ALTERNATIVE MACROALGAL DIETS FOR JUVENILE GREENLIP ABALONE ( HALIOTIS LAEVIGATA ) IN THE LATER NURSERY PHASE

Greenlip abalone (Haliotis laevigata) are a highly valued fishery resource, grown in aquaculture facilities around the southern states of Australia. These commercial farms have a nursery system that utilises natural algal diets adhered to vertical plates for rearing postlarval and juvenile abalone. However, as the juvenile abalone grow (5 – 15 mm shell length), the current commercial nursery diet of the green alga Ulvella lens plus the diatom Navicula cf. jeffreyi does not supply adequate algal biomass to maintain commercially viable abalone growth rates at the required stocking densities. The focus of this research was to identify, develop and evaluate alternative macroalgal diets to overcome the restrictions in algal biomass during the later nursery phase. Macroalgae as an alternative natural diet for juvenile abalone can increase the algal biomass supplied on the vertical plates given its fast, 3-dimensional growth; while also considered a suitable nutrition source, as it is the primary feed of wild adult abalone. Australian abalone species have a preference for red macroalgae and to accommodate this, propagation methods including carpospore liberation, protoplast production and vegetative propagation; were assessed to determine the fitness of Rhodophyta species as an alternative diet for juvenile abalone in the later nursery phase. Protoplasts were readily isolated from red macroalgal species, however due to their limited regeneration, high cost of production and significant expertise required, the method was deemed unsuitable for utilisation in a commercial abalone nursery. Vegetative propagation successful established fragment culture of several red macroalgal species. The development of an artificial adhesion protocol, utilising active immobilisation processes through gel entrapment by the natural polysaccharide agar; enabled macroalgal fragments to be presented to juvenile abalone on the vertical plates in the nursery system. A diet of Laurencia sp. fragments adhered to the plates with agar produced juvenile abalone growth rates (50 μm.day-1) comparable to the current commercial nursery diet of U. lens and N. jeffreyi. The grazing resistance of the Laurencia/agar diet was low and fragments did not regenerate; so regular re-application was required, making artificial adhesion protocols unsuitable for use in the development of juvenile abalone diets within the nursery system. Instead of integrating alternative macroalgal diets in the nursery system, a different abalone management (weaner) system utilising an artificial diet, was able to produce significantly greater juvenile abalone growth rates and weight gain for abalone larger than 8 mm shell length. Macroalgal sporelings were incorporated as an alternative diet to remove the need for artificial adhesion protocols, as they can be seeded directly onto the plates whilst still presenting high algal biomass to the juvenile abalone. The morphology and life cycle of the green alga, Ulva allows for the high spore production and sporeling densities required to create a juvenile abalone diet. An Ulva spp. sporeling diet on the nursery plates produced abalone growth rates of nearly 100 μm. day-1 and was comparable to the current commercial nursery diet (U. lens/N. jeffreyi). However, the Ulva sporeling diet was unable to maintain suitable growth rates for abalone greater than 8 – 9 mm shell length and consequently, did not overcome the biomass limitation of natural algal diets in the nursery system. Given the Ulva sporeling diets ability to produce commercially viable growth rates for juvenile abalone less than 8 – 9 mm shell length and Australian abalone preference for red macroalgae, a composite green and red macroalgal sporeling diet was identified as an alternative diet for juvenile abalone in the later nursery phase. To incorporate Rhodophyta species into the diet, propagation via carpospore liberation was achieved for several red macroalgal species by temperature, dark and osmotic pressure induction treatments, with Hypnea sp. liberating the greatest number of carpospores (67.23 ± 10.19 x103 carpospores.g-1). Therefore, the combination Hypnea and Ulva sporeling diet was developed, which also reduced the biomass of red macroalgal carposporophyte required compared to that needed for creating a monospecies diet. This composite sporeling diet produced larger juvenile abalone (15 mm shell length), faster growth rates (87 μm.day-1) and weight gain (2.5 μg.day-1), when compared to the current commercial diets in the nursery (U. lens/N. jeffreyi) and weaner (artificial feed) systems. The addition of new seeded plates for all nursery diets during the trial allowed the composite sporeling diet to provide sufficient algal biomass. The Hypnea/Ulva sporeling diet was able to overcome the biomass limitations of algal diets and accommodate the juvenile abalone (<15 mm shell length) high grazing pressure, while producing commercial viable growth rates throughout the entire later nursery phase. This composite sporeling diet has been incorporated into a detailed feeding regime for Australian commercial abalone nursery practices, to help improve juvenile Haliotis laevigata culture and increase overall farm production of this highly valued resource.