Spencer Gulf Research Articles

Evolutionary history of birds across southern Australia: structure, history and taxonomic implications of mitochondrial DNA diversity in an ecologically diverse suite of species

Recent phylogeographic studies have examined the location and timing of putative Plio-Pleistocene bio-geographical barriers in moulding present-day patterns of diversity in southern Australian vertebrates. We previously investigated the divergence history of an assemblage of southern Australian birds. Here we more explicitly incorporate idiosyncratic effects of dietary ecology and habitat on the long-term demographic stability and the presence or absence of genetic structure on each of 12 species of bird. Our data suggest that the Eyrean Barrier (Flinders Ranges—Lake Eyre Basin) has been critical in shaping present-day diversity, especially that of mesic and mallee environments, and that other barriers (e.g. Murchison Barrier, Nullarbor Barrier) have played a lesser role. In contrast, nectarivory and occurrence in patchily distributed habitats are correlated with weak or no phylogeographic structure. Population expansions were most prevalent in western parts of species’ ranges. Substantial genetic divergences accompanied by moderate or no phenotypic divergence challenge traditional approaches to taxonomy in the Blue Bonnet (Northiella haematogaster), Chestnut Quail-thrush (Cinclosoma castanotum), White-eared Honeyeater (Nesoptilotis leucotis) and Scarlet Robin (Petroica boodang) and we suggest taxonomic treatments for each of these species that accommodate existing data. We elevate some taxa to species rank and note where further analyses of gene flow will be useful in clarifying remaining issues. Further phylogeographic study in the region of the Mount Lofty Ranges, Flinders Ranges, Spencer Gulf and Gulf St Vincent) of South Australia is warranted.

Prokaryotic and diazotrophic population dynamics within a large oligotrophic inverse estuary

The ecology of microbial assemblages inhabiting classical (positive) estuaries has been well documented. However, we know relatively little about the microbial ecology of inverse (negative) estuaries, which exhibit different physical and hydrodynamic properties, including oligotrophy and hypersalinity. We investigated the dynamics of bacterioplankton communities in Spencer Gulf, an inverse estuary in temperate South Australia. We characterised patterns in the overall diversity and composition of the resident microbial assemblage, and tested the hypothesis that pelagic nitrogen-fixing bacteria (diazotrophs) could be an important functional group in the nutrient limited waters of the region. Prokaryotic and diazotrophic communities were evaluated using 16S ribosomal DNA and nifH amplicon tag pyrosequencing, respectively. Significant heterogeneity in microbial community composition and diazotrophic population structure was observed, which was driven by shifts in the relative importance of temperate vs. subtropical and oceanic vs. coastal ecotypes of Cyanobacteria throughout the inverse estuary. The globally significant unicellular cyanobacterium, UCYN-A 'Candidatus Atelocyano-bacterium thalassa', was the dominant diazotrophic phylotype. Temperature, chlorophyll a and nitrogen availability were all significant drivers of bacterioplankton dynamics within the gulf. These results demonstrate the heterogeneous microbiology of inverse estuaries, indicating that specific abiotic and biotic characteristics select for discrete microbial communities, and that pelagic nitrogen fixation may be important in this temperate oligotrophic system.

Evidence for a broad-scale decline in giant Australian cuttlefish (Sepia apama) abundance from non-targeted survey data

Little is known about the population trajectory and dynamics of many marine invertebrates because of a lack of robust observational data. The giant Australian cuttlefish (Sepia apama) is IUCN-listed as Near Threatened because the largest known breeding aggregation of this species in northern Spencer Gulf, South Australia, has declined markedly since the turn of the century. We used by-catch records from long-term trawl surveys to derive abundance data for S. apama and commercial cuttlefish harvest data as a measure of exploitation. Using Bayesian hierarchical models to account for zero-inflation and spatial dependence in these abundance counts, we demonstrated a high probability of broad-scale declines in the density of S. apama, particularly surrounding the primary aggregation site, which supports the recent closure of the entire S. apama fishery in northern Spencer Gulf. Historical harvest data were positively correlated with S. apama density estimated from the trawl surveys, suggesting that the commercial cuttlefish catch tracks the species abundance. Our results also indicated the possibility that the known S. apama breeding grounds might be supplemented by individuals that were spawned elsewhere in northern Spencer Gulf.

