Spatial variation in abundance of recently settled rocky reef fish in southeastern Australia: implications for detecting change

Abstract

Spatial variation in abundance of recently settled fish was assessed uslng replicated visual census during 3 surveys of rocky reefs < 12 m deep in New South Wales Two surveys compared abundances on reefs up to 29 km apart. The third, larger-scale survey compared abundances at 3 levels: regions (100 to 200 km) locations (5 to 12 km) and reefs (1 to 3 km). There were differences in abundance of recently settled individuals among reefs for many species, but these differences were masked by great variability in abundance within reefs. A realistic increase in replication would have greatly increased the chance of detecting significant dfferences in abundance among reefs for some species. In the larger-scale survey, 9 species settled. Most of the variability in abundance occurred among or within reefs, rather than among locations and regions. The great vanabhty in abundance within reefs at all spatial scales has consequences for assessment of environmental impact on settlement of rocky reef fish. We propose several ways of collecting data that should increase the likelihood of detecting such impact, including selection of control reefs with similar habitats to the impacted reef(s), and counting fish within habitats rather than from transects crossing habitats.