1. INTRODUCTION
2. CALCIUM AND PHOSPHORUS
3. MAGNESIUM
4. OTHER ESSENTIAL INORGANIC ELEMENTS
5. REFERENCE
K. W. Chow
Food and Agriculture Organization
Rome, ItalyW. R. Schell
University of Washington
Seattle, Washington
1/ Lecture was presented by W.R. Schell
Mineral elements have a great diversity of uses within the animal body. The following mineral elements are recognized as essential for body functions in fish: calcium, phosphorus, sodium, molybdenum, chlorine, magnesium, iron, selenium, iodine, manganese, copper, cobalt and zinc. To these may be added fluorine and chromium which have also been shown to be essential for land animals.
The prominence of each mineral element in body tissues is closely related to its functional role. As constituents of bones and teeth, minerals provide strength and rigidity to skeletal structures. In their ionic states in body fluids they are indispensable for the maintenance of acid-base equilibrium and osmotic relationship with the aquatic environment, and for integration activities involving the nervous and endocrine systems. As components of blood pigments, enzymes and organic compounds in tissues and organs they are indispensable for essential metabolic processes involving gas exchange and energy transactions.
2.1 Distribution
2.2 Absorption and Metabolism
2.3 Deficiency Symptoms
2.4 Calcium and Phosphorus in Feeds
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.
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 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.
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.
3.1 Distribution
3.2 Absorption and Metabolism
3.3 Deficiency Symptoms
3.4 Magnesium in Feeds
Magnesium is closely associated with calcium and phosphorus both in its distribution and its metabolism.
The bulk of magnesium in fish (60 percent in the carp) is stored in the skeleton. Magnesium constitutes a little over 0.6 percent of the ash content of bones compared with 30 percent calcium and 15 percent phosphorus. The remaining 40 percent of the body's magnesium is distributed throughout the organs and muscle tissues (where it plays vital roles as enzyme co-factors, and as an important structural component of cell membranes) and in extracellular fluids.
Fish are capable of extracting magnesium from the environment, although studies with the common carp showed that, in this species, gill extraction of this element is very limited. Due to the comparatively low concentrations of the mineral element in fresh water, non-marine fish appear to depend upon dietary sources to meet their requirement of magnesium.
In the common carp, as well as in the rainbow trout, dietary magnesium levels do not affect calcium and phosphorus composition in the whole body or skeleton despite sharp reductions of up to 50 percent of tissue magnesium when this mineral element was lacking in the diet (80 ppm) and retarded growth and behavioral abnormalities observed.
The symptoms of magnesium deficiency in the common carp and rainbow trout are very similar to those described for magnesium deficient land animals: loss of appetite, poor growth, sluggishness and convulsion followed by tetany. Mortality is often high. Histo-logical changes have also been observed in muscle, pyloric caeca and gill filaments of trout fed magnesium-deficient diets.
Although natural waters are a good source of dissolved magnesium, fish do not extract this mineral element in sufficient quantities to meet dietary needs. Natural foods, as well as most artificial feed ingredients of both animal and vegetable origin, are adequate sources and deficiency under ordinary rearing conditions has not been observed to date.
Dietary requirements of fish for most of the trace mineral elements have not been established. Iron deficiency in the red sea bream results in a form of microcytic, hypochromic anaemia similar to iron deficiency anaemia in land animals. Common carp fed a semi-purified diet without supplementary iron grew normally but exhibited sub-clinical symptoms of hypochromic microcytic anaemia. Iodine deficiency produces a goitrous condition in trout. Rainbow trout fed a semi-purified diet deficient in zinc (1 ppm) had increased mortality rate, cataracts in the eyes and erosion of the fins and of the skin. Protein digestibility was also reduced. Manganese has also been shown to be essential for growth and survival of Tilapia mossambica and the rainbow trout.
The roles of trace elements in fish, although not clearly defined, are probably similar to those described for land animals. Fish in their natural habitats are probably adequately provided for to meet the requirements for all the mineral elements. However, the intensive culture of certain fish species in man-made ponds and raceways, together with reliance on artificial feeding, make it necessary to incorporate adequate quantities of mineral nutrients in the feed. For the most part, where exact requirements are not known, levels are arbitrarily based on land animal requirements.
A summary of available information on mineral requirements of fish is given in Table 1.
National Research Council, 1977 Subcommittee on Warmwater Fishes, Nutrient requirements of warmwater fishes. Washington, D.C., National Academy of Sciences, (Nutrient requirements of domestic animals) 78 p.
Table 1 Summary of Information on Mineral Requirements of Fish
|
Mineral element |
Principal metabolic activities |
Requirement symptoms |
Requirement / kg dry diet |
|
Calcium |
Bone and cartilage formation; blood clotting; muscle contraction |
not defined |
5g |
|
Phosphorus |
Bone formation; high energy phosphate esters; other organo-phosphorus compounds |
Lordosis, poor growth |
7g |
|
Magnesium |
Enzyme co-factor extensively involved in the metabolism of fats, carbohydrates and proteins |
Loss of appetite, poor growth, tetany |
500 mg |
|
Sodium |
Primary monovalent cation of inter cellular fluid; involved in acid-base balance and osmoregulation |
not defined |
1-3g |
|
Potassium |
Primary monovalent cation of intra-cellular fluid; involved in nerve action and osmoregulation |
not defined |
1-3g |
|
Sulphur |
Integral part of sulphur amino acids and collagen; involved in detoxification of aromatic compounds |
not defined |
3-5g |
|
Chlorine |
Primary monovalent anion in cellular fluids; component of digestive juice (HCl); acid-base balance |
not defined |
1-5g |
|
Iron |
Essential constituent of haeme in haemoglobin, cytochromes, peroxidases, etc. |
Microcytic, homochronic anaemia |
50-00 mg |
|
Copper |
Component of haeme in haemocyanin (of cephalopods); co-factor in tyrosinase and ascorbic acid oxidase |
not defined |
1-4g |
|
Manganese |
Co-factor for arginase and certain other metabolic enzymes; involved in bone formation and erythrocyte regeneration |
not defined |
20-50 mg |
|
Cobalt |
Metal component of cyanocobalamin (B12). Prevents anaemia; involved in C1 and C3 metabolism |
not defined |
5-10 mg |
|
Zinc |
Essential for insulin structure and function; co-factor of carbonic anhydrase |
not defined |
30-100 mg |
|
Iodine |
Constituent of thyroxine; regulates oxygen use |
Thyroid hyperplasia (goiter) |
100-300 mg |
|
Molybdenum |
Co-factor of xanthine, oxidase, hydrogenases and reductases |
not defined |
(trace) |
|
Chromium |
Involved in collagen formation and regulation of the rate of glucose metabolism |
not defined |
(trace) |
|
Fluorine |
Component of bone appatite |
not defined |
(trace) |
