Interdecadal changes in the community, population and individual levels of the fish fauna of an extensively modified estuary

This study examined inter-period changes over two to three decades in the fish fauna of an urbanized estuary experiencing rapid population growth and a drying climate (Swan-Canning Estuary, Western Australia). Responses were compared at the fish community level (species composition; 1978-2009 in the shallows and 1993-2009 in deeper waters) and at the population and individual levels of an estuarine indicator species, black bream Acanthopagrus butcheri (biomass-abundance and per capita mass at age, respectively; 1993-2009). All three levels showed distinct shifts from earlier to later periods, but their patterns, sensitivity and breadth differed. Community composition changed markedly in the shallows of the lower-middle estuary between the late 1970s and all later periods and moderately between more disparate periods from 1995 to 2009. Several species trends could be linked to the increasing salinity of the estuary or declining dissolved oxygen levels in its middle-upper reaches. Community changes were, however, small or insignificant in the shallow and deeper waters of the upper estuary and deeper waters of the middle estuary, where environmental perturbations are often most pronounced. This may reflect the resilience of the limited suite of species that typify those reaches and thus their lack of sensitivity in reflecting longer-term change at the coarser level of mean abundance. One such species, the selected indicator, A. butcheri, did, however, show marked temporal changes at both the population and individual levels. Biomass decreased markedly in deeper waters while increasing in the shallows from earlier to later periods, presumably reflecting an onshore movement of fish, and per capita body mass in the 2+, 3+ and 4+ year classes fell steadily over time. Such changes probably indicate deteriorating habitat quality in the deeper waters. The study outcomes provide support for a multifaceted approach to the biomonitoring of estuaries using fishes and highlight the need for complementary monitoring of relevant stressors to better disentangle cause-effect pathways

The importance of regional, system-wide and local spatial scales in structuring temperate estuarine fish communities

An extensive literature base worldwide demonstrates how spatial differences in estuarine fish assemblages are related to those in the environment at (bio)regional, estuary-wide or local (within-estuary) scales. Few studies, however, have examined all three scales, and those including more than one have often focused at the level of individual environmental variables rather than scales as a whole. This study has identified those spatial scales of environmental differences, across regional, estuary-wide and local levels, that are most important in structuring ichthyofaunal composition throughout south-western Australian estuaries. It is the first to adopt this approach for temperate microtidal waters. To achieve this, we have employed a novel approach to the BIOENV routine in PRIMER v6 and a modified global BEST test in an alpha version of PRIMER v7. A combination of all three scales best matched the pattern of ichthyofaunal differences across the study area (ρ = 0.59; P = 0.001), with estuary-wide and regional scales accounting for about twice the variability of local scales. A shade plot analysis showed these broader-scale ichthyofaunal differences were driven by a greater diversity of marine and estuarine species in the permanently-open west coast estuaries and higher numbers of several small estuarine species in the periodically-open south coast estuaries. When interaction effects were explored, strong but contrasting influences of local environmental scales were revealed within each region and estuary type. A quantitative decision tree for predicting the fish fauna at any nearshore estuarine site in south-western Australia has also been produced. The estuarine management implications of the above findings are highlighted

Predicting the resilience and recovery of aquatic systems: A framework for model evolution within environmental observatories

Maintaining the health of aquatic systems is an essential component of sustainable catchment management, however, degradation of water quality and aquatic habitat continues to challenge scientists and policy-makers. To support management and restoration efforts aquatic system models are required that are able to capture the often complex trajectories that these systems display in response to multiple stressors. This paper explores the abilities and limitations of current model approaches in meeting this challenge, and outlines a strategy based on integration of flexible model libraries and data from observation networks, within a learning framework, as a means to improve the accuracy and scope of model predictions. The framework is comprised of a data assimilation component that utilizes diverse data streams from sensor networks, and a second component whereby model structural evolution can occur once the model is assessed against theoretically relevant metrics of system function. Given the scale and transdisciplinary nature of the prediction challenge, network science initiatives are identified as a means to develop and integrate diverse model libraries and workflows, and to obtain consensus on diagnostic approaches to model assessment that can guide model adaptation. We outline how such a framework can help us explore the theory of how aquatic systems respond to change by bridging bottom-up and top-down lines of enquiry, and, in doing so, also advance the role of prediction in aquatic ecosystem management

Validation of the Fish Community Index of Estuarine Condition and development of a monitoring regime for the Swan-Canning Riverpark

