Fish are the primary food of captive marine mammals except for the herbivorous sirenians. The purchase and subsequent proper storage and handling of high-quality fish are the most important aspects of feeding cetaceans and pinnipeds.
On receipt, fish should always be inspected for quality. The following are useful for evaluation of quality:
The boxes should be checked to see whether catch dates are indicated.
Overall appearance of the fish should be good.
Gills should be red (light pink gills indicate considerable time may have elapsed before the fish were frozen after being caught).
Eyes should not be sunken, indicating dehydration.
Flesh of thawed fish should be firm, skin should be intact and not discolored, and there should not be a bad odor.
There should not be excess water and blood pooled on the bottom of frozen cases, which indicates the fish have thawed and been refrozen.
Ideally, the lenses of frozen fish should be cloudy, which indicates the fish have been properly stored at or below –30°C before purchase (higher temperatures often result in clear lenses).
To minimize peroxidative damage and nutrient destruction, fish should be stored at or below -20°–25°C. Most fish species should not be stored >6 months if at all possible. A maximum of 3–4 months is recommended for fatty fish such as mackerel; lean fish such as smelt may remain in good condition for up to 9 months. Ideally, fish should be thawed overnight under refrigeration. If this is not possible, thawing at room temperature is preferable to thawing in water, which can cause significant nutrient leaching. Thawing temperature should not be >10°C. Individually quick frozen fish are preferred by many zoos, because proper quantities can be thawed without waste.
As a general rule, marine mammals should be given marine fish. Composition of marine fishes can vary greatly between species and even within species depending on age, season, and catch location. Fish that have been used successfully include Atlantic and Pacific herring; Atlantic, Pacific, and Spanish mackerel; bluerunner; capelin; and anadromous smelt. Squid are readily consumed by many pinnipeds, and clams can be included in walrus diets. No commercial substitute for fish has been developed that will be accepted by cetaceans, but such products have been used with some success for pinnipeds. The regular diet of any marine mammal should consist of two or more fish species to help ensure a balanced diet. It is not recommended to feed fish that are threatened in the wild. For example, Marine Stewardship Council (MSC) fish should be fed.
Thiamine should be added (at 30–35 mg/kg fish, as fed daily) to any marine mammal feeding program because of the possibility of thiamine destruction by thiaminases found in several fish species. Supplemental vitamin E helps compensate for oxidative destruction of natural vitamin E in fish during storage and helps protect against the deleterious effects of peroxides formed in stored fish. Oily fish such as mackerel, which are high in unsaturated fatty acids, are particularly susceptible to vitamin E destruction and peroxidative damage. Vitamin E at 100 IU/kg fish, as fed per day, is generally recommended whenever fish are fed. Regularly, vitamins A and D are also added, but there is discussion on whether this is necessary
Although subject to some debate, salt (NaCl) supplementation of pinnipeds maintained in freshwater is sometimes recommended to prevent hyponatremia; 3 g salt/kg fish should be adequate. Although supplemental vitamin C is frequently given to captive cetaceans, there is no conclusive evidence it is beneficial. Evidence indicates hepatic vitamin A levels in captive dolphins are often much lower than in their wild counterparts. Although specific recommendations cannot be made, vitamin A supplementation of some captive cetacean diets may be desirable.
Food intake in marine mammals varies considerably, depending on fat content of fish, water temperature, and activity. Performing Atlantic bottlenose dolphins generally eat 7–10 kg fish/day. Adult seals and sea lions consume ~5%–8% of their body wt in fish/day. Captive sirenians can be maintained on a diet of lettuce, cabbage, alfalfa, and aquatic plants (eg, water hyacinth).