Mitigating the bycatch of giant cuttlefish Sepia apama and blue swimmer crabs Portunus armatus in an Australian penaeid-trawl fishery

Trawlers in Spencer Gulf, Australia, target western king prawns Melicertus latisulcatus, with nearly all other species discarded; two of the latter have evoked significant, although quite different, concerns. The first is the giant cuttlefish Sepia apama, which has undergone a drastic decline in numbers in Spencer Gulf in recent years, and is listed as 'Near Threatened' by the International Union for the Conservation of Nature. S. apama are susceptible to trawling during their annual spawning migration through Spencer Gulf. The other bycatch problem involves large quantities of blue swimmer crabs Portunus armatus, which currently are separated inside the trawl using a large-meshed liner. Due to the additional handling required and the physical damage caused by the crabs' exoskeletons on the soft-bodied M. latisulcatus and Sepia spp., P. armatus would ideally be able to escape of their own accord during towing. This study examined the utility of mechanical-separating bycatch reduction devices (BRDs) for reducing unwanted bycatches of Sepia spp. and P. armatus. We compared (against conventional codends) the smallest and largest practical sizes of Nordmore-grids (the latter to maximise sorting area), with correspondingly large and low grid angles. The large Nordmore-grid significantly reduced Sepia spp., P. armatus and total bycatch (by 33-50%), but had no effect on catches of M. latisulcatus. Whilst additional research is required, a modified Nordmore-grid should help to resolve the bycatch of P. armatus and S. apama in this fishery, with minimal commercial impacts.

Spatial variation in shape of otoliths for southern garfish Hyporhamphus melanochir – Contribution to stock structure

Otolith morphometrics and shape analyses were used to delineate the sub-population structure and determine the level of mixing of southern garfish in South Australian waters. Adult garfish from the 2+ age class were collected from three sites within each of six regions along the South Australian coast: Northern Gulf St Vincent, Southwest Gulf St Vincent, Kangaroo Island, Northern Spencer Gulf, Southern Spencer Gulf and the West Coast of Eyre Peninsula. This study adopted a multi-stage statistical approach exploring the spatial variation in gross otolith morphology and through elliptical Fourier analysis separately, and in combination, over both broad (regional) and fine (site) spatial scales. Stepwise discriminant function analysis revealed significant separation across both spatial scales; however, overall classification success of garfish to their site of capture was poor (<45%). Despite this, the results indicated some level of structuring within South Australia's southern garfish population, as fish were correctly classified at rates that were 32% better than would be expected by chance. At least six putative regional components with various levels of intermixing were detected based on differences in otolith shape. This result refines the spatial resolution of the fishery and consequently highlights the vulnerability of these regional components to localized depletion, particularly as the resource is currently considered to be over-exploited.

Carrying capacity for finfish aquaculture, Part II – Rapid assessment using hydrodynamic and semi-analytic solutions

The carrying capacity for aquaculture cage farming in Spencer Gulf (South Australia) is based in part on guidelines that the maximum feed rates and nutrient flux into the lease regions are determined such that the maximum nutrient concentration c does not exceed a prescribed value (say cP) to ensure ecosystem health – ecological carrying capacity. The goal of this study is to allow the rapid estimation of maximum nutrient fluxes and feed rates at new lease sites. Spencer Gulf is chosen as a case study although the methodology should find application in other regions around the world. In part I of this study, semi-analytic solutions were obtained to show that to a good approximation the maximum nutrient flux (feed rate) F can be simply estimated from: F=cP/T* where T* is a flushing time scale of the cage or lease region. In this study a 3-dimensional hydrodynamic model for Spencer Gulf is used to determine the parameters needed to estimate T* and thus F and feed rates at every model cell in the gulf. The parameters needed include the vector mean speed (U), r.m.s. tidal amplitude (UK) and the mean shear dispersion diffusivity (KS). As a case study, these parameters and T*, are estimated by three-monthly, winter averages. Results show the vector mean speed to be very small (U∼0.01m/s), tidal velocities large (UK∼0.3–1m/s) and the associated shear dispersion coefficients very large (KS∼10–100m2/s). Flushing at the scale of the lease (600m) and in the upper gulf is generally dominated by diffusive affects for which the maximum nutrient flux (and feed rates) is largest. The results should find application in other finite source flux problems in the coastal oceans including desalination plants and ocean outfalls.

