Feeding our domestic cats is strongly associated with fish. The clichéd image of the cat as a seafood lover is everywhere—whether it’s the cat fishing in the goldfish bowl, the “tuna addict,” or the alley cat snatching fish scraps from a bin. Countless pet products cater to this stereotype with a wide variety of fish-based and fish-flavoured treats and diets.

Cats certainly seem to enjoy fish, and caregivers the world over are happy to oblige. But fish is not a natural food for domestic cats. Our cats are direct descendants of the African wildcat, which evolved in the Sub-Saharan savannahs—not on the shores of the Mediterranean. Every physical attribute of the domestic cat is finely tuned for hunting and eating small mammals. This is well documented by biologists.

According to the latest species classification based on both molecular genetics and morphological data, the domestic cat (Felis silvestris catus) is one of four recognised subspecies of Felis silvestris. If we want to understand the truly natural diet of F. s. catus, we must look at the diets of its wild cousins: the European wildcat (F. s. silvestris), the Asian steppe cat (F. s. ornata), and the African wildcat (F. s. lybica). None of these eat fish. If we expand our view to the entire Felis genus, we find that only one species—the jungle cat (Felis chaus)—regularly includes fish in its diet, and even then, its primary prey is still rodents. All other members, including the black-footed cat (F. nigripes), the sand cat (F. margarita), and the Chinese mountain cat (F. bieti), do not.

Tinned tuna is one of the most popular fish foods fed to companion cats, and many cats develop a strong preference for it. As a result, many cats refuse all other foods. These cats are often referred to by veterinarians as “tuna junkies”. But feeding tinned tuna as a staple food has long been known to cause nutritional diseases. One of the most common is steatitis, or yellow fat disease—an inflammation of fat tissues caused by a deficiency of vitamin E. This is most commonly seen in cats fed tuna or other canned fish packed in vegetable oil. These products are high in polyunsaturated fatty acids, which degrade vitamin E, and they’re also poor sources of the vitamin to begin with. In fact, any fish-heavy diet can put cats at risk of developing this condition. Tinned fish—whether packed in oil or water—is not a complete diet. At best, tinned fish can be used to flavour other food to entice a cat to eat, or be used as a source of Omega 3 fatty acids.

What about whole, raw fish? Would that be more suitable? Some feral cats, especially in coastal regions or the Mediterranean, do supplement their diets with whole, raw fish. But we have no reliable data on their health. Raw fish does come with problems of its own. It contains an enzyme called thiaminase, which destroys vitamin B1 (thiamine) and can lead to neurological issues and general physical decline due to appetite loss. Cooking destroys this enzyme, but also reduces the nutritional value of the food overall. Raw fish can also carry parasites, which poses another health risk.

Fish is not recommended as the primary meat in a homemade recipe. Even with a premix and additional supplementation to offset its low levels of trace minerals—particularly iron, zinc, copper, and manganese—fish remains too nutritionally and biologically distant from what small cats evolved to eat. It must also be cooked to be fed safely, which further reduces its nutritional value.

Despite the popularity of fish among domestic cats, it is not a natural part of their ancestral diet. Why do so many cats like fish, then? It could be the scent, which may trigger an acquired craving. Many cats are raised on commercial food containing fish meal, fish oil, or fish-based flavours, as well as fish-based treats and scraps. This early exposure can shape their food preferences. In fact, when cats raised without exposure to fish are offered it for the first time later in life, many will refuse to eat it. Perhaps it’s people who teach cats to like fish by buying into the cultural stereotype—and cats, clever as they are, play along.

If you want to include fish in your cat’s diet, it’s best to do so only occasionally—as a treat, a supplement, or to tempt a sick cat to eat. Beyond that, the relationship between cats and fish is more fairy tale than fact.

Lastly, there’s the environmental impact to consider. Today, 90% of the world’s fisheries are either fully exploited or overfished. Overfishing, habitat destruction, and unsustainable practices are pushing marine ecosystems to the brink of collapse. The pet food industry, driven by profit and increasing global demand, is not innocent in this crisis. As the oceans are emptied faster than nature can replenish them, we must ask ourselves if feeding fish to cats is truly sustainable—or even ethical—in the long term.