7.4 Minerals

Fish have more flexibility when discussing minerals in fish nutrition than mammals in acquiring the needed quantities. Fish nutrition is supported by absorbing several minerals across gills, skin, and some epithelial linings from the outside aquatic environment. Calcium (Ca), copper (Cu), iron (Fe), magnesium (Mg), potassium (K), selenium (Se), sodium (Na), and zinc (Zn) are absorbed from water as a rule of thumb – if the water environment contains the minerals. Please keep in mind that outside aquatic environments can range from mineral-depleted freshwater to the mineral abundance of marine environments. When managing the mineral access for fish, it is necessary to know the mineral values in the surrounding water. Dietary availability can support fish needs in low-mineral water. Brackish and marine species are not often affected by mineral imbalances. Fish absorb dietary minerals much as mammals.

Minerals, i.e., inorganic elements, are required for many normal life functions of fish. Minerals are involved in skeletal formation and maintenance, osmoregulation, and being part of enzymes and hormones. Fish have several complex mechanisms to control mineral acquisition. Hence, there is a unique ability for this class of vertebrates to occupy the largest vertebrate range of environments.^[1]

Minerals are categorized as macrominerals or micronutrients (trace minerals). The minerals are essential to fish since the body cannot generate minerals and requires certain levels to maintain growth, health, and reproduction activity. There are six minerals listed as macrominerals. The macrominerals include minerals such as calcium, phosphorus (phosphates), magnesium, sodium, chloride, and potassium.^[2] Soft water sources can create nutritional challenges for some fish in some situations. Therefore, the water source for a production facility is best examined for the mineral content of elements absorbed by gills. Macrominerals are needed in larger amounts than trace minerals.

Macrominerals

Calcium

Calcium is a major component of both skeletons and scales. Scale calcium can amount to 19-24% of the total body dry weight.^[3] Dietary levels of Ca are usually met with the ingredients of the formulated feed if the water level of Ca is adequate.

Fish can have the same relationship as mammals between Vitamin D and Ca balance for skeletal growth and maintenance.

Phosphorus

Fish gills can absorb phosphorus from water and can also be acquired from the diet. Fish are better at utilizing animal sources of phosphorus rather than plant sources. Meat and bone meal work well in fish diets. Plant sources are not well used when P is bound with phytic acid. Calcium phosphate is well utilized. Formulated diets need about 0.45% available P in a diet depending on species similar to monogastric mammals. Some species have demonstrated a balanced ratio between Ca:P, such as 1:2 in red seabream^[4] and 1:1 in eel.

Magnesium

Magnesium (Mg) is considered a critical divalent cation that plays essential roles in many functions in the body. Extracellular Mg is involved in muscle function, nerve conduction, and many tissue metabolic functions. The major proportion (50–70%) of Mg in the body of fish is located in skeletal tissues and scales^[5]. Aquatic livestock access to Mg is generally adequate in freshwater when Mg is present in water above 46 mg per liter.^[6] Fish in saltwater will not need Mg in the diet. Mg toxicosis is possible with high Mg and low protein diets. When on formulated diets, Mg deficiency can present with anorexia, reduced growth, lethargy, and reduced tissue magnesium content in fish tissues. Trout will present with renal calcinosis as a consequence of Mg deficiency. ^[7].

Salt (sodium chloride)

Freshwater fish plasma has higher concentrations of sodium chloride (NaCl).than the surrounding water. Elevated plasma sodium chloride leads to diffusive salt loss across the gill epithelium. This outflow can create serious challenges for fish living in low-ion waters. Feed formulations for freshwater generally provide 0.07% Na and 0.18% Chloride in the final feed analysis to counterbalance the difference in most cases. There has been a thought to produce brackish species in freshwater by adding more salt to the diet to bolster the electrolyte levels needed in brackish species. Salt is widely used in production and hauling water as a symptomatic treatment for stress. The benefit is replacing salt loss from stressors. Supplemental salt added to the water allows the fish to balance the homeostatic osmolality pressure that hauling activities cause fish.

Potassium

Potassium is often described along with the sodium and chloride components because all are easy for fish to acquire from their surrounding water. Potassium is also one of the most abundant elements in the body. It is typically readily absorbed from water, and the abundance of potassium in natural ingredient feedstuffs used in fish diets means they need not be supplemented. Potassium was found to be needed for chemically defined diets. Signs of potassium deficiency have been anorexia, tetany, and death.^[8].

Trace Minerals

Trace elements in fish diets have numerous minerals, but not all have nutritional requirements. Without listing specific fish species, the following microminerals have dietary requirements: iron, copper, manganese, zinc, cobalt, selenium, and iodine.^[9] Trace metals are mostly required in small amounts of less than 100 mg kg¹. The dietary requirements are nonetheless absolutely required for normal development and growth. Some of the trace minerals are described in the following discussion. Iron is widely used in fish for hemoglobin, ferritin, and enzymes such as cytochromes. Deficiency leads to anemia in fish, just as in other vertebrates. The gut mucosa absorbs the great majority of iron for fish needs. Copper is involved in enzyme activity, and various copper proteins and chelates are involved in metabolic activities. Copper toxicity is more common than deficiency and is seen as anorexia and poor growth. The range of copper adequate for most fish rests between 3 to 5 mg Cu /kg diet. Copper can have interactions with zinc since they have similar valence qualities. Manganese is important in many enzyme activities of fish. There is a restriction of growth when manganese is inadequate. Dwarfism has been reported with manganese deficiency.^[10].