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 the determination of the quantitative dietary requirements of minerals (reviewed by Lall, 2003).
Many trace elements are required in such 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, which varies in salinity from 0 to 35 ppt in fresh, brackish, and seawater.
Although significant progress has been made in the mineral nutrition of fish in the past two decades, environmental concerns of air and water pollution have prompted more research on the metabolism and functional role of toxic elements.
The ability to regulate abnormally high concentrations of minerals varies among the aquatic organisms. Certain fish and crustaceans are able to excrete high proportions of excessive metal intake and consequently regulate body mineral concentration at relatively normal levels (Handy, 1995).
The soluble trace elements in water are more toxic than higher dietary intake of minerals such as copper, zinc, and iron. The toxicity mechanisms of metal ions include blocking of essential biological functional groups of enzymes, displacing the essential metal ion in the biomolecule (enzyme or protein), and modifying the active conformation of the biomolecule.
Molecular studies conducted on mammals, plants, and yeasts show high sequence homology in key function regions for the metal transporters of iron, copper, zinc, and some other trace elements but the role of these transporters in fish remains to be investigated. The characterization of homologues of metal transporters (Cu-ATPase, ferroportin, ZnT-1, and DMT1) in fish indicates that essential mineral uptake mechanisms in fish have been conserved through evolution (Bury et al., 2003).
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 the main functions of minerals, which include skeletal structure maintenance, cellular respiration, oxygen transport, protein stability, and regulation of acid-base equilibrium, as well as important components of hormones, enzymes, and activators of enzymes.
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.
Quantitative dietary requirements have been reported for nine minerals (Ca, P, Mg, Zn, Fe, Cu, Mn, I, and Se) for selected fish species (Lall, 2003). The calcium requirement of fish is met largely by their ability to absorb these ions directly from water. Unlike terrestrial animals, bone is not the major site of calcium regulation in fish.
Gas exchange across gills in fish provides a 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 gills. In the marine environment, fish derive calcium and magnesium from seawater, thus making supplementation of these minerals unnecessary (Lall and Bishop, 1977).
Stanniocalcin is the predominant hormone that regulates calcium and phosphate metabolism in freshwater (FW) and seawater (SW) salmon as well as several other FW fish (Verbost et al., 1993; Wagner et al., 1998). The role of calcitonin and vitamin D metabolites in calcium and phosphorus homeostasis is not clearly established.
For optimal growth a low concentration of calcium (0.34% 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 FW and SW. The regulation of phosphate is considered more critical than that of calcium because fish must effectively absorb, store, mobilize, and conserve phosphate in both FW and SW environments. Dietary phosphorus requirements for fish range from 0.4 to 0.9%.
Many studies with monogastric animals have shown that an optimum dietary Ca:P ratio is important; and increasing the Ca:P ratio interferes with the absorption of phosphorus and, conversely, a high P:Ca ratio may restrict calcium absorption. However, studies on the Ca:P ratio in fish diets are limited. The optimum Ca:P ratios observed in the diet of red sea bream and eel are 1:2 and 1:1, respectively.
Studies on quantitative requirements of sodium, potassium, and chloride have not been undertaken because fish readily absorb these elements from water. Trace element deficiency is not commonly observed in fish cultured under practical conditions; however, it may be readily produced experimentally in certain fish fed low amounts of these minerals. The following ranges of dietary trace element requirements (mg/kg) have been reported for fish: iron, 30-170; copper, 1-5; manganese, 2-20; zinc, 15-40; cobalt, 0.05-1; selenium, 0.15-0.5; iodine, 1-4 (reviewed by Lall, 2003).
Mineral deficiency signs in fish include reduced bone mineralization, anorexia (K), lens cataracts (Zn), skeletal deformities (P, Mg, Zn), fin erosion (Cu, Zn), nephrocalcinosis (Mg, Se toxicity), tetany (K), thyroid hyperplasia (I), muscular dystrophy (Se), and hypochromic microcytic anemia (Fe). Several types of skeletal deformities including vertebral and spinal malformations like kyphosis (e.g., humpback), lordosis, scoliosis, and platyspondyly (short-tail, compressed vertebrae) have been linked to mineral and/ or vitamin deficiencies.Mineral toxicity
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, and oxidation state of the mineral components, and the redox environment of the system. The soluble forms are usually ions (simple or complex) or un-ionized organometallic chelates or complexes.
