LAUSR

Abstract The Essential Amino Acid (EAA) requirements of many cultured species remains unknown and therefore aquafeeds used in new industries are often based on the requirements of similar species or those from a similar trophic level. One such example is the formulation of aquafeeds for Australian Yellowtail Kingfish Seriola lalandi (YTK), which are historically based on the published EAA requirements of Japanese Yellowtail Seriola quinqueradiata. The requirements of YTK for sulphur-amino acids as well as taurine have recently been quantified. These studies indicated that cultured YTK may have different EAA requirements to Japanese Yellowtail which has raised doubts about the adequacy of other EAAs in commercial feeds for YTK. Of these, the adequacy of histidine was considered particularly important due to concerns about cataract formation and poor growth. Therefore, the digestible histidine requirement of YTK was quantified using a dose-response design that employed a fishmeal-free basal diet containing 0.61% digestible histidine. Supplemental crystalline L-histidine was added to the basal formulation in 0.5% increments resulting in four additional diets with a digestible histidine content of 0.96, 1.32, 1.61 and 2.0%, respectively. Each dietary treatment was assigned to triplicate saltwater tanks containing 10 × 80 g YTK which were fed to apparent satiation for seven weeks at a temperature of 20 °C. Despite the incremental addition of histidine there were no differences in growth rate, feed intake, condition factor and feed conversion ratio of YTK in response to dietary treatment. In contrast, evaluation of whole carcass revealed histidine deposition rate increased in response to digestible histidine content before declining at the highest dietary level (i.e. >1.61%), suggesting YTK had reached their maximum rate of histidine deposition under our experimental conditions. A factorial analysis of the retention data from the current study was compared to and combined with previous factorial models on histidine utilisation by YTK. The combined model indicated that while the maintenance demand for histidine varied with experimental conditions, the unit cost of histidine deposition in YTK was constant at 1.99. A conservative estimate of the variable digestible histidine requirement for maintenance at temperatures between 15 and 24 °C was found to be 64.2 mg kgBW-0.7d−1. Feed ration models generated from the factorial data sets indicate YTK should be fed diets that contain between 1.6 and 2.0% digestible histidine in order to meet their daily histidine requirement. These amounts are slightly higher than generally recommended for Japanese Yellowtail.