South-eastern Australian Estuaries Research Articles

Strong philopatry in an estuarine-dependent fish.

Understanding fish movement is critical in determining the spatial scales in which to appropriately manage wild populations. Genetic markers provide a natural tagging approach to assess the degree of gene flow and population connectivity across a species distribution. We investigated the genetic structure of black bream Acanthopagrus butcheri across its entire distribution range in Australia, as well as regional scale gene flow across south-eastern Australia by undertaking a comprehensive analysis of the populations in estuaries across the region. We applied genome-wide sampling of single-nucleotide polymorphism (SNP) markers generated from restriction site-associated DNA sequencing. Genetic structure and potential gene flow was assessed using principal component analyses and admixture analyses (STRUCTURE). Using 33,493 SNPs, we detected broad scale genetic structuring, with limited gene flow among regional clusters (i.e. Western Australia, South Australia and western Victoria; and eastern Victoria, Tasmania and New South Wales). This is likely the result of unsuitable habitats, strong ocean currents (e.g. the Leeuwin Current and the East Australian Current), large water bodies (e.g. Bass Strait) and known biogeographical provinces across the continent. Local-scale genetic structuring was also identified across the south-eastern Australian estuaries sampled, reflecting that the coexistence of both migratory and resident individuals within populations (i.e. partial migration), and the movement of fish into coastal waters, still results in strong philopatry across the region. Instances of movement among estuaries at this spatial scale were primarily found between adjacent estuaries and were likely attributed to lone migrants utilising inshore coastal currents for movement beyond nearby habitats. Targeting SNP markers in A. butcheri at this continental scale highlighted how neither spatial proximity of estuaries nor black bream's ability to move into coastal waters reflects increased gene flow. Overall, our findings highlight the importance of location-specific management.

Estuarine movements in a sparid hybrid complex

Context Movements of purebred and hybrid complexes of species show the interactions that facilitate hybridisation and genetic introgression. Aims This study combines genetic analysis of Acanthopagrus spp. and acoustic tracking to understand the spatial ecology of this species complex. Methods Acanthopagrus australis (yellowfin bream) and the Acanthopagrus hybrid complex of A. australis and Acanthopagrus butcheri (black bream) were tracked using acoustic telemetry within a south-eastern Australian estuary. Key results Movements between A. australis and Acanthopagrus hybrids showed similarities, fish displayed high levels of residency and site fidelity, with peak distributions occurring 15 and 32 km upstream of the river entrance. Offshore movements were recorded for 43% of A. australis and 38% of Acanthopagrus spp. hybrids where fish did not return to the study estuary. Estuarine movement patterns in A. australis and A. spp. hybrids were significantly related to conductivity, freshwater flow, temperature, genetic classification, and capture location. Repetitive spawning migrations were not observed for either A. australis or A. spp. hybrids. Overlap in distributions throughout the spawning period did occur. Conclusions This study highlighted the complexity of estuarine movement patterns in A. australis and Acanthopagrus hybrids because they appear to be dependent on freshwater flow, temperature, and ancestry. Implications A. australis and A. spp. hybrids may be capable of spawning within estuaries, and adult offshore movements may play a role in the genetic mixing of populations.

Bioenergetics of blue swimmer crab (

Stock enhancement is a contemporary management method employed to support fisheries productivity. Blue swimmer crab (Portunus armatus) is a widely distributed species that has been identified as a candidate for stock enhancement; however, the release strategy and ecological impact of releases have not yet been assessed. Here, we (1) quantify the bioenergetics of blue swimmer crab, (2) estimate consumption rates, and (3) develop these relationships in a stocking model to assess appropriate release densities and associated trophic impacts. Static respirometry was used to measure the resting metabolic rate (RMR) of blue swimmer crab at three temperatures (19, 24, and 29°C). RMR was found to increase with temperature (Q10 = 2.32), and was 73% higher when crabs were at a premoult or moulting stage. Parameters derived from respirometry experiments were applied to estimate blue swimmer crab stocking density in a south-eastern Australian estuary, by adapting an existing production-based simulation model. The model estimated a median stocking density of ~1.2 crabs per 100 m2. A sensitivity analysis showed that the growth rate was the most influential parameter in this model, showing the importance of this parameter when assessing stocking scenarios.