Fish Community Indices were developed for the nearshore, shallow waters of the Swan-Canning estuary and also for its deeper, offshore waters. They integrate information on various biological variables (metrics), each of which quantifies an aspect of the structure and/or function of estuarine fish communities and responds to a wide array of stressors affecting the ecosystem. Given the well-known responses of these fauna to environmental stressors, these fish-based indices therefore provide a means to assess an important component of the ecology of the system and how it responds to changes in estuarine condition. The present report describes a follow-up study which aimed to validate index sensitivity and robustness and to develop a monitoring regime to enable the condition of the Swan-Canning Estuary to be reliably quantified and reported into the future. The scope of this report was extended in 2012 to include a review of alternative approaches for determining estuarine condition grades/categories

Responses of fish-based, estuarine health indices to algal bloom events of differing severity

We have recently developed fish-based, multimetric indices for assessing and monitoring the health of the nearshore ( 2 m depth) waters of the permanently-open Swan Estuary, Western Australia. Here, we examine index scores calculated from fish community samples collected before, during and after two blooms of the harmful dinoflagellate, Karlodinium veneficum which have occurred in this system in the last decade. The historical (March 2004) and more recent (May 2011) algal blooms differed in their spatial extents, temporal persistence and severity of impact, and thus enabled us to assess the sensitivity of the estuarine health indices to ecological perturbations of differing intensities. The responses of these indices to the algal blooms confirmed that they are sufficiently sensitive to quantify ecological health responses to local-scale environmental perturbations, and to track the subsequent recovery of the system following their removal. In addition, the consistency of nearshore index scores across sampling occasions prior to the May 2011 bloom provides further confirmation that the nearshore health index is consistent and robust (i.e. is not overly sensitive to natural, background variability). Most interestingly, our results elucidate the avoidance movements of certain fish species and the vital role of nearshore habitats in this system as refugia for fish during severe algal bloom events. These indices, which are the first such tools to be developed for Australian estuaries, provide managers with a reliable and cost-effective, quantitative method for assessing and communicating the health of the Swan Estuary, and potentially of other estuaries across south-western Australia

Establishing reference conditions for multimetric health indices: the benefits and challenges of historical data sets

We describe the development and some preliminary results of fish-based, multimetric indices for assessing the ecological health of nearshore (1.5 m) waters of the Swan Estuary, Western Australia. A suite of fish community characteristics (metrics) were first selected via a weight of evidence approach, based on their sensitivity to detect inter-annual change in estuarine condition. Seasonally-adjusted reference conditions for each selected metric were then established for each region of the Swan Estuary using 30 years of historical fish assemblage data, providing a best available standard against which the current and future health of the estuary may be assessed. The nearshore data first required standardisation to minimise the effects of gear-induced biases that were attributable to differences in the characteristics of the seine nets used to sample the nearshore fish fauna of this system since the mid-1970s. A net selectivity study was thus conducted during two seasons in two main regions of the estuary, to compare the compositions of fish samples collected using each of the different seine nets employed historically, i.e. 21.5, 41.5 and 133 m-long seines. The resultant data were subjected to generalised linear modelling techniques to derive net equivalence factors for quantitatively standardising fish abundance estimates, thereby minimising the effects of sampling biases. Scoring thresholds determined statistically from the 5th and 95th percentiles of the standardised, composite nearshore and offshore fish assemblage data sets enabled each metric in each sample to be scored according to the extent of its deviation from an appropriate, ‘best-available’ reference condition. We focus on some interpretations of preliminary trends in the resulting index values, to highlight both the benefits of historical data sets and the challenges they present when constructing reference conditions

Inter-relationships between benthic macroinvertebrates and habitat types in Broke Inlet, south-western Australia. Report to the Ernest Hodgkin Trust for Estuary Education and Research, September 2008

