By Jean Hofve, DVM
A lot of cats love fish, but it’s really not a good idea to feed it to your cat! Why not? Because it is simply no longer safe to feed to cats (and humans should be very careful about eating it themselves, as well as feeding it to their children!).
* The fish used in canned pet foods comes from “trash fish,” the unsavory leftovers of the seafood industry. It usually includes bones, and is high in phosphorus and magnesium, which can be an issue in cats with a history of urinary tract disorders or kidney disease. In practice, I have seen quite many cats develop urinary tract infections and blockages if they eat much fish–even boneless fish like canned tuna.
* Many cats are sensitive or even allergic to fish; it is one of the top 3 most common feline food allergens.
* Fish-based foods contain high levels of histamine, a protein involved in allergic reactions.
* While cats’ gut bacteria can synthesize their own Vitamin K from most food sources, fish-based foods may not support sufficient Vitamin K synthesis. Vitamin K is necessary for proper blood clotting. The most common synthetic Vitamin K supplement, menadione, has toxicity issues. We do not recommend feeding any cat food containing menadione.
* There is a known link between the feeding of fish-based canned cat foods and the development of hyperthyroidism in older cats, which is now at epidemic levels.
* Predatory fish at the top of the food chain, such as tuna and salmon, may contain very elevated levels of heavy metals (including mercury) as well as PCBs, pesticides, and other toxins. Tilefish (listed on pet food labels as “ocean whitefish”) are among the worst contaminated, along with king mackerel, shark, and swordfish. These fish are so toxic that the FDA advises women of child-bearing age and children to avoid them entirely; and recommends only 1 serving of albacore tuna per week due to its high mercury levels (yellow or “light” tuna is far safer for us, but still inappropriate for cats). If these fish are dangerous to children, cats are at even higher risk!
* A substance called domoic acid is a very stable, heat resistant toxin produced by certain species of algae that are becoming more common in coastal regions due to climate change. (Coastal regions are, of course, exactly where the world’s fish farms are located.) Domoic acid particularly accumulates in mussels, clams, scallops, and fish. Because it is so dangerous, the FDA limits the amount of this neurotoxin in seafood. However, new research indicates that domoic acid causes damage to the kidneys at concentrations 100 times less than the amount that causes brain toxicity. This is especially concerning for cat guardians, because not only can the legal level of domoic acid in any seafood harm the kidneys, but fish that are condemned for human consumption due to excessive domoic acid may instead be processed directly into pet food. Could contaminated fish in cat food be a hidden factor in the high rate of chronic kidney disease in older cats, who may have been eating this toxin every day for years?
* Fish and other seafood in the Pacific Ocean have been exposed to leaking radiation from the damaged Fukushima nuclear power facility in Japan for nearly three years. While the authorities continue to assert that there is (so far) no danger from eating Pacific seafood, the plant is still releasing 300 tons of highly toxic radioactive water into the ocean every day, with no end in sight. The first part of the initial radioactive plume has already reached U.S. shores; and low levels of Fukushima-specific radioisotopes have been found in West Coast seafood. While the Pacific Ocean’s vastness can and does greatly dilute the radioactive materials, the continuing leakage–as well as Japan’s recently-revealed dishonesty about its estimates of the amount of radiation involved–is cause for some concern. A recent meta-analysis found reported significant negative effects on the immune system, and well as increased mutations and disease occurrence even at extremely low levels. (Fortunately, ocean currents largely protect the southern hemisphere’s waters, so products from south of the equator are–so far–unlikely to be affected.)
* A recent exposé revealed the terrible human conditions—including outright slavery—involved in Thailand’s seafood industry, as well as the cheap but foul trash-fish slop that is not only used by fish and seafood farmers around the world to feed their stocks, but also goes directly into pet food. Thailand is the primary source of fish and seafood products used in pet food; some foods (particularly canned cat foods) are made there and shipped to the U.S.
* Salmon is a popular cat food ingredient, but today nearly all of it comes from factory-farmed fish. These unfortunate animals are kept in overcrowded net pens– feedlots–in polluted coastal waters. They’re fed anti-fungals, antibiotics, and brightly-colored dyes to make their flesh “salmon colored”–it would otherwise be gray. Common water pollutants such as PCBs, pesticides, and other chemicals are present in farmed salmon at 10 times the amount found in wild fish. These contaminants will be present in any product made with farmed fish, including cat and dog food.
* “Organic” salmon is also farm-raised, and does not have to comply with USDA organic standards. In fact, there is currently no regulatory agency in the United States that sets organic standards for fish. The contaminant level of organic farmed salmon may be just as high as that of conventional farmed salmon.
* Even “wild-caught” Alaskan and Pacific salmon may have been born and raised in a hatchery.
* Farmed salmon transmit diseases and parasites; those who escape their pens (and they do) outcompete and interbreed with wild salmon.
* A 2006 study confirms that salmon farms are “massive breeding grounds” for sea lice. Under natural conditions, wild adult fish carrying these parasites are not in migration channels at the same time as the defenseless, inch-long baby salmon, so infestation of the young fish is not a problem. But today, in waters near fish farms (which tend to be located at the end of those same migration channels), up to 95% of baby salmon are fatally infested. It is feared that that farmed salmon from nearly 300 fish factories in North America may ultimately decimate the wild population in the Atlantic.
* New research (Dec. 2013) from the University of California raises concerns that the plastics floating in our oceans are absorbing chemical pollutants from the water. Toxins can move up the food chain, starting when fish eat small, contaminated pieces of plastic. Those contaminants enter their tissues, and are transferred to those who eat the fish: including bigger fish (e.g., tuna, mackerel, and tilefish–the fish most commonly referred to as “ocean whitefish”), as well as people and pets.
* Fish tends to be “addictive” to cats. They love it, and will often stage a “hunger strike” by refusing their regular food in favor of fish. Tuna or other fish should be reserved as a rare and special treat. Feed fish no more than once a week, and even then in very small amounts only.
* The meat is unhealthy, and the fishing/aquaculture industry is environmentally destructive–need we say more?
In general, the small amounts of “fish meal” included as a flavoring and/or source of omega-3 fatty acids in cat foods are not a problem, but fish should not be a mainstay of any cat’s diet. Fish should be limited to an occasional–and small–treat.