Environmental Controls on the Geochemistry of a Short-Lived Bivalve in Southeastern Australian Estuaries

Geochemical signals in bivalve carbonate hold the potential to record environmental change over timescales from months to centuries; however, not all bivalves provide reliable proxy records, and modern studies are essential to calibrate these relationships prior to use in palaeo-environmental reconstruction. In this study, 19 shells of the estuarine bivalve Arthritica helmsi, from 14 sites in Southeastern Australia, were obtained from museum collections and analysed for trace elemental (Sr/Ca, Mg/Ca, Sr/Li and Ba/Ca) and stable isotopic ratios (18O/16O and 13C/12C). Mean Sr/Ca and Mg/Ca exhibited significant negative correlations to temperature (R2 = 0.49, p = 0.001; R2 = 0.25, p = 0.02) in agreement with previously published models for trace element partitioning into inorganic aragonite. In addition, the within-shell range of Sr/Ca and Mg/Ca, as measured by laser ablation ICP-MS, correlated to the temperature range (R2 = 0.22, p = 0.03; R2 = 0.46, p = 0.002, respectively). Sr/Li ratios were also negatively correlated to temperature (R2 = 0.34, p = 0.008); however, a significant difference in the model coefficients with previous studies indicates this proxy should be applied with caution. Both oxygen and carbon isotope values exhibited large differences between shells from terrestrial, estuarine and marine waters, suggesting that these stable isotopes hold a potential to record large environmental changes such as sea-level changes or freshening/salinisation in estuarine environments. This study presents the first geochemical study of Arthritica helmsi, highlighting its potential as an environmental tracer.

Lethal and sublethal effects of simultaneous exposure to hypoxia and aluminium on juvenile eastern school prawn

Estuarine waters are heavily influenced by catchment land use and run-off, and acid sulfate soils directly affect both dissolved oxygen (DO) and aluminium concentrations. This can affect harvested species (including benthic crustaceans) and can have concomitant effects on their productivity. In this study we evaluated the effects of hypoxia (DO concentrations <2mgL–1) on eastern school prawn (Metapenaeus macleayi) under enhanced and ambient aluminium concentrations (mean±s.d., 0.112±0.009 and 0.015±0.002mgL–1 respectively). There was a significant increase in survival with increasing DO and LC50 was ~0.9mgL–1, although survival decreased at higher DO concentrations in the presence of enhanced aluminium concentrations. Total fatty acid and amino acid concentrations were significantly greater in prawns subjected to hypoxia, regardless of aluminium exposure. Docosahexaenoic acid, stearic acid, palmitic acid and behenic acid (fatty acids) and leucine, isoleucine, phenylalanine and asparagine (amino acids) contributed to differences in fatty and amino acid profiles among treatments. Hypoxia and aluminium concentrations in nursery habitats of south-eastern Australian estuaries can exceed the conditions evaluated, and could thus affect survival of M. macleayi. More generally, these results have implications for other crustacean species in estuaries that drain coastal flood plains, and the fisheries that rely on them.

Catchment-derived stressors, recruitment, and fisheries productivity in an exploited penaeid shrimp

Many factors can affect growth, survival, reproduction, and fisheries productivity of estuarine species, including structural and physico-chemical habitats, and freshwater inflow to estuaries. Land-based activities can lead to poor catchment condition, and catchment-derived stressors can adversely impact estuarine systems. Using the Eastern School Prawn (Metapenaeus macleayi) in a south-eastern Australian estuary (Camden Haven Estuary) as a case study, we examine juvenile recruitment and fisheries productivity alongside a comprehensive suite of catchment-derived stressors, and interpret patterns in the context of existing studies of lethal and sub-lethal impacts of these stressors on penaeid prawns. Logged dissolved oxygen data indicated a moderate frequency of hypoxia throughout the system, with occasional periods of anoxia. Dissolved aluminium concentrations remained above the relevant marine water quality guideline for the majority of the study period, and concentrations tended to correlate with estuarine inflow. Hypoxia led to depressed prawn abundance, and both hypoxia and high estuary inflow led to decreased somatic condition in prawns. Long-term commercial catch negatively correlated with estuary inflow, which was the opposite of the expected pattern for the species. These patterns highlight the potential cumulative impacts of a complex array of catchment-derived stressors on an important exploited penaeid species. Similar patterns probably occur for prawn species across other floodplain estuaries across south-eastern Australia, and suggest a hitherto unquantified economic impact of degraded catchments through losses in fisheries productivity.

