Evaluating the growth, feed intake and digestion of plant-based diets supplemented with single and complexed exogeneous enzymes for hybrid abalone (Haliotis rubra × Haliotis laevigata)

Hybrid abalone ( Haliotis rubra × Haliotis laevigata ) perform well when fed plant-based feed ingredients. However, the extent to which they are fully digested and contribute to growth in hybrid abalone remain to be examined. Exogenous enzymes are commonly employed to improve the value of plant-based feeds in other species. Therefore, single and complexed multi-enzyme supplements were used to examine growth indices, feed utilisation and digestive capacity in two simultaneous trials. The complex exogenous enzymes were included at 400 ppm (CE 400 ppm) and 800 ppm (CE 800 ppm) while single application were applied at 320 ppm (IE Protease), 80 ppm (IE Phytase) and 100 ppm (IE Xylanase). Initially, juvenile hybrid abalone weighing 9.03 ± 0.03 g were stocked into triplicate tanks, receiving flow-through seawater and fed for a total of 108 days. The results showed that the complexed enzymes improved the apparent digestibility of the diets overall, however, this was not translated into improved growth indices. The feed intake was not significantly reduced in the CE diet fed abalone treatments, despite a decline in growth response observed at both levels of inclusion. For the IE fed abalone, there were subtle differences observed for apparent digestibility, however, none were significantly improved compared to the control. The exception to this was improved carbohydrate digestibility in the IE Phytase fed abalone, which led to an increase in soft tissue carbohydrate composition. There were no significant differences in any growth indices or feed utilisation parameters for IE fed treatments compared to the control. α-Amylase activity in the digestive gland was reduced in all treatments compared to the control in both experiments. Based on these results, and using the formulation described herein, selecting exogenous enzymes for hybrid abalone may provide only limited benefits to production. An exploratory pooled regression approach was used to further asses the overall feed intake to growth relationship and the results may suggest that Australian hybrid abalone are sensitive to subtle alterations in the sensory profile of formulated diets. Further elucidation of the exact causes contributing to reduced growth performance are warranted.