Investigating effects of dietary protein manipulation and Ulva lactuca supplementation on the postprandial metabolism and nitrogen excretion of Australian hybrid abalone

Abalone diets continue to be refined to suit the physiological and macronutrient requirements of different aged abalone under various seasonal conditions, primarily through dietary protein manipulation. Recently, dietary macro-algae supplementation has gained interest as an promoter of health and growth in abalone. In this study, the effects of three graded levels of dietary protein (35, 41 and 47 %) and 15 % dietary supplementation of dried and ground Ulva lactuca on the digestion and excretion processes in juvenile Australian hybrid abalone (Haliotis rubra x H. laevigata) were investigated through analysis of metabolic oxygen consumption (${\dot{M}}_{O_2}$) and nitrogen excretion (${\dot{M}}_{{\mathrm{NH}}_4^{+}-N}$) during meal digestion and assimilation. Trials were conducted at three farm-relevant acclimation temperatures (12, 17 and 22 °C), and abalone acclimated to the highest temperature were additionally subjected to a simulated heatwave. Within the ranges of 35 and 47 % dietary crude protein, no differences were observed in total postprandial energy use, however, significant differences were observed between the three temperatures. Similarly, differences in nitrogen excretion were observed across temperatures but not across dietary protein treatments. Dietary supplementation of U. lactuca led to significantly higher peak and total ${\dot{M}}_{O_2}$ throughout the postprandial period at 22 °C. Additionally, total ${\mathrm{NH}}_4^{+}-N$ excretion was higher at 17 °C when abalone were fed the U. lactuca supplemented diet. No differences in survival were observed between any of the dietary treatments when juvenile abalone underwent the simulated heatwave, however, the 15 % U. lactuca treatment did exhibit consistently higher ${\dot{M}}_{O_2}$. This information is useful for abalone farmers and feed formulators in developing season-specific diet formulations, particularly regarding macro-algae supplementation.