During their life cycle, wild abalone shift from rasping on a benthic
diet, to grazing on a predominantly macroalgae-based diet
(Kawamura et al., 1998). The protein level (dry) of microalgae can be
up to 35%, while the protein level of macroalgae is signi ficantly lower
(11 –19%) (Mai et al., 1994; Brown et al., 1997; Won et al., 2010). This
shift in feed strategy was suspected to result in a decreased protease activity in cultured abalone throughout the production period. This hypothesis is supported by results from the current study. The trypsin
activity of 2-year old greenlip abalone was signi ficantly downregulated by 53%, compared to 1-year old abalone. The signi ficant
down-regulation of trypsin likely reduced dietary protein utilisation in
2-year old abalone and provides further support to reduce dietary protein level for 2-year-old abalone compared to 1-year old greenlip abalone, as previously recommended by Stone et al. (2013). Uki and
Watanabe (1992) previously investigated the digestibility of heattreated fishmeal by small (13 mm) and large (55 mm) abalone
(Haliotis discus hannai), and reported that smaller abalone were more
efficient at digesting heat-treated fishmeal, and suggested protease activity may decrease with age.