Minerals (CALCIUM AND PHOSPHORUS)

Distribution

Calcium and phosphorus are usually discussed together because they occur in the body combined with each other for the most part and because an inadequate supply of either limits the nutritive value of both.
Almost the entire store of calcium (99 percent) and most of the phosphorus (80 percent) in the fish's body are present in bones, teeth and scales. There appears to be little variation in the composition of bone ash even though bone ash will decrease as a result of dietary deficiency in either calcium or phosphorus. This composition consists of calcium and phosphorus in the ratio of approximately 2:1.
The one percent extra-skeletal calcium is widely distributed throughout the organs and tissues. Calcium in body fluids exists in two distinguishable forms, diffusable and non-diffusible. Non-diffusible calcium is bound to protein whereas the diffusible fraction is present largely as phosphate and bicarbonate compounds. It is this diffusible fraction that is of significance in calcium and phosphorus nutrition. Ionized calcium in the extracellular fluids and in the circulatory system participate importantly in muscle activity and osmoregulation.
Large amounts of extra-skeletal phosphorus are present mostly in combinations with proteins, lipids, sugars, nucleic acids and other organic compounds. These phosphocompounds are vital exchange currencies in life processes and are distributed throughout the organs and tissues of the fish. The skin, like the skeleton, also appears to be an important repository for dietary phosphorus in some species.

Absorption and Metabolism

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.

Deficiency Symptoms

Deficiency symptoms of calcium have not been described in fish although poor growth is observed with diets limited in phosphorus (which leads to reduced accretion of calcium by body tissues). Prolonged feeding of phosphorus-deficient diets to common carps resulted in deformed backs (lordosis) and heads due to abnormal calcification of bone. Bone growth was reduced in the skull and operculum regions. Recent studies with this species and the red sea bream have also shown fatty infiltration of liver and muscle tissues related to dietary phosphorus deficiency.

Calcium and Phosphorus in Feeds

Feed ingredients vary widely in their calcium and phosphorus content. Fish meal, a principal ingredient in fish feeds, is rich in both calcium and phosphorus. On the other hand, feed ingredients of plant origin usually lack calcium and, despite a fairly high content of phosphorus the latter is predominantly in the form of phytin or phytic acid which is not readily available for absorption by fish. Animal sources of calcium and phosphorus are generally better absorbed, although the stomachless carp cannot utilize bone phosphate present in fish meal as well as fish with functional stomachs. Dicalcium phosphate has the highest availability (80 percent). Phosphorus availability of common feedstuffs varies from 33 percent for grains to 50 percent for fish meal and animal by-products. Soybean meal has an intermediate phosphorus availability of 40 percent.