Virally-Mediated Versus Grazer-Induced Mortality Rates in a Warm-Temperate Inverse Estuary (Spencer Gulf, South Australia)

We investigated the seasonal dynamics of flow cytometrically-defined populations of viruses, heterotrophic bacteria, and the picoeukaryotic and prokaryotic phytoplankton at three sites in the temperate oligotrophic inverse estuary of Spencer Gulf (South Australia). We consistently identified two sub-populations of viruses, three sub-populations of heterotrophic bacteria, one population of picoeukaryotic phytoplankton and two populations of prokaryotic phytoplankton (cyanobacteria Prochlorococcus and Synechococcus). Both the cytometric community composition and the abundance of viruses, heterotrophic bacteria and both prokaryotic (Synechococcus and Prochlorococcus) and eukaryotic picophytoplankton were consistent with previous observations conducted in South Australian continental shelf waters. Noticeably LDNA bacteria (i.e. inactive or dormant cells) were consistently significantly the most abundant group of heterotrophic bacteria (totaling from 29% to 68% of total bacterial abundance) and were up to 10-fold more abundant than that previously reported in South Australian continental shelf waters, including the nearby Saint Vincent Gulf. These results suggest an overall low activity of the microbial community, and are consistent with previous evidence that LDNA cells may play a greater role in heterotrophic processes than HDNA cells in oligotrophic waters. In an attempt to further assess the qualitative and quantitative nature of the mortality of these organisms, we used a specific dilution assay to assess the relative contribution of viruses and microzooplankton grazers to the mortality rates of heterotrophic bacteria, and picoeukaryotic and prokaryotic phytoplankton. We consistently reported site-specific, population specific and sea-son-specific viral lysis and grazing rates of heterotrophic bacteria and the picoeukaryotic and prokaryotic (cyanobacteria Prochlorococcus and Synechococcus) phytoplankton across sites and seasons. Specifically, both viral lysis and micro-zooplankton grazing rates of heterotrophic bacteria were consistently relatively low across sites and seasons, even though their seasonality suggested an overall dominance of grazing over viral lysis in both summer and winter. In contrast, no seasonality is found in either lysis or grazing rates of prokaryotic and eukaryotic picophytoplankton, which are comparable to previous observations conducted in oligotrophic waters, suggesting the mortality dynamics of these populations is similar to those encountered in other oligotrophic waters. The observed patterns of mortality rates of heterotrophic bacteria and both prokaryotic and eukaryotic picophytoplankton are consistent with the low chlorophyll concentration and production previously observed in the waters of the Spencer Gulf.

Response of Trophic Groups of Macrobenthos to Organically Enriched Sediments: A Comparative Study between Temperate and Tropical Regions

