Penguins, pelicans, and other fish-eating species in the wild feed primarily on fish, crustaceans, and squid. These food sources vary in their fatty acid, vitamin, and carbohydrate contents. In captivity, squid, smelt, herring, mackerel, and whiting fish are commonly available. One of the most important aspects of feeding piscivorous birds is fish quality. It is advised to feed several fish species, although in some cases one species of fish has been fed for a lifetime. The fish should be in good condition, not emaciated. Captive seabirds develop strong preferences for a particular fish if it is fed exclusively for prolonged periods, which can lead to both nutritional deficiencies and inanition if the feeder fish becomes unavailable.
Supplements commonly given to captive penguins include vitamins A, D, B1, and E. The need for these and the quantity that must be supplemented depends on the quality and content of the primary diet. Supplements can be added to the fish as tablets, as a gel, or as a liquid to be injected in the fish.
Dietary salt (NaCl; 0.5–1 g salt/bird/day) is often provided to birds in freshwater exhibits to help maintain proper functioning of the salt glands.
Most fish are deficient in vitamin E. Clinical signs of vitamin E deficiency in piscivorous birds include weakness and inability to stand or hold the wings in normal posture. Severe generalized myopathy with muscle atrophy, degeneration and necrosis, and replacement with fibrous connective tissue can occur with chronic pronounced vitamin E deficiency. Supplementation with 100 IU vitamin E/kg fish has been proposed. However, oversupplementation (vitamin E at 500–10,500 IU/kg food) may result in decreased growth and coagulation disorders, possibly from creating vitamin K deficiency rather than directly from vitamin E toxicity.
Hand feeding of species and individuals of concern will ensure that each bird receives the proper amount of food and supplement. Some piscivorous species will accept commercial bird-of-prey diets, trout pellets, and/or mice in the diet, as well as fish.