No abstract

No abstract

No abstract

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 southwestern Australia.

Salinities in some normally-closed estuaries in the central south coast of Western Australia are now frequently becoming highly elevated. This is due to: (1) high evaporation rates in water volumes that, by summer, are already low as a result of atypically dry winters; and (2) increased salt run-off following vegetation clearing in the catchments. A few black bream (Acanthopagrus butcheri) died in the basin and lower reaches of the main tributary of Culham inlet when salinities reached ∼ 77 and 67, respectively, and an estimated 1.3 million black bream died in the tributary during the next two months when salinities continued to increase. All black bream in the basin and the lower reaches of the tributary of another estuary were apparently killed when salinities reached ∼ 83–85. It is proposed that A. butcheri becomes stressed at salinities of 60 and typically die before they reach ∼ 85. In both estuaries, a rock bar in the tributary prevented black bream from moving to refugia in upstream areas where salinities were lower.