The sulfur amino acid requirements of juvenile Yellowtail Kingfish (Seriola lalandi)

The dietary methionine and cysteine requirements of Yellowtail Kingfish (YTK) are unknown. Methionine, an essential sulfur-containing amino acid, acts as a sulfur and methyl donor for key metabolites, such as cysteine and taurine. Cysteine, a conditionally essential sulfur-containing amino acid, can spare significant amounts of methionine in the total sulfur amino acid (TSAA) requirement. Both methionine and cysteine are an integral part of YTK aquafeeds, in which methionine supplementation levels are based on the requirements of the closely related Japanese Yellowtail (Seriola quinqueradiata) at approximately 11.1 g kg(-1) diet. However, recent research has demonstrated better growth and feed efficiency in YTK fed diets containing more than 11 g methionine kg(-1), although the precise methionine requirement and potential interactions with cysteine remain unknown. Therefore, the present study was designed to (1) elucidate the methionine requirement of YTK at low and high dietary cysteine contents and to (2) quantify cysteine's sparing capacity for methionine in the TSAA requirement. These requirements and relationships were established by feeding ten isonitrogenous and isoenergetic diets, made from common feed ingredients, to triplicate groups of 12 fish (initial 52.6 +/- 1.0 g fish(-1)) over 54 days. The orthogonal design consisted of two levels of dietary cysteine (5.6 & 13.9 g kg(-1)), crossed with five levels of dietary methionine, increasing from 7.9 to 25.2 g kg(-1). Non-linear regression analysis indicated an average digestible TSAA requirement of 0.70 g kgBW(-1) d(-1) (0.56 g Met kgBW(-1) d(-1) & 0.14 g Cys kgBW(-1) d(-1)) based on feed conversion ratio, specific growth rate, and protein retention efficiency. This approximates to an average dietary sulfur amino acid specification of 24.5 g kg(-1) (18.9 g Met kg(-1) & 5.6 g Cys kg(-1)). Cysteine spared 40.4-49.2% of methionine in the TSAA requirement on an equimolar sulfur basis. Sub- and supra-optimal levels of dietary methionine and cysteine induced inferior growth and feed efficiency. Additionally, fish fed the diet lowest in dietary methionine and cysteine indicated early stages of cataract. This study provides quantitative data on the sulfur amino acid requirements of juvenile YTK and will facilitate the formulation of better diets for this species.

Automated summary

This study investigated the dietary methionine and cysteine requirements of Yellowtail Kingfish, finding that cysteine can spare up to 40.4-49.2% of methionine in the total sulfur amino acid requirement. Optimal levels of these amino acids are crucial for growth and feed efficiency in YTK, with sub-optimal levels potentially leading to cataract development.