Dietary threonine requirement of Atlantic salmon smolts

This study was conducted to determine the threonine requirement of Atlantic salmon (Salmo salar) smolts. Five diets containing 10.6, 11.7, 13.4, 14.9 and 16.6 g kg−1 threonine (presented in protein-bound form of threonine) were fed hourly to satiation to triplicate groups of smolts during the first 7 weeks in seawater. Average initial body weights were 79 ± 2 g per salmon (42 fish per tank). Feed intake was measured daily and growth rates were measured over the trial period. Body composition was analyzed in three groups of 10 fish at the start of the trial and in all of the fish in each group at the end of the trial. Based on the exponential threonine gain response to increasing dietary threonine concentrations, the minimum threonine requirement was found to be 13.1 g kg−1 dry matter (15.4 g kg−1 free form). The efficiency of digestible threonine utilization for growth in the deficient diets was 0.87.

► The minimum dietary threonine requirement of Atlantic salmon smolts was 13.1 g/kg dry matter.
► 87% of the digestible threonine in the deficient diets was utilized for growth.
► No effect of excess threonine was found on fish performance.