Multi-decadal changes in the fish fauna of an extensively modified estuary

This presentation outlines multi-decadal change in the fish fauna of an extensively modified, microtidal estuary in response to the interactive effects of steady population growth and climate change. Drawing on an extensive... [ view full abstract ]

This presentation outlines multi-decadal change in the fish fauna of an extensively modified, microtidal estuary in response to the interactive effects of steady population growth and climate change. Drawing on an extensive and extremely rare data set collated from 1979-2009, we examine multiple aspects of the fish fauna including community composition, an integrated fish-based index of estuarine health and (from 1993-2009) the growth and productivity of a key estuarine species (Black Bream, Acanthopagrus butcheri) to determine their collective ‘signals’ of inter-annual change. Our findings demonstrate prominent but differing inter-period shifts in each of the above aspects of the fish fauna, and highlight the need to take a multifaceted approach to examining ecological indicators of ecosystem health. Whereas fish species composition in the shallows of particularly the lower to middle estuary has changed markedly from earlier to later periods (reflecting increases in several marine-affiliated species and also Black Bream, but declines in others linked with fresher conditions), that in the deeper waters of the middle to upper estuary has shown comparatively little change (though did reflect declines in some species including Black Bream). In contrast, the fish-based index of estuarine health has decreased consistently over time from a ‘report card’ grade of C (fair) to D (poor) in the late 2000s in the deeper waters, but the health of the shallows has remained fair and even increased slightly in recent periods. Moreover, while far greater numbers and biomass of Black Bream now occur in the shallows, apparently reflecting (at least in part) their emigration from the deeper waters, their growth and body condition has declined dramatically since the early 1990s. We then explore the relationships between these biotic responses and a suite of environmental attributes to unravel the key drivers of change. We conclude by considering some potential future scenarios for the fish fauna of this system given current projections of population growth and climate.