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ItemExperimental approaches for understanding density dependence of greenlip abalone (Haliotis laevigata) populations(University of Melbourne, 2011)The aim of the present study was to determine the magnitude of density dependent responses in juvenile mortality and adult growth for greenlip abalone populations in the wild. This was achieved through the conduct of two large-scale, densitymanipulation experiments in the field which also realised commercial outcomes regarding an improved understanding of the response of abalone stocks to fishing and demonstrated a capacity for abalone stock enhancement. The first large-scale experiment was a seeding study where hatchery-reared juvenile Haliotis laevigata were released at eight sites in South Australia covering two geographical regions: Cape Jervis on the Fleurieu Peninsula and Point Marsden on Kangaroo Island. Six reefs were established at each site, each with two boulder layers,as a pilot study showed significant increases in survival two months after release in reefs with two layers of boulders compared to a single layer. Hatchery juveniles were individually tagged and allowed to recover for 5 days. They were released onto reefs between boulder layers to allow spontaneous movement into the reef complex. Three months after seeding almost all juveniles were in cryptic positions between boulders, but six months after seeding 17% of abalone were emergent at 56 � 7 mm. Minimum survival, estimated from abundance surveys, ranged from 0 to 55% after 9 months (mean= 38%). Empty shell collections accounted for 11% of seeded juveniles. A second seeding a year later at one site resulted in similar survival after three months. The survival of these juveniles compares favourably with previous published seeding experiments for abalone species. Considerable variation in survival was observed between regions and was associated with the abundance of a predatory seastar. The asteroid Coscinasterias muricata is a known predator of both Haliotis laevigata and H. rubra populations in Southern Australia. The abundance of C. muricata was negatively correlated with abalone survival at all eight sites. Abundance of this predatory seastar was highest at the Windmill site (near Point Marsden), where apparent survival of abalone was only 1% six months after seeding. Furthermore, the recovery of unbroken shells, evidence of possible predation by C. muricata, was significantly higher near Point Marsden than at sites near Cape Jervis. This study demonstrated that C. muricata can effectively prey upon juvenile abalone in a cryptic environment, causing severe and rapid depletion of a localised population. The findings of this study also supported a previous theory regarding patchy predation by seastars. The development of a multi-state mark-recapture model, using purpose-built software called MSURGE, enabled accurate estimation of survival for the six sites near Cape Jervis. The model accounted for differences in the recapture efficiency of divers and incorporated separate estimates of tag loss for each reef. By demonstrating minimal influence of migration, tag-induced effects and size-specific mortality, unbiased estimates that approximated true survival were obtained for the period between 3 and 9 months post-seeding. This model combined with the robust experimental design enabled detection of density-dependent mortality. This relationship with density was consistent across sites and survey periods despite substantial differences in mortality rates among some sites. The second large-scale field experiment involved the tagging and translocation of adult greenlip abalone. Growth of "stunted" greenlip abalone in areas with low maximum sizes was enhanced over 6 months by reducing their natural density and also by translocation to habitats supporting faster growing abalone. Density reductions significantly increased growth relative to controls, apparently without altering the asymptotic length. Stunted abalone showed a consistent and similar pattern of enhanced growth when translocated to two sites where abalone characteristically grow faster and to larger sizes. When compared with slow growth control abalone, the response of translocated abalone varied with initial length in the same manner as in the experiment where density was reduced. When compared with fast growth controls, translocated abalone had similar trends in growth rate, yet all size categories grew to a consistently smaller size. The fact that reduced density and better quality habitat positively influence growth patterns of greenlip abalone, producing the same short-term response, suggests that food availability may limit growth m stunted populations. The asymptotic length, which appeared unaffected in both experiments, may be determined by long-term conditions, or perhaps by conditions during the onset of maturity. Published evidence for density-dependent responses in juvenile mortality and adult growth in natural abalone populations is very limited. These studies clearly demonstrate that density dependent processes operate on juvenile mortality and adult growth of abalone and that these effects are likely to be compensatory mechanisms that aid regulation of harvested abalone populations. However, the severe episode of natural predation observed in the first large-scale field experiment suggests that the magnitude of these effects can be highly variable and significantly influenced by density independent events. Until we have an improved understanding of the compensatory (and depensatory) processes that affect fish populations and their interactions, we are unlikely to be able to accurately model or predict sustainable production or prevent collapses of local populations from occurring.