Undoubtedly significant progress on the mineral requirements of aquatic animals has been made in the past two decades; overall developments in this field of fish nutrition have been relatively slow. Many gaps still exist in the knowledge of the quantitative requirements of inorganic elements and their physiological functions in most fish. In particular, limited information has been published on trace element metabolism of aquatic organisms…
The minerals are essential chemical elements involved in the normal metabolism of fish. The information currently available is very patchy, the detailed mineral budgets are yet to be worked out and more research has to be done on the uptake, function and biological availability of many minerals. The hurdle lies in the fact that minerals are required only in trace amounts and under experimental conditions, it is difficult to maintain such minimal amounts in formulated diets, apart from keeping the medium devoid of the test element.
There are also indications that the requirement and toxicity of inorganic elements may be influenced by acidic water (low pH). In feed formulations for aquatic animals, one must consider the requirement of the individual element, potential interactions with other inorganic elements and nutrients in the digestive tract and their metabolic level in various tissues, and minerals supplied from the aquatic environment, as well as the species, age, and sex of the fish. Defining the role of inorganic elements in immune response, disease prevention, and reproduction of fish is an important and interesting area of research. Fish may also have requirements for other ultra-trace elements known to be required by terrestrial animals. In recent years, vitamin E and selenium have been implicated in the prevalence of several infectious diseases. The involvement of zinc and manganese in the reproduction of fish is becoming apparent.
Recent research in mineral nutrition is focused around:
• Mineral requirement
• Bioavailability of minerals
• Mineral interaction
– Mineral-mineral interaction
– Mineral-nutrients (protein, lipid, carbohydrate) interaction
– Mineral-vitamin interaction
• Mineral & fish health
Problems associated with the quantification of mineral requirements:
• Identification of potential contribution of minerals from the water
• Leaching of mineral from the feeds prior to consumption
• Availability of suitable test diet
• Limited data on mineral bioavailability & nutritional requirement of many species
• Investigations in fish are comparatively complicated as both dietary intake and waterborne mineral uptake have to be considered in determining the mineral budgets.
• The exchange of ions from the aquatic environment across gills and skin of fish complicates the determination of the quantitative dietary requirements.
• Many trace elements are required in such small amounts that it is difficult to formulate purified diets low in mineral and maintain water sufficiently free of the test element.
• A critical factor in the determination of ultra-trace elements, such as manganese, vanadium, and chromium, is the need for meticulous sample preparation.
• Often normal values of trace elements in fish tissue vary widely in reports from laboratory to laboratory.
• Information available on fish mineral requirement is fragmentary and incomplete.
• Relatively little is known about the uptake, function, and biological availability of many trace elements.
BIOAVAILABILITY OF MINERALS
• Bioavailability of an element can differ markedly when supplied from different feedstuffs and within the same element from feed in different diets.
• Many factors can influence the bioavailability of minerals. These include the intake level of the nutrient, its chemical form, the digestibility of the diet that supplies the element, the particle size, interactions with other nutrients, chelators, inhibitors, physiological and pathological states of the animal, the water chemistry, the type of feed processing, and the species of animal being tested.
• The biological availability of an element in a diet can differ depending on the molecular form in which the element is present, its valence state, the ligands present when the element is ingested from different diets.
• Mechanisms that involve the formation of insoluble and nonabsorbable substances in the gut may either hinder or facilitate the mucosal uptake transport, and metabolism of an element in the body. Certain inorganic elements may compete with the test element for important binding sites during these processes.
A wide range of potential mineral–mineral and mineral–vitamin interactions has also been reported (Hilton, 1989) in fish. Antagonistic relationships occur when elements with a similar electronic configuration and ionic radius compete for binding sites, e.g.
– Zinc and cadmium in metallothionein,
– Magnesium/manganese substitutions at enzyme active sites,
– Synergistic relationships in which one element enhances the role of another, (Mertz, 1986; Davis, 1980) e.g., iron and copper,
– The complex interrelationship among copper, zinc, iron, and calcium, as well as that of copper, molybdenum, and sulfur. Another type of interrelationship involves the interactions between the elements themselves.
Selenium has a high affinity for certain toxic elements such as mercury and silver; hence selenium exerts a protective effect against the toxicity of these metals by forming complexes in vitro, resulting in a decrease in the biological availability of both selenium and the heavy metal.
Minerals also interact with other nutrients. A synergism between dietary selenium and vitamin E is known. Zinc is required for the metabolism of vitamin A. The strong redox potential of vitamin C may alter the valence of copper and iron and thus reduce or enhance absorption. Interactions between minerals usually have negative effects, but they can be beneficial (e.g. small copper supplements can enhance iron utilization) and may depend on the level of supplementation (e.g. large copper supplements can increase iron requirements).
Mutual antagonisms between copper, cadmium and zinc lead to complex three-way interactions. Raising the level of one interactant can lower the status of the other two, as shown for Cd.
MINERALS & FISH HEALTH
Iron has been shown to affect immune system function and host defense against infection. Few studies have evaluated the effect of dietary iron on immune response and disease resistance in fish. Either a deficiency or an excess of iron can compromise the immune system (Beisel 1982; Bhaskaram 1988). Lall et al. (1985) observed that 4.5 mg of iodine/kg of diet was essential to protect Atlantic salmon from bacterial kidney disease infections. Selenium exerts the protective effects against the toxicity of heavy metals such as cadmium and mercury (Lall 1989). Supplementation of the diets with I, F, Fe, Cu, Co, Mn, and Zn resulted in a lower incidence of BKD infections.
Undoubtedly significant progress on the mineral requirements of aquatic animals has been made in the past two decades; overall developments in this field of fish nutrition have been relatively slow. Many gaps still exist in the knowledge of the quantitative requirements of inorganic elements and their physiological functions in most fish. In particular, limited information has been published on trace element metabolism of aquatic organisms. This creates great difficulties in the characterization of deficiency or toxicity symptoms even under controlled environmental conditions. Standard mineral mixtures used in warm-blooded animal experiments have not been effective in supporting optimum growth or preventing nutritional deficiencies in studies designed to investigate the nutrient requirements of finfish and crustaceans. Several pathological conditions and nutritional deficiency signs of unknown etiology have been observed in hatcheries, and aquaculture operations may in fact be due to a dietary mineral imbalance and either limited or excessive uptake of trace elements from the water. Wide differences exist among freshwater, euryhaline, and marine fish species in the absorption and utilization of certain dietary minerals.
About Dr. Amit Ranjan
Dr. Amit Ranjan is working as Assistant Professor in the Department of Fish Nutrition & Feed Technology at the Institute of Fisheries Post Graduate Studies of Tamil Nadu Dr. J. Jayalalithaa Fisheries University (TNJFU), India. He undertakes strategic and applied research in the field of fish and shrimp nutrition. He has good experience in commercial culture of shrimp and freshwater fish. He has published several research papers in international peer reviewed journals and serves as the reviewer of more than 30 international journals.