Expanding the Ingredient Basket in Aquaculture: Growth Performance and Feed Utilization of Australian Hybrid Abalone (Haliotis laevigata × H. rubra) Fed Methanotrophic Single Cell Protein

There is growing interest in the use of single cell proteins (SCPs) derived from methanotrophic bacteria for inclusion in aquafeed to reduce reliance on other, potentially less sustainable proteins. This two-part experiment aimed to investigate first (i) the dose effect of replacing soy protein concentrate with SCP in Australian hybrid abalone diets (Haliotis laevigata Haliotis rubra) and second, (ii) the potential for improved palatability of the SCP by using commercially produced liquid protein hydrolysate (PH). This was assessed in a 2 × 2 factorial experimental design. The diets were formulated to be iso-proteic (~40% crude protein as fed) and iso-energetic (~18 MJ kg⁻¹ as fed). The diets (SCP 0%, SCP 5%, SCP 10%, SCP 20%, SCP 0% + PH, and SCP 20% + PH) were fed to the abalone (~9.25 g initial weight) once daily in the evening for 94 days to apparent satiation. The growth performance and feed utilization of abalone fed with up to 10% SCP was comparable to the control; however, there was a significant reduction in most parameters at the highest inclusion of 20%. In the second experiment, there was a decline in measured protein and energy retention in the abalone fed 20% SCP, with a concomitantly lower apparent biological value. Positively, there were significant improvements in the apparent digestibility of the diet at the 20% SCP inclusion. There was no significant impact of using the PH on growth performance or feed intake. No effects were observed on the digestive enzyme activity (α-amylase, lipase, and trypsin) examined among the treatments. Considering these results collectively, the maximum recommended inclusion of methanotrophic SCP meal should be 10% for hybrid abalone. Reasons for the decline in performance at the highest inclusion may be related to palatability; however, this remains to be fully confirmed.