Читать книгу Clinical Guide to Fish Medicine - Группа авторов - Страница 144

There are 20 amino acids. Ten are essential and cannot be produced by fish. The remainder can be synthesized by the animal, assuming there is enough dietary nitrogen. Thus, dietary sources must provide both essential amino acids and additional nitrogen. The level of dietary protein that is required by fish has been well‐documented for fast‐growing aquaculture species and ranges from 29 to 42% digestible protein (Table A4.1). This requirement has not been well‐documented for other species or other life stages. However, for larval fish, free amino acids may be more important as some lack the enzymatic capability to digest intact protein (Rønnestad and Fyhn 1993).

Essential amino acids for fish include arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine (NRC 2011). Arginine requirements are quite high compared to terrestrial mammals or birds, due to low activity of urea cycle enzymes. Tyrosine and cysteine are considered semi‐essential because they can be synthesized from phenylalanine and methionine, respectively. Taurine, an aminosulfonic acid, appears to be essential for some life stages, notably larval fish (NRC 2011). The relative ratios of amino acids that have been determined to be required by fish are generally proportional to the optimal dietary protein level. However, it is important to note that amino acid requirements have historically been determined with body weight gain as the predictive variable. The use of other functional end points (e.g. optimal immune function) may change the requirement. For example, recent work examining histidine requirements in Nile tilapia (Oreochromis niloticus) found that lowering histidine below previously published requirements improved weight gain, muscle fiber quality, and efficiency of protein utilization (Michelato et al. 2017). This type of refinement of nutrient requirements will continue to improve fish nutrition as well as the impact of excessive feeding on water quality and the environment.

Fish‐based protein sources most closely match the nutritional requirements of fish. However, with sustainability concerns, efforts have been made to identify other protein sources for commercial fish diets, such as marine by‐catch, terrestrial animals, plants, and microbes. Terrestrial animal protein sources lack some essential fatty acids and may be high in fat. Plant‐based protein sources may be less expensive but often lack many essential amino acids and may contain anti‐nutritional factors (discussed below).

As a final caveat, it is important to consider the energy content of the diet for which protein requirements have been determined. The ratio of protein:energy is important as amino acids serve as energy precursors in fish. For most species studied, the optimal ratio of digestible protein:digestible energy ranges from 84 to 105 g/Mcal (NRC 2011).

Table A4.1 Approximate diet composition for some model fish species.

Model species	Common carp, tilapia, channel catfish, rohu	Hybrid striped bass	Rainbow trout, Atlantic salmon, Pacific salmon, European bass	Asian seabass, cobia, Japanese flounder, grouper, yellowtail
Habitat	Fresh or slightly brackish water	Freshwater	Marine or freshwater	Marine
Water temperature	Temperate	Temperate	Cold	Temperate
Feeding strategy	Omnivore	Carnivore	Carnivore	Carnivore
Digestible protein (%)	29–32	36	36–40	36–42
Crude fat (%)	<10%		15–21
Calcium (%)	0.45–0.70
Phosphorus (%)	0.33–0.70	0.50	0.6–0.8	0.6–0.8
Sodium (%)	0.06–0.15
Copper (ppm)	3–5		3–5	5
Iodine (ppm)	1.1		1.0–1.1
Iron (ppm)	30–150		30–60
Manganese (ppm)	2.4–12.0		10–12
Selenium (ppm)	0.25	0.25	0.15	0.70
Zinc (ppm)	15–20	37	15–37	20
Ascorbic acid (ppm)	15–45	22	20	15–54
Thiamine (ppm)	0.5–1.0		1–10	11
Vitamin E (IU/kg)	50–132	28	50–60	115–119

The values are for production animals with the goal of rapid growth. All values are on an as‐fed basis. Source: NRC (2011).

Protein waste products and undigested feed can negatively impact water quality. Nitrogenous waste can be minimized by providing optimal levels of amino acids and reducing the digestible protein:energy ratio such that nitrogen retention efficiency is high (McGoogan and Gatlin 1999).