Friday, June 10, 2016

Protein and amino acid requirements in aquaculture

Protein and amino acid requirements

The minimum dietary requirement for protein or a balanced mixture of amino acids is of primary concern in aquaculture because satisfying this requirement is necessary to ensure adequate growth and health of shrimp, while providing excessive levels is generally uneconomical, as protein is the most expensive dietary component. As such, most studies have been concerned with determining minimum dietary protein requirements of shrimp. Generally, recommended dietary protein levels vary from 30% to 55% for various species. The reported estimate of protein requirement must be carefully examined because it is dependent on quality (e.g., essential amino acid profile and digestibility) of dietary protein, age or physiological state of crustaceans, and environmental factors (e.g. salinity, dissolved oxygen, temperature).

Shrimp require the same 10 essential amino acids (EAA) as other species. The EAAs including arginine, methionine, valine, threonine, isoleucine, leucine, lysine, histidine, phenylalanine and tryptophan, were initially confirmed by Kanazawa and Teshima (1981). Generally, in contrast to the wide range of quantitative crude protein requirements that have been reported for shrimp, their qualitative requirements for amino acids are the same. This similarity provides a foundation for a theoretical approach to the nutritional causes of variation in optimal protein quality (D’Abramo et al., 1997).

Protein, amino acids and immune responses

A reduction in immune function could be expected if shrimp were fed a sub-optimal protein level in conjunction with nutritional stress. Pacific white shrimp Penaeus vannamei juveniles fed with a diet containing only 5% protein had reduced daily growth coefficient compared to shrimp fed diets with 40% protein (Pascual et al., 2004b). An increase in oxyhemocyanin was observed with increasing dietary protein levels indicating that shrimp accumulated protein as hemocyanin. A reduction of hemocytes occurred when shrimp were fed sub-optimal dietary protein levels indicating that zymogens contained in hemocytes, i.e., prophenoloxidase (ProPO) system, penaeidins and their activities (phagocytosis, coagulation), were also reduced. A reduction on respiratory burst was observed indicating that sub-optimal dietary protein level affected the number of cells and the phagocytosis capacity of cells (Pascual et al., 2004b).
Among the 10 EAAs, methionine, lysine and arginine are usually the most limiting amino acids for shrimp in commercial feed formulations and they are indispensable for normal growth and survival of shrimp (Coloso and Cruz, 1980; Pascual and Kanazawa, 1986; Akiyama et al., 1991). Shrimp P. vannamei fed methionine-, lysine- and arginine-deficient diets had significantly less wet weight gain than those fed the control diet, for the three amino acids, arginine appeared to be least limiting (Fox et al., 1995). However, arginine is involved in the synthesis of nitric oxide via nitric oxide synthase, which is inducible in shrimp (Jiang et al., 2004). Arginine has been shown to have numerous beneficial effects on T cell-mediated immunity in various animal models and in humans. Thus its involvement in shrimp health is certainly worthy of further investigation.

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