Bloom drivers of the potentially harmful dinoflagellate Prorocentrum minimum (Pavillard) Schiller in a south eastern temperate Australian estuary

Harmful algal blooms are an increasing concern in the estuarine reaches of the Hawkesbury-Nepean River, one of the largest coastal rivers systems in south eastern Australia. In the austral spring of 2016, an unprecedented bloom of the harmful mixotrophic dinoflagellate Prorocentrum minimum occurred in Berowra Creek (maximum cell abundance 1.9E+06 cells L−1, 89% of the total phytoplankton community), a major tributary of this river system. In response to this bloom, our study utilises an estuary-wide, thirteen-year time series of phytoplankton abundance and environmental data to examine the spatial and temporal patterns of this harmful alga and its potential bloom drivers in this system. P. minimum cell densities and environmental parameters varied over large spatial scales, with sites located in the main channel of the estuary significantly differing from those in the more urbanized tributary of Berowra Creek. Generalised additive modelling outputs suggested that blooms of P. minimum are complex, but generally corresponded to a spatial gradient of eutrophication and salinity, whereby P. minimum growth and concomitant high chlorophyll-a concentrations were enhanced at sites that were generally less saline and more eutrophic than others. Furthermore, temporal patterns suggested that blooms occurred abruptly and lasted up to three weeks, most often during the austral autumn to spring. While significant correlations were observed between rainfall and nutrients at all other sites, suggesting a pathway for nutrient availability, the association between rainfall and nutrient delivery was generally not observed in Berowra Creek (a 15-m deep site) suggesting that a continual supply of nutrients, coupled with unique bathymetry and water residence time at this site, are the most likely contributing factors to phytoplankton growth. This study presents the most comprehensive examination of P. minimum in any southern hemisphere estuary to date and highlights the importance of continued monitoring of HABs and the important role that anthropogenic inputs have in driving blooms of P. minimum in this oyster-growing river/estuary system.

Environmentally Driven Changes in Fatty Acid Profiles of a Commercially Important Penaeid Prawn

Rainfall, runoff and estuary inflow link catchment conditions to the physical, chemical and biological properties of estuaries. Commercially exploited penaeid prawn species often rely on estuarine nurseries, and thus, the productivity of penaeid fisheries can be enhanced or diminished by catchment conditions. We conducted an initial investigation into changes in the fatty acid composition of school prawn Metapenaeus macleayi following freshwater inflow, under natural environmental conditions. Juvenile school prawns were collected from two south-eastern Australian estuaries during non-flood (normal) and post-flood conditions, and fatty acid concentrations quantified. Total fatty acid concentrations were significantly higher during post-flood conditions, and constrained ordination showed clear separation of samples collected under normal and post-flood conditions on the basis of their fatty acid composition. Docosahexaenoic acid (DHA), docosapentaenoic acid (DPA) and linoleic acid made a significant contribution to the dissimilarity among these groups. There was a significant negative relationship between salinity and linoleic acid concentrations post-flood. Changes observed in fatty acid profiles likely reflect both changes in the estuarine food web, as well as physiological responses to stress. These changes have implications for prawn condition, growth and survival.

