Generally, captive animals fed a diet that is solely or primarily fish are provided dead fish that have been frozen. The logistics and difficulty in providing this diet can lead to some special nutritional concerns. All fish are not of equal nutritional value; diets consisting of a single species of fish are unlikely to provide balanced nutrition for any animal. Similarly, one diet will not serve all piscivores equally. Only fish suitable for human consumption should be fed. (Also see Nutrition: Exotic and Zoo Animals.)
Storage and thawing of frozen fish must be monitored carefully. Feed fish should be held at or below –19°F (–28°C) to slow deterioration of their nutritional value through oxidation of amino acids and unsaturated lipids. Dehydration of frozen fish can also be a problem for animals that obtain their water from food. Fatty fish should not be stored >6 mo. Few fish, with the possible exception of capelin, should be stored >1 yr. To retain optimal vitamin content and reduce moisture loss, frozen fish should be thawed in air under refrigeration. Thawing in water leaches away water-soluble vitamins. Thawing at room temperature encourages bacterial growth and spoilage.
The energy requirements of marine mammals vary with age, environmental temperatures, and condition. Young, growing bottlenosed dolphins and smaller pinnipeds generally require 9%–15% of their body wt in high-quality fish per day. Older animals may need only 4%–9% of their body wt for maintenance. Larger species (whales, elephant seals) generally require less food per unit body weight (2%–5% of body wt) as adults.
Sirenians thrive on a diet of hydroponic grass and various lettuces and vegetables, supplemented with high-protein monkey chow, carrots, bananas, and multivitamin-mineral supplements used particularly to balance calcium to phosphorus ratios. It is thought that sirenians ingest considerable animal protein incidentally during grazing in the wild. Intake requirements have been estimated at 7%–9% of body wt daily. Sirenians are generally fed several times a day to accommodate their grazing feeding pattern.
Sea otters are usually fed diets consisting of various invertebrates (echinoderms, molluscs, occasional crustaceans) and fish. Adult animals require ~25%–30% of their body wt in food each day.
Polar bears in the wild have high-lipid diets, particularly in winter when they subsist heavily on seals. They are considered to have an exceptional dietary requirement for vitamin A, and some dermatologic conditions respond to daily supplementation of 20,000–1,000,000 IU in the diet. Polar bears are commonly fed large amounts of fish in captivity.
Young, unweaned marine mammals are frequently encountered in strandings and must be fed a diet resembling their dam’s milk. In captivity, neonates may be abandoned by their parents and require artificial rearing. The milk of marine mammals has a high lipid content. Most species are carbohydrate intolerant, and neonates fed formulas with carbohydrates develop severe, life-threatening bacterial gastroenteritis. Most neonatal marine mammals also require immense caloric density in replacement milks. Milk replacement formulas based on commercial component-based milk replacers (eg, Zoologic® Milk Matrix) have begun to supplant some of the very complex scratch-made formulas used in the past. When confronted with a marine mammal neonate to raise, contacting one of the major marine mammal rescue centers for advice is recommended.
Phocid and otarid seals can be reared on the same milk replacer−based formulas. Pinniped pups should be fed every 4 hr in their first week of life; gradually, the amount of formula fed should be increased and the feedings dropped to five per day. Harbor seal (Phoca vitulina) pups should be tube fed until 2–3 wk old, when they can be weaned to small pieces of fish. Elephant seal pups require tube feeding until they are 4 wk old, when weaning can begin. California sea lion (Zalophus californianus) pups can be force fed fish as early as 4 wk of age and be free feeding by 6 wk.
Neonatal walruses (Odobenus rosmarus) have been reared on milk replacer–based formulas, as well as on whipping-cream base extended with ground molluscs (clams) rather than fish. They also seem to tolerate carbohydrates reasonably well. Walruses have a much longer nursing period than other pinnipeds.
