Development and validation of fish-based, multimetric indices for assessing the ecological health of Western Australian estuaries

We describe the development of the first fish-based, multimetric indices for assessing and monitoring the health of Australian estuaries, and their application to the nearshore (<2 m depth) and offshore (>2 m depth) waters of the Swan Estuary, Western Australia. Suites of fish community metrics, including measures of species composition, diversity and abundance, trophic structure and life history function, were selected via a novel weight of evidence approach on the basis of their sensitivity to detect inter-annual change in estuarine condition. For each selected metric, seasonally-adjusted reference conditions were established for each spatial management zone of the Swan Estuary using 30 years of standardised historical fish assemblage data. This extensive data set provided a sound basis for determining the ‘best available’ standard of biotic integrity recorded over that time period and thus a reliable benchmark against which the current and future health of the estuary may be assessed and compared. The nearshore and offshore indices were robust to the effects of natural, intra-seasonal variability in environmental conditions, and so provide reliable tools for quantifying and classifying the ecological health of the Swan Estuary and its constituent management zones. The response of the nearshore index to an algal bloom confirmed that it is sufficiently sensitive to quantify ecological health responses to local-scale environmental perturbations and to track the subsequent recovery of the system following their removal. The indices provide managers with a reliable, quantitative method for assessing and communicating the health of the Swan Estuary and, similarly, of other estuaries across south-western Australia.