Feeding our domestic "pussy cat" is strongly associated with fish. The cliche portrait of the cat as a true fish lover is commonplace - fishing in the goldfish bowl, the "tuna addict", and back alley cats snatching fish scraps from trash cans. Countless pet products cater to this image with a wide variety of fish based and fish flavoured treats and staple diets.

The truth is that nearly all domestic cats do indeed fancy fish, and care givers around the world eagerly accommodate this desire of their feline friends. Question is: are we doing our cats a favour with this?

Is fish a component of the cat's natural diet?

The most up to date science of species classification based on molecular genetics as well as morphological schemes indicates, that the domestic cat (felis silvestris catus - often only called felis catus) is one of four sub species of the species felis silvestris. Because domestication has largely influenced and grossly altered the domestic cat's food preference, to explore the truly natural diet of felis catus we should look to the diet of its ancestors: the European wildcat or forest cat (felis silvestris silvestris), the Asian steppe cat (felis silvestris ornata), and the African wildcat or tawny cat (felis silvestris lybica) [1.]
When examining the diet of these three subspecies, none include fish. If we further expand our investigation to include all species of the genus felis, we come to the realisation that only one of its members - the jungle cat (felis chaus) - includes fish in its diet, although still hunts predominantly rodent prey [2.] but none of the others do, including the black-footed cat (felis nigripes), the sand cat (felis margarita), and Chinese mountain cat (felis bleti) [3.]

1. Wild Cats, Status Survey and Conservation Action Plan, IUCN/SSC Cat Specialist Group, Kristine Nowell and Peter Jackson, 1996, pages xxiii and 32
2. Wild Cats, Status Survey and Conservation Action Plan, page 83
3. Wild Cats, Status Survey and Conservation Action Plan, pages 8, 47, and 96

Although a diet including or consisting of fish is not natural for the domestic cat, could a diet of fish nonetheless meet the nutritional needs of the cat?

Canned Tuna is among the most popular food stuff to feed to companion cats, because cats are very fond of it. It is not uncommon for cats, that regularly receive tuna, to refuse all other foods. Cats displaying this addiction-like behaviour are often refered to by Veterinarians as "tuna junkies".
Feeding a mainstay of canned tuna is long known to cause diseases of dietary origin. One of the most prevailing diseases afflicting "tuna junkies" is Steatitis or Yellow Fat Disease - an inflammation of the fat tissue in the body due to a deficiency of vitamin E. A vitamin E deficiency is usually the result of feeding tuna, or any canned fish, packed in vegetable oil. These products are high in polyunsaturated fatty acids which oxidate vitamin E, besides being a poor source of vitamin E to begin with. Currently, a diet consisting of large amount of any type of fish is considered the most common cause of this syndrome, [1.]
Canned fish - tuna or other, packed in water or oil - is not a complete diet for cats. Although it is high in protein, it does not supply the cat with sufficient amounts of certain amino acids, mainly taurine, to maintain health. The Calcium to Phosphorus ratio in canned tuna is 1:14.8 [2.] - providing the cat with too little Calcium to balance Phosphorus, resulting in bone disease caused by a loss of Calcium in the bone due to a deficiency of this mineral in the diet. [3.] The only canned fish providing sufficient Calcium is salmon with bones.
Also, many essential vitamins are not provided in sufficient amounts through a diet of canned fish, such as vitamin A and most B vitamins, like Thiamin, Riboflavin. Pantothenic Acid, Folic Acid, and Vitamin B-12 [4.] Last but not least, canned fish is high in sodium, possibly providing the cat with too much of this mineral.

1. The Cornell Book of Cats, by the Faculty, Staff, and Associates of the Cornell Feline Health Centre, College of Veterinary Medicine, Cornell University, second edition 1997, page 93
2. USDA Nutritional database for standard Reference, release 13
3. The Cornell Book of Cats, page 79
4. USDA Nutritional database
5. Nutrient requirement of Cats, National Research Council, revised edition

Raw, whole fish: Much of the nutritional deficient nature of canned fish can be contributed to the way it was processed. Many nutrients are sensitive to heat, and cooking or canning reduces levels of or even eliminates some nutrients in foods. Also, many nutrients are concentrated in various organs and body parts - like vitamin A in liver and Calcium in bone - and carnivores are provided with a complete diet by consuming the entire prey. Therefore, would a diet consisting of whole, raw fish be adequate for cats? Many feral cats in the Mediterranean do indeed supplement their diet considerably with whole, raw fish, but unfortunately, no statistics are available about their health status. The population appear to be thriving, which may be contributed to the fact that these cats mainly hunt rodents despite the generous availability of fish. [1]
It seems that the idea of raw, whole fish is not entirely without problems. An enzyme found in all raw fish, called thiaminase, can destroy vitamin B-1 (Thiamin), leading to neurological disorders accompanied by a general physical wasting due to loss of appetite. [2] This enzyme can be destroyed by cooking the food, which however reduces overall nutritional density of the food itself as well.

1. Cats in the Sun, Hans Silvester, 1995
2. The Cornell Book of Cats, page 93

Fish, cooked or raw, as part of a supplemented diet: Initially, when evaluating fish meat as a base for a feline diet, species like saltwater halibut and freshwater rainbow trout appear to be nutritionally adequate in all the essential amino acids and fatty acids - when raw or cooked [1]. Supplementation could provide for correct amounts of Calcium within the right ratio with Phosphorus, and could bring levels of vitamins - such as vitamin A, D, E, and complex B, to optimal levels. However, fish meat seems to contain insufficient amounts of the trace minerals iron, zinc, copper, and manganese, making fish meat in the end an unsuitable choice as a base for a staple feline diet, because it would require unreasonable supplementation [2].

1. USDA Nutritional data Base
2. Nutrient requirement of Cats, National Research Council, revised edition

Summery:

Although our feline companions seem have a real taste for all things fishy, fish is in fact not a natural food for the species. Obviously it is the scent that makes these foods so appealing to felines, perhaps triggering some instinctive craving, not unlike a human's craving for greasy, baked goods. However, it could just as well be a learned taste preference. Many domestic cats raised in close contact with humans have exposure to fish from early on, predominantly through commercial foods containing fish meal as a protein source, as well as treats and table scraps*. Also, the stereotype teaches people that cats like fish, who then feed their cats fish, and the cats in turn prove their care giver right by developing a taste for it. That cats don't only eat what is good for them, but also posses the ability to learn taste preferences is indisputable. Every day in households all over the world the true carnivore cat behaves very much like an omnivore, stealing muffins and bread, or indulges in fruit and vegetables, often even ingesting things outright harmful like chocolate or tomatoes.

Care givers have to accept that no favour is done for the cat by feeding fish as a main staple.
Fish, no matter if whole, dressed, raw, cooked, or canned will cause nutritional deficiency of some kind, which in turn will eventually lead to serious disease.
If fish is to be included in the meal plan of the domestic cat, one may only do so sparingly as treats, or to entice a sick cat to eat. Other than that, the story of fish and cats shall remain a fairytale.

(* a large percentage of domestic shorthair cats raise by Feline Future without previous contact to fish as food, will not eat fish when presented with it experimentally)