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, and the potential collapse of a fishery may result in unavailability of some fish species. 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 –28°C (–19°F) 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 months. Few fish, with the possible exception of capelin, should be stored >1 year. 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 bottlenose 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 (eg, whales and elephant seals) generally require less food per unit body weight (2%–5% of body wt) as adults. Obesity can develop in captive animals with excessive caloric intake and reduced exercise; caloric intake should be closely tracked.
Sirenians are herbivores that eat numerous species of marine and freshwater plants in the wild but can thrive on a diet of hydroponic grass and various lettuces, vegetables, and fruits such as carrots and bananas. Feeds can also be supplemented with a manatee-specific pellet of commercially available herbivorous primate pellets, and multivitamin-mineral supplements can be used particularly to balance calcium to phosphorus concentration 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 but vary with age and growth. Sirenians are generally fed at least several times a day to accommodate their grazing feeding pattern, and food should be offered at submerged levels as well as on the surface.
Sea otters are usually fed diets consisting of various invertebrates (eg, echinoderms, molluscs, and occasional crustaceans) and fish. Adult animals require ~25%–30% of their body wt in food each day. Rehabilitating wild sea otters may not take readily to frozen food and may need to be offered live food such as clams and mussels to encourage feeding.
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 U in the diet. Polar bears are commonly fed large amounts of fish in captivity, but their diets are often supplemented with commercial omnivore diets for nutritional and enrichment diversity. Vitamin D should also be supplemented at 1000 U/kg of feed. Renal disease is common in older bears, and in some cases, reduced dietary protein content may be recommended.
Neonatal Nutrition in Marine Mammals
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 generally has a high lipid content, but considerable variation exists and lipid content often varies during lactation. 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 have begun to supplant some of the very complex scratch-made formulas used in the past. Weight should be monitored closely during hand-rearing, and most animals can start weaning and be offered solid food items when teeth have erupted. When confronted with a marine mammal neonate to raise, contacting one of the major marine mammal rescue centers for species-specific advice and updated methods is recommended.
Phocid and otariid seals can be reared on the same milk replacer−based formulas. Pinniped pups should be fed approximately every 4 hours in their first week of life; gradually, the amount of formula fed should be increased and the feedings can be dropped to three to five per day. Transitioning from formula diet to fish diet should be done gradually to ensure sufficient caloric intake during this time; fish oil is commonly added to formulas to increase caloric value. Harbor seal (Phoca vitulina) pups should be tube fed until at least 2–3 weeks old, when they can be weaned to small pieces of fish. Elephant seal pups require tube feeding until they are at least 4 weeks old, when weaning can begin. Otariids generally have a longer nursing period so bottle feeding may be preferable for neonates to reduce handling and risk of accidental feeding tube insertion into the airway. Most otariid pups can be offered fish as early as 4 weeks of age but as they may suckle for a year or more, full transition to fish may take time. California sea lion (Zalophus californianus) pups can be assist-fed fish as early as 4 weeks of age and be free feeding by 6 weeks.
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. Walruses have a much longer nursing period than other pinnipeds, up to 19 months.
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 is 27%, harbour porpoise (Phocoena phocoena) milk is 46%, and mysticete blue whale (Baleanoptera musculus) milk is 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 months. They can be reared on artificial milks with early weaning; however, gastrointestinal problems such as diarrhea, constipation, and enterocolitis are common with inappropriate formulations.
Neonatal sea otters also have been reared successfully from birth on artificial formulas. They should be bottle-fed initially, and solid food can be offered in addition to formula around 4–6 weeks of age. Sea otter hand-rearing is particularly challenging, and as otters are strongly prone to imprinting, specialists should be consulted if hand-rearing is attempted.
Neonatal polar bears are extremely altricial and are a challenge because of an apparently immature immune system. Most hand-reared cubs are given polar bear serum orally and parenterally to support immune response. 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 or using a commercial canine formulation as the major ingredient.
Thiamine Deficiency in Marine Mammals
Because thiamine deficiency can occur in any piscivorous animal, any animal fed a diet of primarily frozen fish should be supplemented with thiamine. 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 develop anorexia, regurgitation, or ataxia. The condition can progress to seizures, coma, and death.
Animals with signs of thiamine deficiency respond rapidly (usually within 24 hours) to injection of thiamine hydrochloride (up to 1 mg/kg, IM), followed by oral supplementation. Control usually involves supplemental thiamine at 25 mg/kg of food, preferably administered 2 hours before a main feeding.
Vitamin E Deficiency (Steatitis, White Fat Disease) in Marine Mammals
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 orally with vitamin E at a rate of as much as 100 mg/kg of feed, which generally maintains high serum concentrations of the vitamin. This does not appear necessary if feeder fish are properly stored and thawed.
Hyponatremia (Salt Deficiency, Addison Disease) in Marine Mammals
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 otariids and other marine mammals, especially if food is thawed in water as sodium is lost in this manner.
Signs include periodic weakness, anorexia, lethargy, incoordination, tremor, and convulsions. Serum sodium concentration can decrease 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 concentrations. Provision of saltwater pools and supplementation of sodium chloride (3 g of NaCl/kg of 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.
Histamine Toxicosis (Scombroid Poisoning, Mackerel Poisoning) in Marine Mammals
Scombroid fish (eg, mackerel and 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 of histamine toxicosis in affected marine mammals. Histamine toxicosis 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; conjunctivitis and increased lacrimation; and cardiac palpitation. Occasionally, vomiting, diarrhea, pruritus, urticaria, or postures indicative of abdominal pain are seen.
Antihistamines, including cimetidine, may provide relief of clinical signs, 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.
Iron Storage Disease (Hemochromatosis) in Marine Mammals
Iron storage disease occurs most commonly in captive cetaceans and pinnipeds and is linked to a high-iron diet. Excessive iron is naturally stored in the liver as ferritin and hemosiderin, and excessive amounts can result in hepatic disease (eg, hepatitis or hemochromatosis). The disease is most commonly noted in bottlenose dolphins and has also been reported in northern fur seals and California sea lions.
Iron storage disease is characterized by high serum iron and ferritin, with transferrin saturation >50%. Activities of liver enzymes (ALT and AST) and concentrations of triglycerides, cholesterol, and globulins are often also increased.
Treatment is reduction of dietary iron, primarily with diet modification, including reducing vitamin C intake. Phlebotomy has been used for more rapid reduction of iron and can be effective short-term, but long-term diet modification is also necessary to prevent recurrence.
