Although their natural diets are rich in calcium, most fish are also capable of extracting dissolved calcium directly from their aquatic environment through the gills. After a 24-hour acclimatization period, channel catfish have been shown to efficiently extract calcium from rearing water containing 5 ppm of the mineral element. On the other hand, gill extraction of phosphorus is negligible and fish rely mainly upon dietary sources for this mineral element. Phosphorus present in plant phytate is poorly absorbed by fish.
Absorption of dietary calcium and phosphorus begins in the upper gastro-intestinal tract. Absorbed calcium is rapidly deposited as calcium salts in the skeleton but absorbed phosphorus is distributed to all the major tissues: viscera, skeleton, skin and muscle. Phosphorus absorption is enhanced by increasing water temperature and by the presence of glucose in the diet. Its recovery from tissues also increases with increasing dietary levels of the element. On the other hand, increasing dietary calcium is not accompanied by correspondingly higher retention of the mineral element in the tissues.
Studies with common carp, Cyprinus carpio, and rainbow trout, Salmo gairdneri, have also shown that the absorption of dietary phosphorus is not affected by calcium in the diet. The level of phosphorus in the diet, however, sets the rate at which calcium is retained in the body. An increasing level of dietary phosphorus will be accompanied by increasing: retention of both mineral elements in body tissues, thus maintaining the ratio of calcium and phosphorus within narrow limits. In the common carp, whole body Ca/P ratio is about 1.4 except when phosphorus is severely lacking in the diet. Vertebral Ca/P ratio is about 2.0. Fish appear to have an ability to balance Ca/P ratios by controlling the absorption and excretion of calcium for optimal utilization of both mineral elements.
The dietary level of phosphorus for maximum growth in the common carp and the red sea bream, Chrysophrys major, has been shown to be 0.7 percent. This corresponds well with the level for maximum conversion of the mineral element in the trout. As stated earlier, most cultured species show efficient gill extraction of calcium from rearing waters although red sea bream fingerlings exhibit better growth with dietary supplementation of calcium. In general, the nutrition of calcium and phosphorus of both salt and freshwater fish species are very similar. Vitamin D plays an essential role in intestinal calcium absorption in land animals. A similar role for fish has not been established. The ability of fish to sequester calcium from water through the gill membrane possibly reduces the importance of such a role.