This study is aimed to detect the level of environmental disturbance caused by farming activities using trophic groups of macrobenthic assemblages, with emphasizes the difference between temperate and tropical regions. The samples of macrobenthic assemblages were taken from 2 (two) farm sites, i.e. Southern Spencer Gulf, South Australia, where farming of southern bluefin tuna ( Thunnus maccoyii ) takes place, representing as a temperate region (Site I) and milk fish ( Chanos chanos ) farms and mangrove area at coastal region of Mngkang Kulon, Semarang, Central Java, Indonesia, representing as a tropical region (Site II). Macrobenthic abundance was categorised based on six major trophic groups. Macrobenthic assemblages dominated by all polychaetes, they are Capitellidae, Cirratullidae, Lumbrineridae, Nephtyidae, and Spionidae at Site I (temperate region), owing to their relatively tolerance to organic enrichment, both at the farm sites and at control sites. Surface deposit feeders (SDF) dominated in abundance at both control and fish farm sites at Site I, whilst subsurface deposit feeders (SSDF) exhibited highest in proportion at Site II over the study period, in particular species of Capittelidae, indicating that both sites has been influenced by fish farm activities. The abundance of deposit feeders was significantly higher at the farm sites than at the reference sites. Organic carbon in the sediment surface is likely to correlate with deposit feeding and sub-surface deposit feeding species richness, whereas total and food particulate matter correlate with species diversity. The result of this comparative study implies that response of trophic groups of macrobenthos to environmental disturbance are considerably similar at both tropical and temperate regions.

Megabenthic biodiversity in two contrasting submarine canyons on Australia's southern continental margin

Submarine canyons are known to play an important role in enhancing productivity in adjacent surface waters, but their influence on the seabed fauna is poorly understood. Here we examine environmental conditions in two canyons with contrasting topographies, and evaluate their relative importance in structuring benthic megafaunal assemblages. Samples of benthic megafauna, sediment and water were collected from five sites at du Couedic Canyon (100–1500 m) and six sites at Bonney Canyon (100–2000 m) off South Australia. Sessile suspension-feeding organisms (primarily sponges, molluscs and cnidarians) dominated samples and comprised 97% of the biomass and 77% of the total species richness (184 spp.). Megabenthic biomass and species richness broadly declined with increasing depth in both canyons, but was markedly higher at du Couedic Canyon in an area subject to organically enriched seasonal outflows from Spencer Gulf. Multivariate analyses showed a strong environmental gradient running perpendicular to the coast in both canyons, and highlighted a progressive shift in species composition. Three station groupings were identified on the shelf (100–200 m), upper slope (500 m) and mid slope (1000–2000 m). These community groupings were largely explained by depth and dissolved oxygen concentration and coincided with depth-related discontinuities in water masses on Australia's southern continental margin.

Five new species of dicyemid mesozoans (Dicyemida: Dicyemidae) from two Australian cuttlefish species, with comments on dicyemid fauna composition

Five new species of dicyemid mesozoans in two genera are described from two Australian cuttlefish species, Sepia apama Gray (giant Australian cuttlefish) and S. novaehollandiae Hoyle (nova cuttlefish): Dicyema coffinense n. sp. from S. apama collected from Coffin Bay, South Australia (SA), Australia; D. koinonum n. sp. from S. apama and S. novaehollandiae collected from Gulf St Vincent (GSV) and Spencer Gulf (SG), SA, Australia; D. multimegalum n. sp. from S. apama collected from Cronulla and North Bondi, New South Wales, Australia; D. vincentense n. sp. from S. novaehollandiae collected from GSV, SA, Australia; and Dicyemennea spencerense n. sp. from S. novaehollandiae and S. apama collected from SG, SA, Australia. Totals of 51 S. apama and 27 S. novaehollandiae individuals were examined, of which all except for four S. apama were infected by at least one dicyemid species. Dicyemid parasites were also observed in host individuals that were held in tanks for 2-3 months prior to examination, including nematogen-exclusive infections, leading to questions about persistence of dicyemids after host death and the mechanism responsible for the switch between a nematogen phase and a rhombogen phase. Variations in host size, calotte shape and collection locality are explored as predictors of differences in observed composition of the parasite fauna. In particular, dicyemid parasite fauna varied with host collection locality. As these parasites are highly host-species specific, their use as biological tags to assess cephalopod population structure using a combined morphological and molecular approach is discussed. This study increases the number of dicyemid species described from Australian cephalopods from five to ten, and from 117 to 122 species described worldwide.