Habitat–fishery linkages in two major south-eastern Australian estuaries show that the C4 saltmarsh plant Sporobolus virginicus is a significant contributor to fisheries productivity

Estuarine fisheries productivity is dependent upon numerous factors, including the productivity of primary producers supporting the food web and the transport of organic matter derived from those primary producers. In this study, we use stable isotope ratios in a Bayesian mixing model to estimate the contribution of primary producers to fully recruited commercial species in two important estuarine commercial fisheries in south-eastern Australia; the Hunter and Clarence estuaries. The C4 saltmarsh plant Sporobolus virginicus had the greatest contribution to consumer diet among almost all sites and times (25–95%), though for prawns the presence of seagrass may be exerting some influence on this calculated contribution in the Clarence estuary. Particulate organic matter (POM; 30%) and fine benthic organic matter (FBOM; 39–41%) also contributed significantly to consumer diet. Mangroves and other C3 sources generally had the lowest contribution to consumers (1–31%). While the exact contributions of each source are uncertain within our Bayesian framework, these results highlight the relatively large role of saltmarsh habitat as a contributor to fishery productivity, especially in estuaries with no seagrasses. Given the anthropogenic threats to saltmarsh habitat, there is potential for loss of fishery productivity with further loss of saltmarsh areal extent.

Geoinformatic analysis of vegetation and climate change on intertidal sedimentary landforms in southeastern Australian estuaries from 1975–2015

Vegetation canopies represent the main ecosystems on intertidal landforms and they clearly respond to changes in coastal environments. Climate change, including temperature, precipitation and sea level rise, are affecting the health and distribution of coastal vegetation, as well as the runoff and sedimentation rates that can impact coastal areas. This study has used the normalized difference vegetation index (NDVI) to investigate vegetation canopy dynamics on three different coastal sites in southeastern Australia over the past 47 years (1975–2015). NDVIs temporal-datasets have been built from satellite images derived from Landsat 1–8. These were then regressed to the climatic and geomorphic variables. Results show clear increases in NDVI at Towamba and Wandandian Estuaries, but a decline at Comerong Island (southeastern Australia). The sedimentation rate has the most significant positive impact on NDVI since it has the potential to provide additional space for vegetation. Temperature and sea level rise have positive effects, except on Comerong Island, but rainfall has no significant effect on the NDVI at any site. Different NDVI trends have been recorded at these three coastal sites reflecting different correlations between the vegetation, climatic and geomorphic (as independent) variables. The geomorphological characteristics of the highly-dynamic intertidal estuarine landforms, which are subject to active erosion and deposition processes, have the largest impact on vegetation cover and, hence, on NDVI. Assessing the vegetation canopy using NDVI as an evaluation tool has provided temporal-dynamic datasets that can be correlated to the main individual environmental controls. Such knowledge will allow resource managers to make more informed decisions for sustainable conservation plans following the evaluation the potential consequences of any environmental changes.

Large wood in the Snowy River estuary, Australia

In this paper we report on 8years of data collection and interpretation of large wood in the Snowy River estuary in southeastern Australia, providing quantitative data on the amount, sources, transport, decay, and geomorphic actions. No prior census data for an estuary is known to the authors despite their environmental and economic importance and the significant differences between a fluvial channel and an estuarine channel. Southeastern Australian estuaries contain a significant quantity of large wood that is derived from many sources, including river flood flows, local bank erosion, and anthropogenic sources. Wind and tide are shown to be as important as river flow in transporting and stranding large wood. Tidal action facilitates trapping of large wood on intertidal bars and shoals; but channels are wider and generally deeper, so log jams are less likely than in rivers. Estuarine large wood contributes to localised scour and accretion and hence to the modification of estuarine habitat, but in the study area it did not have large-scale impacts on the hydraulic gradients nor the geomorphology.

