnesium as a culprit in feline urinary tract disorders was accompanied by the development of specialized 'urinary tract' formula foods. The idea with these foods was to add acid to the food, reasoning that magnesium crystals develop in alkaline, not acid urine. The problem is, cats whose urinary tract acid is too high are prone to another type of crystal--calcium oxylate. Just another reason it's best to feed a cat a diet that nature intended. As Dr. Elizabeth Hodgkins points out in her book, the rise of urinary tract diseases in the cat coincided exactly with the increasing use of dry kibble to feed cats. But I digress.
➜ Strike Four: Addiction.
Heaven knows, most cats absolutely adore the taste of fish. Anyone who's ever opened up a can of fish within a 12-city-block radius of any hungry feline knows that. But you can quickly end up with a 'fish addict' on your hands. And the last thing you need is a cat on a hunger strike refusing to eat anything but an inferior fish diet.
➜ Strike Five: Heavy metals.
There is a great deal of persuasive research suggesting that predatory fish (those at the very top of the food chain and the same ones often found in pet food or used as 'treats' for cats) have extremely high levels of heavy metals such as mercury--in addition to pesticides and other toxins. A 2004 study published in Acta Neuropatholgica discovered neurological disturbances in young kittens fed daily with tuna containing the US FDA-approved level of mercury (0.5 ppm).
➜ Strike Six: Possible link to hyperthyroidism.
A US Environmental Protection Agency (EPA) study in 2007 revealed a possible disturbing link between feline hyperthyroidism and the chemicals in fire retardants--that mimic thyroid hormones--and cats' consumption of canned food with fish. In the study, cats eating canned fish were exposed to polybrominated diphenyl ethers (PBDEs) at levels five times higher than cats eating poultry or beef canned foods.
➜ Strike Seven: Vitamin E depletion.
Felines love tuna, but eating it long term can deplete a cat's stores of vitamin E and create conditions that lead to an extraordinarily painful condition called steatitis, with symptoms such as hypersensitivity to touch and loss of appetite. Huuuuuge bummer for the cat and for you. You'll find tuna in lots of cat foods for the very reason that it's tasty to cats and draws them to the food. But it has nothing to do with healthy, safe, or necessary nutrition for cats.
➜ Strike Eight? Allergenic.
Fish are allergenic. To my mind, it just makes little sense to feed something that is more likely to create a allergic reaction than something that isn't.
Dr. Jean Hofve wisely advises against feeding fish and suggests that it be reserved as a very occasional and special treat--certainly no more than once a week.
If you're determined to give your cat a treat once in awhile, go for something like small bits of dehydrated chicken liver or freeze-dried chicken hearts. But skip the fish. If you must give treats, make it from something that had fur or feathers, not fins. It's just much kinder to and safer for your carnivore.