First descriptions of dicyemid mesozoans (Dicyemida: Dicyemidae) from Australian octopus (Octopodidae) and cuttlefish (Sepiidae), including a new record of Dicyemennea in Australian waters

Three new species of dicyemid mesozoans are described for the first time from Australian octopus and cuttlefish species. Dicyemennea floscephalum sp. n. is described from Octopus berrima Stranks et Norman (southern keeled octopus) collected from Spencer Gulf and Gulf St. Vincent, South Australia, Australia and represents the first description of a species of Dicyemennea Whitman, 1883 from Australian waters. Dicyema papuceum sp. n. and D. furuyi sp. n. are described from Sepia papuensis Hoyle (Papuan cuttlefish) collected from Shark Bay, Western Australia, Australia. Dicyemennea floscephalum sp. n. is a medium to large species that reaches approximately 4.9 mm in length. The vermiform stages are characterised by having 23-28 peripheral cells, and a disc-shaped, flower-like calotte in larger individuals. An anterior abortive axial cell is absent in vermiform embryos and verruciform cells were not observed in nematogens and rhombogens. Infusoriform embryos comprise 37 cells; one nucleus is present in each urn cell. Dicyema papuceum sp. n. is a small species that reaches approximately 1.1 mm in length. The vermiform stages are characterised by having 30-33 peripheral cells and a relatively small, cap-shaped calotte. An anterior abortive axial cell is absent in vermiform embryos and verruciform cells were occasionally observed in nematogens. Infusoriform embryos comprise 37 cells; two nuclei are present in each urn cell. Dicyema furuyi sp. n. is a large species that reaches approximately 5.3 mm in length. The vermiform stages are characterised by having 22-24 peripheral cells and an elongate calotte. An anterior abortive axial cell is absent in vermiform embryos and verruciform cells were not observed in nematogens and rhombogens. Infusoriform embryos comprise 37 cells; one nucleus is present in each urn cell. Three secondary nematogens were also observed in the right renal appendages of two host individuals, confirming the occurrence of this form.

Linking near- and far-field hydrodynamic models for simulation of desalination plant brine discharges

A desalination plant is proposed to be the major water supply to the Olympic Dam Expansion Mining project. Located in the Upper Spencer Gulf, South Australia, the site was chosen due to the existence of strong currents and their likely advantages in terms of mixing and dilution of discharged return water. A high-resolution hydrodynamic model (Estuary, Lake and Coastal Ocean Model, ELCOM) was constructed and, through a rigorous review process, was shown to reproduce the intricate details of the Spencer Gulf dynamics, including those characterising the discharge site. Notwithstanding this, it was found that deploying typically adopted 'direct insertion' techniques to simulate the brine discharge within the hydrodynamic model was problematic. Specifically, it was found that in this study the direct insertion technique delivered highly conservative brine dilution predictions in and around the proposed site, and that these were grid and time-step dependent. To improve the predictive capability, a strategy to link validated computational fluid dynamics (CFD) predictions to hydrodynamic simulations was devised. In this strategy, environmental conditions from ELCOM were used to produce boundary conditions for execution of a suite of CFD simulations. In turn, the CFD simulations provided the brine dilutions and flow rates to be applied in ELCOM. In order to conserve mass in a system-wide sense, artificial salt sinks were introduced to the ELCOM model such that salt quantities were conserved. As a result of this process, ELCOM predictions were naturally very similar to CFD predictions near the diffuser, whilst at the same time they produced an area of influence (further afield) comparable to direct insertion methods. It was concluded that the linkage of the models, in comparison to direct insertion methods, constituted a more realistic and defensible alternative to predict the far-field dispersion of outfall discharges, particularly with regards to the estimation of brine dilution in the immediate vicinity of an outfall location.

The effects of temperature on larval size in the western king prawn, Penaeus (Melicertus) latisulcatus Kishinouye, from Spencer Gulf, South Australia: implications for fishery management

Temperature, salinity and food availability are generally considered to have the most influence on larval growth and survival in penaeid species. Larval size has previously been used as a measure of animal quality in a range of crustacean species. The aim of this project was to investigate the effects of temperature (17°C, 20°C, 22.5°C and 25°C) on stage-specific larval size in Penaeus latisulcatus collected from the Spencer Gulf, in South Australia. Five different measurements of larval size were assessed: body length, body width, antenna length, carapace length and abdomen length. Results showed that larval growth rate was dependant on temperature, with growth rate greater at higher temperatures. Despite this, larvae reared at 20°C reached the largest size, while those at the temperature extremes (17°C and 25°C) were smallest. When considering average annual temperature, the spawning time that would most likely maximise larval size in the Spencer Gulf is approximately December. This coincides with current management arrangements for the fishery that protect the spawning biomass at this time of year.

