Investigating the appropriate mode of expressing lysine requirement of fish through non-linear mixed model analysis and multilevel analysis.

Accurate estimates of lysine requirement are essential to fish feed formulation. However, controversy exists regarding the most appropriate mode to express lysine requirement. In the fish nutrition literature, essential amino acid (AA) requirement has been expressed as a percentage of diet, a percentage of dietary crude protein or a ratio to dietary digestible energy (DE). The controversy lies in the different assumptions regarding the effects of dietary protein and DE on lysine requirement. Non-linear mixed model analysis and multilevel analysis were carried out to investigate whether dietary protein or DE affected lysine requirement of fish. The non-linear mixed model analysis suggests that expressing lysine requirement as a percentage of dietary protein provides a better goodness of fit to the modelling dataset than expressing requirement as a fixed concentration of diet, which in turn is generally better than expressing requirement as a ratio to DE. Results from the multilevel analysis confirm that dietary protein content has a significant effect on lysine requirement, while DE does not. The findings of the present study could contribute to a better understanding of the underlying dietary factors that affect AA requirements of fish. The results of the present study could also be useful for developing nutritional guidelines and feed formulations for fish.