Amino acid requirements of fish larvae and post-larvae: new tools and recent findings

This paper reviews methodologies and recent findings in the study of the amino acid (AA) metabolism of fish larvae and post-larvae, in order to better understand the AA requirements. The larval indispensable AA (IAA) profile can be used as index of the IAA requirements. When turbot larvae and live food IAA profiles are compared, the profile of the latter seems to be deficient in some IAA. However, the larval IAA profile is only a rough indicator of AA requirements. A more precise estimate of the ideal dietary IAA profile implies the knowledge of the relative bioavailabilities of the individual AA, in particular, eventual differential rates of absorption and catabolism. Metabolic budgets (including unabsorbed AA, AA oxidation and AA retention) can be estimated using an in vivo method based on controlled tube-feeding of AA mixes containing a 14C-labelled AA. Results with fasted post-larval Senegal sole (Solea senegalensis) and fasted herring (Clupea harengus) larvae show a high retention of labelled doses of IAA (>60%) in the body, compared to catabolism as measured by liberated 14CO2 (<25%). In contrast, dispensable AA (DAA) show a higher catabolism (>40%) and a lower retention (<57%). So, from the onset of exogenous feeding, fish larvae have high catabolic losses of AA, but use DAA preferentially to IAA as energy substrates. A new method combining the use of 13C-labelled live food and 13C-NMR spectroscopy can be used to study simultaneously the relative bioavailability of several individual AA in fish larvae. In larval gilthead seabream (Sparus aurata) fed on rotifers, relative bioavailabilities (a combined measure of absorption efficiency and rate of catabolism) vary between AA being high for aspartate, glutamate and lysine and low for threonine. These estimates of relative biovailability of individual AA together with the IAA profiles of the larval seabream indicate that rotifers are deficient in threonine and leucine for larval seabream, threonine being the first limiting AA for protein synthesis. In order to define ideal IAA profiles for larval fish, further studies are needed on the factors affecting the relative bioavailability of IAA, such as species, age, developmental stage, temperature and the dietary nitrogen molecular form(s). Estimates of relative bioavailability of individual AA together with the IAA profile of the larval protein allow to determine the ideal dietary IAA profile for a given species.