Broad‐scale movements and pelagic habitat of the dusky shark Carcharhinus obscurus off Southern Australia determined using pop‐up satellite archival tags

AbstractWe investigated the broad‐scale movements and pelagic habitats of large juvenile dusky sharks (Carcharhinus obscurus) off Southern Australia. Pop‐up satellite archival tags (PSATs) were deployed on three large juvenile dusky sharks (∼2.2–2.6 m total length) for 6 months in Spencer Gulf during 2010. Tagged dusky sharks all migrated westward and across the Great Australian Bight (GAB) during autumn to offshore shelf waters off Western Australia. Estimated minimum distances travelled ranged from 1760 to 2736 km. Depths occupied by tagged dusky sharks ranged from the surface to 355 m. The most common thermal ranges occupied were 19–22°C. Broad‐scale movements of large juvenile dusky sharks across the continental shelves combined with periods of residency in semi‐protected gulf waters indicated that a multi‐jurisdictional management approach may be appropriate for this species.

The spawning dynamics of snapper (Chrysophrys auratus) in northern Spencer Gulf, South Australia

This study investigated the inter- and intra-annual variation in the spawning dynamics of snapper (Chrysophrys auratus). Reproductive samples from northern Spencer Gulf, South Australia were collected over three seasons (2005/06, 2006/07 and 2007/08) and were analysed macro and microscopically. Spawning activity was determined by calculating estimates of spawning fraction and batch fecundities. The onset of spawning occurred in November but varied between years and corresponded with times when water temperature was between 18 °C and 20 °C. The length of the spawning season also differed between years. In each year the peak spawning activity occurred during December when fish spawned almost daily. Spawning frequency and relative batch size did not differ between the first two spawning seasons but, in the third season, batch size was considerably greater and spawning fractionally lower. These results are contrasted with a known recruitment of 0+ over the three years of the study.

Two New Species of Dicyemid (Dicyemida: Dicyemidae) from Two Australian Cephalopod Species:Sepioteuthis australis(Mollusca: Cephalopoda: Loliginidae) andSepioloidea lineolata(Mollusca: Cephalopoda: Sepiadariidae)

Two new species of dicyemid parasites from Dicyema are described from 2 species of Australian cephalopods, i.e., Dicyema calamaroceum n. sp. from Sepioteuthis australis Quoy and Gaimard, 1832 (southern calamary) collected from Spencer Gulf (SG) and Gulf St Vincent (GSV), South Australia (SA), Australia, and Dicyema pyjamaceum n. sp. from Sepioloidea lineolata Quoy and Gaimard, 1832 (striped pyjama squid), collected from SG, SA, Australia. Dicyema calamaroceum is a medium sized species that reaches approximately 2,400 μm in length. The vermiform stages are characterized by having 31-34 peripheral cells, a conical calotte, and an axial cell that extends to the propolar cells. An anterior abortive axial cell is absent in vermiform embryos, and verruciform cells were not observed in nematogens and rhombogens. Infusoriform embryos consist of 39 cells; 2 nuclei are present in each urn cell, and the refringent bodies are solid. Dicyema pyjamaceum is smaller than D. calamaroceum, with a body length that reaches approximately 1,950 μm. The vermiform stages are characterized by having 20-23 peripheral cells, a cap-shaped calotte that forms a cephalic swelling together with the parapolar cells, and an axial cell that extends to the propolar cells. An anterior abortive axial cell is absent in vermiform embryos. Verruciform cells and granules in propolar cells were observed in nematogens and rhombogens. Infusoriform embryos consist of 37 cells; 2 nuclei are present in each urn cell, and the refringent bodies are solid. This represents the first description of dicyemid parasites from Australia.

