Reevaluation of the Quantitative Dietary Lysine Requirements of Fish

Studies on the quantitative lysine requirements of fish are numerous and have employed a range of methods. In this review, dose-response dietary lysine requirements are shown to have considerable intraspecies variation that cannot be explained by the particular laboratory variables between experiments (dietary formulation, fish growth rate, statistical model, and response criteria). Alternatively, it is demonstrated that marginal lysine intake in dose-response experiments is utilized for liveweight gain (LG) with an equivalent efficiency (54.1 mg LG.mg-1 lysine intake). Equivalent lysine utilization for LG means the lysine requirements of fish are not dissimilar when expressed relative to LG - and estimated to be 18.5 g lysine.kg-1 LG. Expressed relative to LG, lysine requirements of rainbow trout range between 15.7 and 21.1 g lysine.kg-1 LG, which represents an order of difference of only 34%. Due to a constant lysine requirement for LG, dietary lysine concentration is determined by feed efficiency ratio (kg LG.kg-1 feed intake). Consequently, increased feed efficiency with increased dietary energy results in a constant requirement expressed as lysine to energy ratio. It is recommended that future amino acid requirements of fish be expressed relative to gain (LG or protein) to improve current variation between studies.