The benthic macroinvertebrate fauna of Broke Inlet, a seasonally-open estuary located in the South coast of Western Australia was sampled seasonally between spring 2007 and autumn 2008 at 20 nearshore sites throughout the estuary. These sites were chosen to represent five of the twelve habitat types that were identified quantitatively on the basis of their differences in a suite of enduring environmental criteria that reflected either location within the estuary, exposure to wave activity or the amount of submerged aquatic vegetation. Sampling yielded 5,519 individuals that represented 25 species. Most of which belonged to the class Polychaeta (10 species and 63.5% of the individuals), followed by those representing the classes Amphipoda, Anthurida and Bivalvia contributed 14.8, 8.2 and 6.8% respectively to the total number of individuals. The number of species, density and Shannon diversity of the benthic macroinvertebrate fauna were found to be significantly influenced by both habitat type (p<0.001) and season (p<0.001), with the greatest values in the majority of habitat types occurring in spring and summer. The composition of benthic macroinvertebrate assemblages were significantly influenced both by habitat type (R=0.418) and season (R=0.304), with the greatest differences among habitat types occurring in spring (R=0.493). Furthermore the pattern of spatial differences among habitat types, as exhibited by the benthic macroinvertebrate fauna, statistically matched that among the suite of enduring environmental variables used to distinguish each of those habitat types in both spring 2007 (Rho=0.441) and summer 2008 (Rho=0.367). Possible reasons for the non significant matching between the benthic macroinvertebrate fauna in autumn 2008 and the enduring environmental variables are also discussed

Developing a multimetric estuarine health index for the Swan Estuary, Western Australia: An approach for selecting fish community metrics in the absence of independent measures of ecological condition

We describe a novel, weight of evidence-based approach for selecting fish community metrics to assess estuarine health. In the absence of reliable measures of estuarine quality against which to test the sensitivity of candidate metrics, objective, multivariate statistical analyses and multimodel inference were employed to select metric subsets likely to be most sensitive to inter-annual changes in ecosystem condition. Novel pre-treatment techniques were first applied to down-weight the influence of highly erratic metrics and to minimise the effects of seasonal and spatial differences in sampling upon metric variability. A weight of evidence approach was then adopted to select those metrics which responded most consistently across multiple analyses of nearshore and offshore fish abundance data collected between 1977 and 2009. Sets of 11 and seven metrics were selected for assessing the nearshore and offshore waters of this system, respectively. Metrics represented species composition and diversity, trophic structure, life history and habitat functions and, in the case of the nearshore index, a potential indicator species. These metric sets are currently being used to construct a multimetric health index for the Swan Estuary, which is, to our knowledge, the first such tool to be developed for assessing the health of estuaries in Australia

The development and validation of an estuarine health index using fish community characteristics

Multimetric indices, incorporating information on a range of fish community attributes, have been shown to provide an effective, simple and sensitive method for evaluating the health or integrity of aquatic environments worldwide. We outline the development of a fish-based, multimetric index of estuarine health for the Swan Estuary, Western Australia, and focus on recent advances in metric selection. An extensive suite of fish community characteristics (metrics), including species composition, diversity and abundance, and measures of trophic structure and life history function, were tested for their suitability for incorporation into an Estuarine Health Index (EHI). Meaningful reference conditions for each selected metric will be established for each region of the Swan Estuary, using historical fish assemblage data (1977-2009). Established reference conditions will thus incorporate natural spatial and temporal variability in fish assemblage characteristics, and together represent a best available standard of biotic integrity against which the current and future health of the estuary may be assessed and compared. Values for the EHI are calculated for each main region of the Swan Estuary for each season and year, from data collected over the period 2007-2009, to establish and monitor trends in current estuarine health, and to validate the sensitivity and reliability of the index. It is envisaged that the index developed will provide managers with a reliable, practical and cost-effective method for assessing and communicating the health of the Swan Estuary

The use of benthic macroinvertebrates to establish a benchmark for evaluating the environmental quality of microtidal, temperate southern hemisphere estuaries

Establishment of a benchmark against which deleterious changes to an estuary can be evaluated requires validating that it has not been subjected to detrimental anthropogenic perturbations and then identifying the biological features which are indicative of a pristine condition and can thus be employed as indicators for detecting and monitoring departures from the natural state. The characteristics of the benthic macroinvertebrate fauna of an essentially pristine, seasonally-open estuary in Western Australia (Broke Inlet) have been determined and compared with those previously recorded for a nearby eutrophic, seasonally-open estuary (Wilson Inlet). Density was far lower in Broke than Wilson. Compositions differed radically at all taxonomic levels, with polychaetes contributing less, and crustaceans more, to the abundance in Broke. Average taxonomic distinctness was greater for Broke than both Wilson and 16 other temperate southern hemisphere estuaries, whereas the reverse was true for variation in taxonomic distinctness, emphasizing that Broke Inlet is pristine