DETERMINING THE NUTRITIONAL STATUS OF FISH
Nutritional deficiency signs, like those listed in Table 7-5, are useful in diagnos-ing feed problems. However, the absence of visible signs does not necessarily mean that the fish is in optimal or desirable nutritional health. Increasing the dietary level of all essential nutrients result in a heightened physiological re-sponse up to the point at which the dietary requirements are met. Beyond a certain level of nutrient availability, concomitant increase in physiological re-sponse is not seen. This is the dietary level that supports optimal nutritional status. At some level below that requirement, visible deficiency signs appear in the fish. The range of dietary intake between that resulting in visible signs of deficiency and that resulting in no attendant increase in physiological function is an area referred to as subclinical deficiency. Fish receiving this level of an essential nutrient do not show signs of deficiency, yet they may be nutritionally compromised and less able to resist infectious disease or cope with suboptimal water quality conditions. These points are illustrated by Hardy (1991) in Figure 7-1.
Diagnosis of nutritional diseases requires knowledge of the following:
a. External and internal disease signs:
b. Quantitative clinical chemistry of blood or tissue of diseased fish;
c. Histopathological examination;
d. Histochemical analysis; and
e. Proximate analysis of the diet given.
Information on the feeding practices, feeding rates, storage conditions of feeds, and feeding behavior of the fish are also important.
Diagnosis of dietary carbohydrate-related pathology can be made by blood glu-cose and liver glycogen analyses. Histopathological examination of stained tis-sue sections can be useful in the diagnosis of nutritional diseases. Diagnosis of vitamin deficiencies is difficult. Gross signs, blood chemistry, hematology, tis-sue vitamin assay, gross pathology, histopathology and ration vitamin assay may be necessary for making a diagnosis.
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