Thea Linke

This presentation will focus on a quantitative scheme for classifying nearshore habitat types and predicting their fish faunas, which has been developed for a range of estuaries in south-western Australia that differ widely in their geographical and geomorphological characteristics. This scheme provides a reliable framework for managers and ecologists to (i) investigate relationships between habitat types and faunal composition at local to regional scales and under different geographic and estuarytype scenarios, (ii) establish benchmarks against which the impact of future environmental change can be assessed and (iii) predict the habitat type and characteristic fauna of any nearshore site of interest within those systems. The habitat classification scheme has been based on the use of enduring environmental criteria that can be easily measured in GIS from mapped data sources such as satellite imagery and bathymetric charts, and has employed novel applications of relatively new multivariate routines. The results of this classification scheme, and the relationships between the derived habitat types and their fish assemblages, will be presented for five estuaries across south-western Australia, namely the permanently open Swan-Canning and Peel-Harvey estuaries, the seasonally-open Broke and Wilson inlets and the normally-closed Wellstead Estuary. Questions regarding the importance of geographical location and estuary type in structuring habitat types and their fish faunas will also be addressed.

Bottlenose dolphins are valued feature of the Swan Canning river system. Sighting data indicate that dolphins use the rivers year-round and range throughout their extent. The 2009 mortality event emphasised the need to improve our understanding of the connections between dolphins and the estuary ecosystem. We address three elements of an ecosystem-based approach to dolphin conservation: (1) environmental influences on dolphins; (2) interactions between dolphins and other biological components; and (3) dolphins as a means to increase community understanding about river health. To investigate environmental influences, we are investigating: (a) the prevalence and severity of tattoo skin disease lesions on known dolphins from the river system and adjacent waters (e.g. Cockburn Sound); (b) spatial and temporal patterns in lesion occurrence; and (c) associations with salinity and other environmental factors. A preliminary investigation into dolphin foraging ecology in 2009 indicated differences between the stable isotope ratios of dolphins associated with the river system and dolphins associated with two coastal sites, and also supported behavioural observations indicating that dolphins associated with the river system are likely to feed on both marine and estuarine-based prey. Climate change and population growth will continue to alter the Swan Canning river system, suggesting that the long-term retention of dolphins as functioning ecological components of this ecosystem will on-going adaptation and change, both by dolphins and by human managing the environment around them.

Estuaries are highly productive and often urban ecosystems. It is of great interest to managers and ecologists to understand the trophic linkages within a system. In this study, the traditional stomach content analysis approach was used to identify predator-prey relationships. This was complemented with biochemical methods to elucidate the pathway of energy transfer from primary producers into three abundant and important fish species with different feeding regimes, i.e. Acanthopagrus butcheri (omnivore), Leptatherina wallacei (pelagic feeder) and Pseudogobius olorum (benthivore), in a permanently-open (Swan-Canning) and a seasonally-closed (Wilson Inlet) estuary. Three complementary, quantitative approaches are being adopted. 1) Analysis of the dietary items consumed by the fish and variation in diet with body size, habitat and season. (2) Determination of stable isotope ratios of C13/C12 and N15/N14 for fish and their prey. (3) Determination of fatty acid biomarkers of fish and dietary items. Stomach content data indicated that the overall diets differed significantly between species. Size-related changed in dietary composition were evident for each of the three species. Stable isotope ratios of N15/N14 suggested that three trophic levels exist in each system, while the C13/C12 ratio enabled differentiation between food chains based on detrital material or plankton. In the Swan-Canning estuary, fatty acid data revealed trophic markers for dinoflagellates in A. butcheri and L. wallacei, while trophic markers characteristic for diatoms were present in P. olorum. These findings suggest that inter- and intra-specific resource partitioning occurs among the three fish species and between estuaries, and different organic matter sources support both pelagic and benthic food chains. The resultant data can be used in quantitative ecosystem modelling.

This study aims to identify the key primary producers that support abundant fish species with different feeding regimes, i.e. Acanthopagrus butcheri (omnivore), Leptatherina wallacei (pelagic feeder) and Pseudogobius olorum (benthivore), in the Swan-Canning estuary. Although there is extensive knowledge of stomach content composition for many fish species in this highly important urban estuarine system, there is poor understanding of the energy sources and trophic interactions. Three complementary, quantitative approaches were adopted. 1) Analysis of the dietary items consumed by the fish and variation in diet with body size, habitat and season. (2) Determination of stable isotope ratios of C13/C12 and N15/N14 for fish and their prey. (3) Determination of fatty acid biomarkers of fish and dietary items. Stomach content data indicated that the overall diets differed significantly between species. For each of the three species, ontogenetic changes were apparent and seasonal differences were most pronounced in spring. Stable isotope ratios of N15/N14 suggested that three trophic levels exist in the system, while the C13/C12 ratio made a differentiation possible between food chains based on detrital material or plankton. Fatty acid data revealed trophic markers for dinoflagellates in A. butcheri and L. wallacei, while trophic markers characteristic for diatoms were present in P. olorum. These findings suggest that inter- and intraspecific resource partitioning occurs among the three fish species and different organic matter sources support both pelagic and benthic food chains in this estuary. The resultant reliable data can be used in ecosystem modelling.