Aquatic animals are unique among vertebrates in their ability to absorb minerals not only from their diets but also from water. The exchange of ions from the aquatic environment across gills and skin of fish complicates determination of the quantitative dietary requirements of minerals (reviewed by Lall, 2003).
Many trace elements are required in small amounts and it is difficult to formulate purified diets low in minerals and maintain the water sufficiently free of the test element.
The biochemical mechanisms of mineral metabolism in fish are generally similar to those of terrestrial animals with the exception of mineral uptake from the water and maintenance of osmotic balance between body fluids and the aquatic environment.
Although significant progress has been made in mineral nutrition of fish in the past two decades, environmental concerns have prompted more research on the metabolism and functional role of toxic elements. The ability to regulate abnormally high concentrations of minerals varies among aquatic organisms.
Many gaps exist in the knowledge of mineral requirements, physiological functions, and bioavailability from feed ingredients. Studies on vertebrate nutrition and metabolism have been useful to confirm main mineral functions.
Since an excessive intake of minerals from either diet or gill uptake causes toxicity, a fine balance between mineral deficiency and surplus is vital for aquatic organisms to maintain homeostasis either through increased absorption or excretion.
Calcium requirement of fish is met largely by their ability to absorb these ions directly from water. Gas exchange across gills in fish provides continuous access to an unlimited calcium reservoir. The regulation of calcium also occurs at gills, fins, and oral epithelia although the most important site is the gill. In the marine environment, fish derive calcium and magnesium from seawater, hence making supplementation of these minerals unnecessary (Lall and Bishop, 1977).
For optimal growth, a low concentration of calcium (0.34 percent or less) is required in the diet of some fish such as carp, eel, tilapia, and catfish, particularly in culture systems where calcium concentration is low in water.
Food is the main source of phosphorus because the phosphate concentration is low in freshwater (FW) and seawater (SW). Regulation of phosphate is considered more critical than that of calcium because fish must effectively absorb, store, mobilise, and conserve phosphate in both FW and SW environments. Dietary phosphorus requirements for fish range from 0.4 to 0.9 percent.
Many studies with monogastric animals have shown that an optimum dietary Ca:P ratio is important; and increasing the Ca:P ratio interferes with phosphorus absorption and conversely, a high P:Ca ratio may restrict calcium absorption. However, studies on the Ca:P ratio in fish diets are limited.
Fish are able to accumulate and retain toxic minerals from their aquatic environments (Taylor, 1996). The solubility of trace metals in natural waters is principally controlled by pH, type and concentration of ligands and chelating agents, oxidation state of mineral components, and the redox environment of the system.
A large variation exists in the mineral composition of feed ingredients, test diets, and commercial feeds. Unlike feeds for terrestrial animals, aquaculture feeds contain a high proportion of fishmeal and marine by-products.
Increasing demands of the world's aquaculture producers upon the finite quantity of this high-quality protein source necessitates that fish feeds become increasingly comprised of alternate highly digestible protein sources of plant and/or animal origin that support similar fish performance and concurrently have little or no adverse effects upon the environment.
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Article made possible through the contribution of Alltech and Nottingham University Press.