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.
2 Absorption and Metabolism
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.
3 Deficiency Symptoms
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.
4 Magnesium in Feeds
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.