Long-term perspective on the relationship between phytoplankton and nutrient concentrations in a southeastern Australian estuary

Sixteen years (1997–2013) of physicochemical, nutrient and phytoplankton biomass (Chlorophyll-a (Chl-a)) data and a decade (2003-2013) of phytoplankton composition and abundance data were analyzed to assess how the algal community in a temperate southeastern Australian estuary has responded to decreased chronic point source nitrogen loading following effluent treatment upgrade works in 2003. Nitrogen concentrations were significantly lower (P<0.05) following enhanced effluent treatment and Chl-a levels decreased (P<0.05) during the warmer months. Temperature and nutrient concentrations significantly influenced temporal changes of Chl-a (explaining 55% of variability), while salinity, temperature, pH and nutrient concentrations influenced phytoplankton abundance and composition (25% explained). Harmful Algal Bloom (HAB) dynamics differed between sites likely influenced by physical attributes of the estuary. This study demonstrates that enhanced effluent treatment can significantly decrease chronic point source nitrogen loading and that Chl-a concentrations can be lowered during the warmer months when the risk of blooms and HABs is greatest.

Trophic relay and prey switching – A stomach contents and calorimetric investigation of an ambassid fish and their saltmarsh prey

Trophic relay is an ecological model that involves the movement of biomass and energy from vegetation, such as saltmarshes, within estuaries to the open sea via a series of predator-prey relationships. Any potential for trophic relay is therefore affected by water movements within an estuary and by the ability of a predator to “switch” prey in response to fluctuating abundances of those prey. Saltmarsh-dwelling grapsid crabs, which feed on saltmarsh-derived detritus and microphytobenthos, release zoeae into ebbing tides that inundate saltmarshes during spring-tide cycles within tidally-dominated estuaries, such as Brisbane Water Estuary, therefore providing an opportunity to examine whether prey-switching and/or trophic relay may occur in fish that feed on those zoeae (such as the highly abundant estuarine ambassid, Ambassis jacksoniensis). This model was examined by sampling A. jacksoniensis near saltmarshes in a large, temperate south-eastern Australian estuary during flood and ebb tides on days of saltmarsh inundation and non-inundation over four spring-tide events in 2012. Stomach fullnesses of A. jacksoniensis were generally highest during ebb tides on days of saltmarsh inundation, implying that feeding was most marked at these times. Caridean decapods dominated diets during flood tides and on days of no saltmarsh inundation, while crab zoeae dominated diets during ebb tides and on days of inundation, suggesting that, when saltmarsh-derived zoeae became abundant, A. jacksoniensis switched to feeding on those prey. Three potential zooplankton prey (calanoid copepods, caridean decapods and crab zoeae) did not differ calorimetrically, indicating that switching of prey by A. jacksoniensis is not directly related to their preying on energetically greater prey, but reflects opportunistic feeding on more abundant and/or less elusive prey. As A. jacksoniensis is able to switch prey from estuarine caridean decapods to saltmarsh-derived crab zoeae, this very abundant ambassid would be well-placed to promote any trophic relay, via further water movements or other predator-prey relationships, to the adjacent marine environment.

Genetic structure of Carcinus maenas in southeast Australia

The European shore crab Carcinus maenas is a highly successful marine invader, and has displayed rapid range expansion following its introduction to many parts of the world. In Australia, it was first reported in Port Phillip Bay, Victoria in the late 1800s. Despite predictions that it would expand its range northward, its distribution has remained limited to the southeast coast of the mainland and to Tasmania. Using microsatellite loci and mitochondrial DNA, we assessed whether low connectivity among southeastern Australian estuaries might be contribut- ing to the limited distribution of this invasive species. Under this hypothesis, we expected that sampling of C. maenas from 6 estuaries, roughly evenly spaced along the southeast coast, would reveal: (1) greater genetic variability among than within estuaries; (2) increasing genetic dissimi- larity with distance from Port Phillip Bay; and (3) in the absence of human-mediated dispersal, declining genetic variation with distance from Port Phillip Bay. Contrary to these predictions, we found that genetic variability was no greater among than within mainland southeast Australian estuaries — indicating significant gene flow. Some slight genetic differentiation was, however, evident between Tasmania and the mainland. Multiple introductions appear to have contributed to the Australian population. The high connectivity of populations among southeast Australian estuaries suggest that management strategies focused on eradication of C. maenas from individ- ual localities will not be effective. Factors other than limited connectivity appear to be responsible for the slow range expansion up the east Australian coast.

