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"Ecological interactions between abalone, urchins, and benthic habitat in a temperate reef ecosystem : implications for ecosystems based management"

The Tasmanian blacklip abalone (Haliotis rubra) fishery supplies -25% of the world market and, with a value of -AUD$120 M p.a., is the State's most valuable fishery. Fishing pressure is intense and management has been based on 'single-species' methods. However, with clear recognition that the fishery depends on the ecosystem that supports it, there is growing demand for a broader ecosystems based management approach (EBM). This thesis examined the ecological interactions between H. rubra and the invasive urchin ( Centrostephanus rodgersii), and whether fishing of H. rubra leads to a shift in the benthic community structure to a habitat configuration that becomes less favourable for abalone. Previous surveys demonstrated a negative relationship between densities of C. rodgersii and H. rubra. We used manipulative experiments to examine the effect of competition on behaviour, movement, growth, and survival. Removals of H. rubra had no detectable effect on C. rodgersii, so there is no evidence that fishing of abalone contributed to the urchin invasion success. In contrast, introducing C. rodgersii to intact algal beds caused H. rubra to flee and seek shelter in cryptic microhabitats, and reduced their growth and survival rates, all of which would negatively impact on the abalone fishery. H. rubra only ventured into small C. rodgersii barrens after urchins were removed and these areas revegetated. This suggests that lack food and/or loss of biogenic habitat structure explain the absence of abalone on barrens. The combined research suggests that EBM of C. rodgersii to optimise the H. rubra fishery requires complete removal of urchins and regrowth of barrens. Surveys at a range of spatial scales, depths and locations demonstrated a positive relationship between cover of encrusting red algae (ERA) and densities of H. rubra, and a negative relationship between densities of abalone and cover of filamentous algae, sessile invertebrates and the sediment matrix. Removal of H. rubra, to simulate heavy fishing, resulted in overgrowth of ERA by sessile invertebrates and a matrix of filamentous algae and sediment. Emergent H. rubra actively avoided areas overgrown by the matrix, which may negatively impact the fishery. Because H. rubra ostensibly prefer an open bottom dominated by ERA, these changes create a positive feedback loop. Above some threshold, more fishing is likely to render a greater proportion of the benthos unsuitable for abalone. Since H.