The effects of density on the behaviour and growth of juvenile blacklip abalone (Haliotis rubra)

The growth of juvenile abalone in aquaculture is known to be affected by density as a result of competition for food and decreases in water quality. Our results suggest that behaviour is also affected by density and this also has a significant impact on the growth of individual abalone. 1800 juveniles of Haliotis rubra were individually tagged and reared for 5 months in 12 gravity-fed tanks at two levels of density. The experimental design allowed the differentiation of the direct from the indirect effects of density. The abalone growth and distribution was monitored monthly along with the water quality. The distribution of abalone during daytime was closely related to the availability of preferred shelter space. The percentage of abalone stacked on the top of others increased with density. Preferred shelter space was characterized by low light intensity and a corner or edge for the abalone to rest against. Hides at the ends of the tanks were not occupied as much as others. Abalone were found crawling on the side of the tank during daytime when tanks were shaded. Abalone juveniles of 15-60 mm showed fidelity to their resting shelter during daytime but this fidelity was significantly reduced at the higher density. Shading of the tanks totally changed the distribution of the abalone and their daytime behaviour. Competition for shelter space reduced growth more than water quality.