Distribution and ontogenetic shifts in habitat and abundance of the temperate western blue groper, Achoerodus gouldii (Richardson)

Densities of juveniles western blue groper Achoerodus gouldii on exposed coasts of South Australia were slightly greater on granite than other substrata and increased with increasing bottom relief and declined with depth and increasing exposure. Sub-adults were in higher density on schists than on other substrata, and densities increased with increasing bottom relief, and declined with depth, increasing exposure and increasing algal canopy cover. Adults were in higher densities on schists, and abundances increased with depth and bottom relief. At a mesoscale A. gouldii was absent from gulfs and sheltered bays, except near their entrances, while at offshore islands juveniles were rare, sub-adults were in lower abundance, and adults in greater abundance, compared with mainland coasts. It is hypothesized that sub-adults may swim up to 60 km from settlement locations to exposed areas and to offshore islands. At a geographic scale, abundances of all three size classes of A. gouldii declined significantly from west to east over 2000 km of coast from the centre of the species' range to its eastern edge. Although A. gouldii is a protected species over part of its range, abundance of sub-adults and adults was significantly reduced by fishing, as shown by an index of fishing intensity. The mean size of sub-adult A. gouldii was also correlated with this index, suggesting that the observed geographic trend in sub-adult and adult abundance may be contributed to by fishing. Simulation showed that a high instantaneous fishing mortality rate of 0·8 per annum could explain the maximum observed reductions in mean size of sub-adult A. gouldii .