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Minerals
ОглавлениеMinerals in the diet are often subdivided into:
Macrominerals (calcium [Ca], phosphorus [P], sodium [Na], potassium [K], magnesium [Mg]).
Trace minerals or microminerals (copper [Cu], zinc [Zn], iron [Fe], manganese [Mn], selenium [Se]).
Ultra‐trace minerals (iodine [I], cobalt [Co], chromium [Cr], molybdenum [Mo]).
Some minerals can be acquired by fish from the water, including Ca, Cu, Fe, K, Mg, Na, Se, and Zn (Terech‐Majewska et al. 2016). It is generally assumed that Ca, K, Mg, and Na requirements can be met by the water, particularly seawater, although low pH does restrict absorption (Terech‐Majewska et al. 2016). However, most commercial fish diets also meet minimum requirements. As with other nutrients, these recommended levels are generally determined for optimal performance of aquaculture species and should be used as a guide only (Table A4.4). Finally, many trace minerals interact with each other and with vitamins or other elements in the water, making it hard to determine their bioavailability to fish.
Phosphorus is likely the most discussed mineral in fish as dietary sources are required, and because of the variability in bioavailability and the environmental impact of excreted phosphorus. Phosphorus may be present in the diet from plant, animal, or chemical sources. Plant‐based forms are often complexed with phytic acid (e.g. phytate‐P) which generally has very low bioavailability to fish. A notable exception is Nile tilapia which can digest some phytate‐P (Kumar et al. 2012). Undigested phytate‐P can form complexes with other nutrients (e.g. Zn), reducing their bioavailability, and can contribute to algal blooms when excreted. Efforts to improve phytate‐P digestibility have been a focus of extensive research in fish nutrition. Exogenous phytase enzymes may be provided in the diet to improve phytate‐P digestibility, although efficacy varies. For example, agastric species such as the common carp (Cyprinus carpio) have a higher GI pH and most phytase enzymes have little effect unless dietary acidifiers or neutral pH‐active phytase enzymes are used (Kumar et al. 2012; Lemos and Tacon 2016).
Selenium is a trace mineral involved in antioxidant and housekeeping functions. Deficiencies of selenium reduce antioxidant function and can result in vitamin E or vitamin C based pro‐oxidation (Hamre et al. 2016). Deficiencies may be seen in captive‐bred larval fish, as rotifers and other common aquarium food items have much lower levels than wild food items (Hamre et al. 2008; Penglase et al. 2010). Selenium can also be toxic; excess can reduce growth rates and increase mortality and has been associated with renal calcinosis (NRC 2011).
Table A4.4 Dietary mineral requirements for teleosts and elasmobranchs.
Sources: Janse et al. (2004), NRC (2011), and Hamre et al. (2013). © John Wiley & Sons.
| Nutrient | Juvenile teleost | Larval teleost | Elasmobranchs |
|---|---|---|---|
| Calcium (%) | 0.87 (0.34–2.00) | 0.5 | |
| Phosphorus (%) | 0.61 (0.33–0.80) | 0.70 | |
| Sodium (%) | 0.11 (0.06–0.15) | (0.1–0.3) | |
| Magnesium (%) | 0.05 (0.04–0.06) | 0.05 | |
| Potassium (%) | 0.53 (0.26–0.80) | (0.1–0.3) | |
| Chloride (%) | 0.16 (0.15–0.17) | (0.1–0.5) | |
| Copper (ppm) | 4.3 (3.0–5.0) | (1–4) | |
| Iodine (ppm) | 1.1 (1–1.1) | (100–300) | |
| Iron (ppm) | 88 (30–150) | (50–100) | |
| Manganese (ppm) | 9 (2–12) | 40 | (20–50) |
| Selenium (ppm) | 0.34 (0.15–0.70) | (1.4–3.0) | (0.15–038) |
| Zinc (ppm) | 23 (15–37) | (15–100) |
All values are on a dry matter basis and represent the mean (range) from published literature.
Iodine is important for all fish and functions as a component of thyroid hormones. The requirements of elasmobranchs for iodine are well‐described and deficiency is a common cause of goiters (Janse et al. 2004). Ozone filtration reduces the bioavailability of water‐based iodine (iodide) due to oxidation to the unavailable iodate (IO3−). Additionally, high nitrate levels or other goitrogenic compounds can reduce iodide uptake (Morris et al. 2011).
Mineral (and vitamin) requirements of elasmobranchs are not well‐defined. Few comprehensive studies have examined the vitamin and mineral content of their wild prey. A recent study of lemon shark prey items found that the daily Zn intake exceeded the minimum requirements for teleosts, while daily intake of Fe, Cu, and Mn was much lower than the minimal requirements for teleosts (Pettitt‐Wade et al. 2011). Daily intake was calculated as a percentage of total diet at 6.6% Ca, 1.1% Na, 1.0% K, 0.3% Mg, 0.008% Zn, 0.005% Fe, 0.0007% Cu, and 0.0001% Mn.
