Abstract

The dietary management of fish eating mammals is greatly affected by numerous variables. The relative importance of these factors to a specific animal or group may vary tremendously based on the knowledge level of the management staff, the recognition of the variables involved, and the ability to clarify impacting factors.

The potential of an animal to grow and prosper depends on the availability of a balanced diet consisting of proper levels of moisture, fat protein, carbohydrate, minerals and vitamins. It is rare for any one food source to properly supply all of these factors over a seasonal period.

Fish prey species may vary in composition because of their dependency on similar variables such as food availability, temperature variation and reproductive need. These seasonal variations will present not only different caloric levels to their predator species but also different compositions based on their food source. A decreased availability of fish species could potentially lead to short term and long term health implications.

Most fish species vary in caloric content seasonally with the main fluctuation occurring in fat content. Protein varies slightly in caloric contributions mostly secondary to the abundance or decrease of fat. Fat content also influences the volume of water presented to the marine mammal.

Herring caught during different times of the year have levels of fat ranging from 3.1% to 15.5%. Capelin fat content ranges from 3.9% to 11.3% with the highest levels occurring in the fall. Mackerel may range from 1.6% to 10.5%. Sardines have ranged from 2.4% to 14%. The size of the fish does not correlate with the fat content though there may be some visual clues to those with increased fat content.

Another factor not often considered with the presence of low fat fish is palatability. Observation of fish acceptance correlated with fat content show that many individuals will show varying acceptance behavior associated with new lots of fish that are of lower fat content. Immediate reactions can include spitting out lower fat fish in favor of higher fat fish, playing with food, holding it for short periods in the mouth before swallowing. If a low fat fish is presented with a higher fat fish the behavior may be explained as a result of satiation or new fish texture. Playing or spitting out fish may be viewed by trainers as a problem of satiation requiring a food base decrease. For many animals this behavior will only last a few days to a week. Then food adjustments should be based on the animals weight as well as behavior.

When multiple fish type are low calorie at the same time (spring, summer) many animals will eat less bulk possibly related to palatability and can eventually experience secondary weight loss. If these individuals are young, pregnant, nursing or enter this period already dun there can be major health complications encountered. These can include failure to thrive in young animals, stunting, dehydration and immune system compromise.

To help understand these problems managers should be aware of caloric content of food fish and that it does vary. Employees should be well educated to identify visual cues of thinness and all animals should be regularly weighed.

A working knowledge of caloric relationships might allow year long planning of diets to spread out fatty fish availability rather than using it all during one portion of the season. Higher fat fish should be available for ill, young, pregnant or nursing animals during lean periods. Supplementation of lean fish from dim groups with fish oils may be another consideration. If diet plans do not include fat availability consideration then animals should be allowed to bulk up during winter periods to offset future lean periods in spring and summer. Managers should also strive to provide varied diets which do not depend on only one to two fish types.