This study used underwater visual census techniques to quantify the distribution, abundance, fork lengths and biomass of scarid fishes on multiple reefs across the continental shelf of the northern Great Barrier Reef. Spatial patterns in fish distribution were examined over a cross shelf environmental gradient using a hierarchical sampling design that covered a spectrum of scales ranging from metres to tens of kilometres. The design included replicate reefs within mid and outer continental shelf positions and replicate sheltered and exposed sites within each reef. Most of the 21 species surveyed were found to be widely distributed across mid and outer shelf reefs and the number of species did not change significantly between reefs, exposures or across the shelf. Despite a similar complement of species, scarid assemblages differed markedly in the relative and absolute abundance of taxa between exposed and sheltered habitats on mid and outer shelf reefs. Principal components and cluster analyses indicated that changes in the numerical abundance of taxa resulted in consistently different assemblages from 9 sheltered mid shelf sites and 9 exposed outer shelf sites at opposite ends of the surveyed environmental gradient. Variance components calculated for scarid abundance and biomass across 4 spatial scales revealed differences in the scales at which significant variability occurred between sheltered and exposed reef habitats. On exposed reef crests significant variation occurred at both scales of tens of kilometres (shelf position) and hundreds of metres (sites), while on sheltered back reefs significant variation was attributable only to the site scale. Mean scarid abundance (± SE) for exposed outer shelf sites (3060 ± 460 ha -1 ) was on average 4 times higher than in all other habitat zones surveyed, while mean biomass (± SE) was only 3 times higher (920 ± 190 kg ha -1 ). The discrepancy between abundance and biomass estimates is explained by a 30% average decline in parrotfish fork lengths on exposed outer shelf reef crests relative to other habitats. Log-linear analysis indicated that at least 8 species of scarids displayed significant changes in body size according to their location across the continental shelf and/or the prevailing exposure regime. The observed patterns of variation in density and length frequency suggest density-dependent processes and that changes in mortality or growth rates may exist between habitats across the continental shelf.

This study describes changes in the timing of maturation and sex reversal and shifts in the sexual composition of unfished local populations of protogynous reef fishes across the continental shelf of the Great Barrier Reef. On outer shelf reef crests, both Chlorurus sordidus and Scarus frenatus matured as females and underwent protogynous sex reversal at reduced sizes and ages relative to fish from equivalent midshelf habitats 20 km away. Earlier maturation at smaller body sizes on the outer shelf is likely to be an adaptive response to the higher mortality rates in these habitats compared with the middle shelf. Furthermore, reduced size and age at sex reversal on the outer shelf is consistent with the reduced growth and increased mortality regimes in these locations. The sexual composition of local populations changed markedly across the shelf for C. sordidus but not for S. frenatus. High densities of C. sordidus on the outer shelf were characterized by threefold higher proportions of both terminal-phase and primary males than on midshelf reefs. Differences between local populations separated by tens of kilometres highlight the danger of assuming homogeneous sexual compositions and life histories when managing exploited reef fishes over larger spatial scales.

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Genetic diversity generally underpins population resilience and persistence. Reductions in population size and absence of gene flow can lead to reductions in genetic diversity, reproductive fitness, and a limited ability to adapt to environmental change increasing the risk of extinction. Island populations are typically small and isolated, and as a result, inbreeding and reduced genetic diversity elevate their extinction risk. Two island populations of the platypus, Ornithorhynchus anatinus, exist; a naturally occurring population on King Island in Bass Strait and a recently introduced population on Kangaroo Island off the coast of South Australia. Here we assessed the genetic diversity within these two island populations and contrasted these patterns with genetic diversity estimates in areas from which the populations are likely to have been founded. On Kangaroo Island, we also modeled live capture data to determine estimates of population size. Levels of genetic diversity in King Island platypuses are perilously low, with eight of 13 microsatellite loci fixed, likely reflecting their small population size and prolonged isolation. Estimates of heterozygosity detected by microsatellites (HE= 0.032) are among the lowest level of genetic diversity recorded by this method in a naturally outbreeding vertebrate population. In contrast, estimates of genetic diversity on Kangaroo Island are somewhat higher. However, estimates of small population size and the limited founders combined with genetic isolation are likely to lead to further losses of genetic diversity through time for the Kangaroo Island platypus population. Implications for the future of these and similarly isolated or genetically depauperate populations are discussed.

Summary1. Environmental assessments of coastal aquaculture are concerned mostly with direct impacts on natural assemblages in the vicinity of shellfish or fin-fish farms. As the size and density of farmed sea space increases, there is greater potential for indirect effects on food webs beyond the immediate culture area. 2. We investigated the potential indirect effects of long-line mussel Perna canaliculus farms on the demography of an important benthic predator, the sea star Coscinasterias muricata . Surveys beneath four active farms, an abandoned farm and three unfarmed areas of seabed in Pelorus Sound, New Zealand, described the direct effects of mussel culture on the distribution and abundance of sea stars and other benthic consumers. These data were used to calibrate a model that simulated the fertilization success of sea star populations in farmed and unfarmed areas of the bays. 3. Deposits of living mussels and mussel shells covered up to 55% of the seafloor beneath farm sites, but were absent from soft sediments at unfarmed sites. Mean densities of sea stars were up to 39 times larger at active farm sites than in unfarmed areas and were correlated with the abundance of living mussels on the seafloor. 4. Within individual farms, the distribution of sea stars was highly aggregated at small spatial scales, with most (63%) individuals occurring within 2 m of their nearest neighbour. In unfarmed areas, sea stars were widely dispersed (< 1 80 m − 2 ). 5. Our simulations indicate that, because of the extremely clumped distributions of sea stars, spawning individuals at farm sites would on average have substantially greater fertilization success ( c. 90% of eggs) than those foraging in areas where farms are absent (<< 2% of eggs), and total zygote production could be as much as 1500 times greater than in unfarmed areas. 6. Synthesis and applications . This study demonstrates the potential for significant bottom-up effects of aquaculture on surrounding ecological assemblages. If, as has been suggested, sperm limitation is a major constraint on recruitment of asteroids and other invertebrate predators, supplemental provisioning from increased farm development could result in occasional outbreaks of populations, over a broader area than the farmed location. Without appropriate monitoring such events are likely to be dismissed as rare, natural phenomena rather than a consequence of shellfish culture.