Large-scale patterns in habitat structure on subtidal rocky reefs in New South Wales

Aerial photography was used to estimate the representation of shallow subtidal habitats in New South Wales. Sixty sites, each between 4 and 5 hectares, were mapped with Geographical Information Systems software using ortho-rectified images digitized from 1:8000-scale photographs and `ground truthed' in the field by divers. Barrens habitat covered an estimated 50% (s.e. = 3.9) of nearshore reefs between Port Stephens and Disaster Bay. Coverage of barrens habitat was greatest in Disaster Bay (68%, s.e. = 6.7) and least south of Disaster Bay (1%, s.e. = 0.3). There were clear differences among localities in the area of reef within the mapped sites; those at Cape Howe, Nadgee, and Turingal were significantly smaller in area than all others. There was no clear latitudinal trend in these differences but there was evidence of sand inundation at a site at Nadgee, where the reef was small. Differences in the densities and size-structure of the sea urchin Centrostephanus rodgersii at 27 of the mapped sites provide a basis for testing relationships between the demography of this species and the persistence of the barrens habitat. The extensive coverage of the barrens habitat in New South Wales is likely to limit the productivity of the abalone industry. The development of a sea urchin fishery may have large impacts on habitat representation on nearshore reefs.