These minerals are absorbed by aquatic organisms through gills and body surfaces and from ingestion of zooplankton, fish, and water. However, their ability to regulate abnormal concentrations varies with the species. Experimentally induced sub-lethal effects of minerals cause morphological, physiological (growth, swimming performance, respiration, and reproduction), and behavioral changes.Source and bioavailability
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 fish meal and marine by-products. Some minerals in these marine byproducts are found in concentrations above the recommended requirement levels.
Excessive amounts of minerals, particularly calcium and phosphorus, reduce zinc bioavailability and have also been linked to cataract formation in juvenile salmonid fishes (reviewed by Lall, 2003). The 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.
Alternate plant proteins, particularly soybean meal and canola meal or their concentrates, show significant potential to replace fish meal in fish feeds (Higgs et al., 1995; Storebakken et al., 2000). Although fish meal is considered an adequate source of dietary minerals, supplementation of fish meal-based diets with certain trace elements is necessary for optimum growth and bone mineralization. In diets containing high levels of plant protein, mineral supplementation is necessary to improve growth and bone mineralization of carnivorous salmonid and marine fishes.
Animal nutrition studies have clearly demonstrated that the bioavailability of an element differs markedly when supplied from different feedstuffs and with the same element from feedstuffs of different dietary composition (McDowell, 2003). Similar approaches with aquatic animals indicate that factors that influence the bioavailability of minerals include intake level of the nutrient, its chemical form, digestibility of the diet, particle size, interaction with other nutrients, chelators, inhibitors, physiological and pathological states of the animal, water chemistry, type of feed processing, and the fish species.
Certain mechanisms, which involve the formation of insoluble and non-absorbable substances in the gut may either hinder or facilitate mucosal uptake, transport, and metabolism of an element. For example, zinc is better absorbed from animal protein supplements than from plant protein sources. Cereals and other plant feedstuffs contain a number of substances, particularly phytate, which can bind zinc, and make it unavailable for absorption. The bioavailability of iron is influenced not only by its chemical form, but also by interactions between iron and other dietary components. Ascorbic acid enhances iron absorption whereas phytate and tannic acid may decrease its absorption.
Chelated minerals have been successfully used in terrestrial animals. In some fish species these chelated minerals show higher bioavailability than inorganic forms.
Potentially the use of chelated minerals with higher bioavailability may allow lower supplementation and reduce waste from unassimilated minerals. Amino acid chelates of zinc and copper appear to be more readily available than inorganic sources of these minerals in rainbow trout (Apines et al., 2003).
Chelated forms of copper, iron, manganese, selenium, and zinc (proteinates) were shown to have higher bioavailability to catfish in purified and practical diets (Paripatananont and Lovell, 1997). An improvement in the availability of minerals in chelated rather than inorganic forms in catfish diets has been attributed to a greater number of inhibitory compounds such as phytate and fiber in feedstuffs for which chelated minerals are less affected.
Organic forms of selenium including selenomethionine and Sel-Plex® selenoyeast have been shown to have higher bioavailability than inorganic sodium selenite for channel catfish (Wang et al., 1997). No apparent differences in the bioavailability of zinc sulphate and zinc methionine were noted in catfish (Li and Robinson, 1996). Use of citric acid and amino acid-chelated minerals in red sea bream diets improved growth, feed utilization, nutrient retention, and lowered nitrogen and phosphorus excretion (Sarkar et al., 2005).
New developments, particularly improvements in the sensitivity and specificity of analysis of mineral concentration in feeds and animal tissues and identification of biochemical indices to characterize mineral deficiency, have been useful in mineral nutrition research of fish. Advances in the knowledge of bioavailability, homeostasis, and appropriate indices of toxicity in aquatic animal nutrition will increase the use of plant proteins in aquafeeds as well as the major concerns for the impact of minerals on aquatic environment quality. These areas of research provide challenges and opportunities to address the pertinent environmental and health concerns of mineral nutrition in fish.
Author: SANTOSH P. LALL
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National Research Council of Canada, Institute for Marine Biosciences, Halifax, Nova Scotia, Canada