Neonatal cetaceans have longer nursing periods than pinnipeds. Success at bottle rearing has improved with experience, and individuals from species ranging from common dolphins (Delphinus delphis) to gray whales (Escrichtius robustus) have been reared successfully. The fat content of cetacean milks varies considerably; bottlenose dolphin (Tursiops spp) milk is ~17% fat (half that of most pinniped milks); beluga whale (Delphinapterus leucas) milk, 27%; harbour porpoise (Phocoena phocoena) milk, 46%; and mysticete blue whale (Baleanoptera musculus) milk, 42%. Formulas based on commercial component milk replacers supplemented with ground fish and oils have been fed successfully to bottlenose dolphins and harbour porpoises using a lamb’s nipple or stomach tubing.
Neonatal sirenians begin to nibble sea grasses shortly after birth but may continue to nurse up to 18 mo. They can be reared on artificial milks with early weaning. Neonatal sea otters also have been reared successfully from birth on artificial formulas. Neonatal polar bears are extremely altricial and are a challenge because of an apparently immature immune system. Polar bear milk is high in fat (31%) and contains minimal lactose. Polar bears have been successfully reared on formulas with a whipping-cream or oil base.
This can be seen in any piscivorous animal. Thiamine in the food is destroyed by the activity of thiaminase enzymes or antithiamine substances in the fish being fed. These active enzymes can also destroy supplemental thiamine placed in fish if the fish sits for long periods before feeding. Clinical signs of thiamine deficiency are primarily of CNS disturbances. Affected animals may show anorexia, regurgitation, or ataxia. The condition can progress to seizures, coma, and death.
Animals with signs of thiamine deficiency respond rapidly to IM injection of thiamine hydrochloride (up to 1 mg/kg body wt), followed by oral supplementation. Control usually involves supplemental thiamine at 25 mg/kg food, preferably administered 2 hr before a main feeding.
The antioxidant properties of vitamin E are believed to play an important role in maintaining the integrity of cellular membranes. Oxidative processes during the storage of fish destroy vitamin E and other antioxidants. Steatitis has been induced experimentally in phocid seals, and relationships between vitamin E deficiency and hyponatremia are suspected. Captive piscivores commonly are supplemented PO with vitamin E at a rate of as much as 100 mg/kg of feed, which generally maintains high serum levels of the vitamin. This does not appear necessary if feeder fish are properly stored and thawed.
Hyponatremia in pinnipeds is closely related to adrenal exhaustion and development of Addison disease, which links the syndrome to environmental stressors rather than to a simple primary salt deficiency. It is most common in pinnipeds maintained in freshwater exhibits but can be seen in animals kept in saltwater. It is more common in phocid seals but occurs in otarids and other marine mammals. Signs include periodic weakness, anorexia, lethargy, incoordination, tremor, and convulsions. Serum sodium levels can fall to <140 mEq/L. Severely affected animals may collapse in an Addisonian crisis, which can be fatal.
Emergency therapy consists of sodium chloride infusion and replacement corticosteroids. Longterm management of advanced cases requires mineralocorticoid supplementation in conjunction with oral sodium chloride supplements and periodic monitoring of serum sodium levels. Provision of saltwater pools and supplementation of sodium chloride (3 g/kg food) in the diet of captive pinnipeds maintained in freshwater pools should be considered a poor second choice. Animals on salt supplementation should have continuous access to freshwater.
Scombroid fish (mackerel, tuna) and other dark-fleshed fish have a short shelf life, even when frozen at low temperatures. A complex of substances, including histamine formed by bacterial decarboxylation of the large amount of histidine found in the flesh of the fish, is responsible for the signs seen in affected marine mammals. The toxicity can also occur with nonscombroid fishes, including poorly handled herring, anchovies, or pilchard. It is most common in pinnipeds but is seen in other marine mammals. Clinical signs include anorexia; lethargy; a red, inflamed mouth or throat; and conjunctivitis and increased lacrimation. Occasionally, vomiting, diarrhea, pruritus, urticaria, or postures indicative of abdominal pain are seen. Antihistamines, including cimetidine, may provide symptomatic relief, but the condition is generally self-limiting, and the animal begins feeding within 2–3 days. In more severe or acute cases, epinephrine is effective in counteracting the histamine reaction. Cortisone and diphenhydramine hydrochloride can be beneficial in the face of respiratory difficulty. Control consists of avoiding scombroid fish in the diet or paying careful attention to their quality, storage, and handling when used.