Nutritional and interspecies effects on the estimation of threonine and lysine requirements in salmonid fry with a highlight on methodological impacts

In fish nutrition, researchers and industrial aquafeed producers alike need accurate estimates of indispensable amino acid requirements that are valid for their own particular conditions. However, published values for single indispensable amino acids manifest a wide range of values even for the same species. Researchers recognize the influence of several factors on the variation of indispensable amino acid requirements in fish; however, the order of their importance, the magnitude of their influence and their interactions are still poorly understood. In this context, the present innovative study focused on understanding the impact of specific nutritional, methodological, genetic and physiological factors on indispensable amino acid requirements in salmonids. Dietary protein level is an important nutritional factor known to affect growth, feed intake and efficiency. Its precise influence on indispensable amino acid requirements in fish was controversial. The present work revealed that the dietary protein level (310 or 469 g/kg dry matter) had no effect on the lysine (an indispensable amino acid) requirement of rainbow trout (Oncorhynchus mykiss, W.) fry when expressed in either absolute (mg lysine intake/kg MBW per day) or relative (g dietary lysine/100 g protein) terms. Consequently, the present work demonstrated the pertinence of these two complementary ways of expressing indispensable amino acid requirements. Existing comparative studies between rainbow trout and Atlantic salmon (Salmo salar, L.), two salmonid species, concerned only feed efficiency and the protein productive value. The present study established that the dietary threonine (an indispensable amino acid) requirement of trout and salmon fry was different when expressed in absolute terms (mg/kg MBW per day). In addition, the results showed the determining role of feed intake in explaining this difference. Another original aspect of the present work is that it offers practical advice and recommendations for dose-response experiments concerning the use of crystalline amino acids in experimental diets and the methodological choices of model and response criterion. The overall conclusion drawn from this study is that nutritional and methodological choices operated throughout the experimental procedure for dose-response experiments in fish have a decisive impact on the subsequent indispensable amino acid requirement estimations. This work will guide methodology standardization for indispensable amino acid requirement estimation in fish that is necessary to avoid common pitfalls and allow for better and more valid comparisons to be made between species and between studies. It opens interesting future prospects for the elaboration of deductive models to predict the indispensable amino acid requirements simultaneously for these phylogenetically related species.