Sedimentation, elevation and marsh evolution in a southeastern Australian estuary during changing climatic conditions

Mangrove and salt marsh vertical accretion and surface elevation change was measured at Kooragang Island within the Ramsar-listed Lower Hunter estuarine wetlands in New South Wales, Australia, using surface elevation tables and marker horizons over a ten-year period. We surveyed mangrove, salt marsh and a zone of mangrove encroachment into salt marsh. The period of analysis was dominated by El Niño (drought) climatic conditions, though included a series of east coast low pressure systems and associated storms over the central coast of NSW in June 2007. The storms may have initially caused scouring of sediments in the mangrove zone, followed by significant accretion within both the mangrove and salt marsh during the six months following the storms, with most of this accretion corresponding to spring tides several months after the storms. These accretion events were not accompanied by an equivalent elevation change, and robust elevation trends over the study period in mangrove and salt marsh indicate that the storms may have had little impact on the longer-term elevation dynamics within both the mangrove and salt marsh at Kooragang Island. Elevation dynamics in these zones appear to be regulated by vertical accretion over longer time periods and modulated by hydrology at shorter temporal scales. Elevation declined in the mangrove encroachment zone despite continued vertical accretion and we propose that this discrepancy may be associated with expansion of tidal creeks near the zone of mangrove encroachment or loss of salt marsh vegetation. This pattern of encroachment is consistent with observations from sites throughout the region and may be related to climatic perturbations (El Niño Southern Oscillation) rather than directly attributed to the storms.

Post-release mortality of angled sand mullet ( Myxus elongatus: Mugilidae)

Grey mullets (Mugilidae) are important to recreational fisheries throughout the developed world. In Australia, several species are angled and then released in large numbers; all with virtually unknown fate. In response to the need for such data to facilitate effective stock management, this study sought to quantify the post-release mortality and key causal factors for sand mullet ( Myxus elongatus). A total of 125 fish were conventionally angled, and then released along with 50 controls into floating cages in a south eastern Australian estuary, where they were monitored for four days. Five treatment fish died, providing a non-significant mortality of 4%. The few fatalities were mostly explained by a significant positive relationship with the length of trace (between the float and hook) used, and bleeding during release. While sand mullet appear quite tolerant of catch and release, their welfare nevertheless could be improved through simple changes to fishing strategies.

Mitigating discard mortality from dusky flathead Platycephalus fuscus gillnets

The mortalities and contributing parameters were estimated for key species discarded during commercial gillnetting (80 mm mesh) targeting dusky flathead Platycephalus fuscus in a southeastern Australian estuary. Bycatches (1470 individuals from 16 species over 11 deployments) were assessed for their immediate mortalities onboard the gillnetter, before subsamples (570 individuals from 11 species) were discarded into cages and monitored for their short-term fate over 4 d. Appropriate controls were concurrently caged and monitored. Blood samples were taken from some live meshed-and-discarded yellowfin bream Acanthopagrus australis and luderick Girella tricuspidata and analysed for plasma cortisol and glucose. Concomitantly angled fish were similarly sampled (to provide baseline estimates of blood physiology). The immediate mortalities of the abundant species ranged between 0 (undersize blue swimmer crab Portunus pelagicus <6 cm carapace length) and 70% (undersize P. fuscus <36 cm total length [TL]). Water temperature had a statistically significant positive relationship with the immediate mortality of G. tricuspidata and large-tooth flounder Pseudorhombus arsius, and TL had a significant negative relationship with the immediate mortality of black sole Synaptura nigra. Compared to baseline estimates, mean plasma cortsiol concentrations in meshed-and-discarded G. tricuspidata and A. australis were significantly greater, and approached levels comparable to most teleosts after peak stress. Mean glucose concentrations were not concomitantly elevated, possibly reflecting limited time between stress and sampling for some individuals. Short-term mortalities occurred throughout the entire 4 d monitoring period for most species and ranged from 0 (yellowfin leatherjacket Meuschenia trachylepis) to 29% (A. australis). Water temperature and TL were identified as having significant impacts similar to those described above on the delayed fate of A. australis and G. tricuspidata. The partitioned mortalities were combined to provide estimates of overall mortality (+/- SE) for the main species that ranged between 5.9 +/- 3.3% (P. pelagicus) and 76.9 +/- 7.8% (undersize P. fuscus). Discard mortality in this fishery could be mitigated by allowing fishers to retain a small percentage of undersize P. fuscus, restricting the deployment of nets in water temperatures >16 to 17 degrees C, and encouraging the careful removal of catches from meshes.

