Changes in diversity in the fish assemblage of a southern Australian embayment: consistent spatial structuring at decadal scales

Comprehensive assessment of spatio–temporal variation in assemblages, particularly relating to management and conservation efforts, should include examination of variation across scales. The present study investigated spatio–temporal variation at various scales in the fish fauna of Port Phillip, Australia, over 17 years. There were significant increases in diversity and changes in faunal composition in the most recent study, compared with 17 (+38%) and 7 (+151%) years ago. No significant year-to-year differences and no fortnightly differences within a season were found, supporting the notion of long-term changes. However, inter-seasonal variation was significant, with diversity highest in summer and lowest in winter (42.3% of summer diversity), illustrating substantial variation only at particular scales. The spatial structuring of assemblages was consistent at all temporal scales over 17 years. Fish assemblages and diversity varied significantly among sites and regions, but diversity was always highest on reefs in the eastern region of Port Phillip. However, the majority of spatial variation occurred among replicate transects (up to 75% of overall variation). Despite the high degree of small-scale spatio–temporal variability, the results provide evidence of long-term changes in faunal composition and diversity within the bay. Moreover, the results underline the necessity for multi-scalar approaches in ecological studies like abundance assessments.