Of Cats and Fish

Published Jul. 26, 2013

I was eavesdropping on a conversation between pet owners a few days ago that got me to thinking. The question they were debating was, “Why do we feed our cats fish?”

The practice doesn’t make a lot of sense from a natural history standpoint. Domestic cats evolved from desert dwelling ancestors. Last time I checked, the world’s deserts were not exactly teeming with fish. The African wildcat, the most likely ancestor of today’s house cats, eats primarily mice, rats, and rabbits with the occasional bird or reptile thrown in for good measure.

I’m not debating the point that domestic cats like fish; mine certainly does. She is elderly and suffers from heart disease that is causing her to lose significant amounts of muscle mass. My goal from a nutritional point of view is to encourage her to eat as much high quality food as possible to slow the inevitable decline in her body condition. To this end, she has access to both wet and dry cat food at all times. I’ve noticed that when I introduce a new variety of soft cat food, she initially gobbles it down, but after a week or so on that diet, her appetite wanes. When I switch to another flavor, she’ll dig back in again (I don’t mind spoiling her rotten at this point in her life). I find that I can eke out a longer period of time with the same diet if it is fish flavored; I assume this is because she likes these the best.

But fish isn’t always an optimal food for cats. When cats eat a diet that consists primarily of raw fish (not commercially prepared foods that contain fish), they are at risk for developing thiamine deficiency. Symptoms include a loss of appetite, seizures, and possibly death. Thiamine can also be broken down by heat but is added to cat foods after processing to ensure that it is present in appropriate amounts. It is important to note that the manufacturers of canned tuna meant for human consumption do not add thiamine to their products. Cats can eat small amounts of canned tuna as a treat every now and then, but if it makes up a large part of their diet, they too are at risk for thiamine deficiency.

Fish is also responsible for a large percentage of food allergies in cats. In one study of 56 cats with identifiable food allergies, fish was a responsible ingredient in 13 (23%) of those cases. This puts fish in third place for potential allergic reactions behind only beef and dairy products (16 cases [29%] each). Come to think of it, just like fish, neither beef nor dairy are really “natural” parts of an adult cat’s diet, are they? Maybe we’re on to something here.

I don’t mean to imply that all owners should avoid feeding foods that contain fish to their cats. As long as the cat is not allergic to fish and it is included as part of a nutritionally complete diet, fish is a good source of protein. I simply find it intriguing that domestic cats have developed a taste for a prey species that wasn’t a major part of their ancestors' diets.