Importance of saltmarsh to fish species of a large south-eastern Australian estuary during a spring tide cycle

The dietary compositions of fish species over saltmarsh in a large south-eastern Australian estuary (Brisbane Water) were explored to ascertain the importance of this habitat type to the fish community of that estuary. Following tidal inundation, 12 fish species (&lt;120 mm total length) were obtained using fyke nets, with Ambassis jacksoniensis being particularly abundant. The stomachs of all fish contained undigested prey, implying that they fed while on the saltmarsh. Three species (A. jacksoniensis, Atherinosoma microstoma and Redigobius macrostoma) fed nearly exclusively on the crab zoeae that were released in high numbers by saltmarsh crabs. The diets of these three species significantly differed from those of the other species, which comprised mainly foraminiferans, copepods, crabs, polychaetes, terrestrial insects and/or detritus. Substantial partitioning of food resources also occurred between the other nine species. With increasing body size, A. jacksoniensis fed more on benthic prey (polychaetes and copepods) and less on crab zoeae. This study highlights the importance of saltmarsh as providing both protection and food resources for fish species during even short periods of tidal inundation by its provision of a superabundant food source (crab zoeae) and other prey, and underscores the need for recognition of saltmarsh in management plans for fish.

Sampling estuarine fish and invertebrates using demersal otter trawls: Effects of net height, tow duration and diel period

Trawl surveys are done in many parts of the world but few studies have used pilot experiments to examine the effects of gear design and sampling practices on retained samples. We did two separate experiments in two south-eastern Australian estuaries to test the hypotheses that samples of fish and invertebrates taken using a demersal otter trawl would be affected by: (i) vertical net height (standard 0.8 m vs. high-rise 1.2 m) and tow duration (5 vs. 10 vs. 20 min); and (ii) diel period (day vs. night) and tow duration. Mean catch-per-unit-effort (standardised to numbers of individuals caught 5 min −1) was significantly larger in the high-rise net for some variables, but in most cases, CPUE was correlated proportionately between the two heights of net. There were no differences in the structure and composition of assemblages between net heights or among tow durations. The size-frequencies of fish differed between net heights and among tow durations, but there were no clear patterns in the proportions of fish caught across different size classes. Several species were caught mostly, or in larger numbers, at night and contributed to differences in assemblages of fish and invertebrates between diel periods. We stress the importance of using pilot studies to develop more reliable and cost-effective surveys and conclude that future sampling with our trawl should be done at night using the standard net and short (5 min) tows.

Metals in fish and shellfish from Lake Illawarra, New South Wales, Australia

Fish (dusky flathead – Platycephalalus fuscus and luderick – Girella tricuspidata) and shellfish (Anadara trapezia and Saccostrea commercialis) samples from Lake Illawarra in south-eastern Australia were collected in 1993 and 2000 and analysed for a range of metals. Sediments with which the shellfish were in contact were also examined. The results were compared with shellfish data collected in 1976 and in an independent study in 1993. No reported data were available for comparison with the fish metal results. The results showed that trace metal concentrations in both fish and shellfish were generally low and represented minimal health risk. The dusky flathead, a predator species, had detectable concentrations of mercury, while the luderick, a herbivore, had values below detection limits. Metal concentrations in shellfish were in general agreement with those found in other studies, and showed that no apparent changes were occurring over time, except those that could be explained by changes in the age of the shellfish. Only copper in Lake Illawarra oysters showed greater concentrations than found in other south-eastern Australian estuaries. It is recommended that future studies focus on organic and microbial contamination.