Seawater desalination: an environmental regulator’s perspective

abstract The Port Stanvac seawater reverse osmosis desalination plant will be capable of producing 300,000 m3 of potable water per day when fully complete. This is the first large-scale desalination project in South Australia. A second desalination plant has recently received development approval for the Upper Spencer Gulf in South Australia. The Port Stanvac plant was initiated by the South Australian Government in response to a wide spread and prolonged drought in Australia. The plant was planned and built to drought proof Adelaide, a city of over one million people. The timescale for the plant from the first proposal to site selection, design and build was compressed due to the urgency of the situation. There were significant environmental concerns in the construction and operation of a desalination plant in the location chosen. These included protection of cliffs and high-value intertidal reefs during construction and energy use and the protection of the marine environment during the operating life of the plant. The environment protection authority was charged with regulating the environmental effects of the construction and ongoing operation of the desalination plant. This was the first such plant ever constructed in South Australia, so there were a lot of new concepts to understand to ensure the highest level of protection could be obtained. A lot of effort was made to ensure that the plant could operate in an environmentally sustainable manner in a sensitive location. Ensuring that this could be demonstrated to the public was an important factor in the way the plant was regulated. The plant is now operational and the monitoring system is in place. As the plant ramps up from producing 30,000 m3 of potable water to 300,000 m3 per day, the ongoing challenges of monitoring the discharge has been complex.

The use of food resources by 0+ snapper, Chrysophrys auratus, from northern Spencer Gulf, South Australia

Food availability is an important factor in survival and growth of juvenile fish and has been proposed as a major factor shaping the patterns of distribution and abundance of 0+ snapper, Chrysophrys auratus, in New Zealand and Japan. Thus, we tested the hypothesis that diet of 0+ snapper would exhibit spatial variation related to their abundance within a major nursery region. Therefore, we described the stomach contents of 0+ snapper collected from within a nursery region in areas of low and high abundance of 0+ snapper. The data also provided the opportunity to consider sized based differences in diet and temporal feeding patterns. 0+ snapper were generalist carnivores that preyed on a wide range of invertebrates and fish. No evidence of sized based dietary variation was identified but feeding either ceased or was much reduced during the hours of darkness. The major finding was that more polychaetes were present in the stomachs of 0+ snapper in the area of high 0+ snapper abundance than in the areas of low 0+ snapper abundance. The spatial differences in diet observed supports the hypothesis that prey availability is important as a driver of habitat selection by 0+ fish.

The salinity response of an inverse estuary to climate change &amp; desalination

Abstract Just as runoff primarily governs the tidally-averaged salinity at a point within a classical estuary, this paper examines the extent to which the salinity of an inverse or negative estuary (one that experiences salinities higher than the adjacent ocean) responds to loss of freshwater. The hypothesis that ’the mean salinity of an inverse estuary varies predictably from the effects of net freshwater loss, and that the mean salinity at any time reflects the integration of these effects during a constant, measurable prior period’ is tested using observations spanning 27 years from Spencer Gulf in South Australia. It is shown that, on any day, the mean salinity of the whole (∼500 km2) of northern Spencer Gulf (NSG), deduced from broad oceanographic surveys on fourteen separate occasions between 1982 and 2009, mirrors the integration of net evaporation for a prior period of approximately six months. Linear regression (with a correlation coefficient of 0.89) indicated that a sustained change of one millimetre per day of net evaporation caused the mean salinity of NSG to change by 0.23. It is also shown that the regression relationship deduced using only 1980s data, remained entirely consistent with new data collected in 2008 and 2009, despite substantial longer-term trends during the intervening decades. Since net evaporation is essentially a continuous variable (measured approximately daily) its use as a proxy measure for mean salinity permits deductions about NSG’s long-term salinity behaviour. For example, the Gulf is inferred to reach its annual maximum salinity three months after the summertime net evaporation maximum, and the annual salinity cycle is almost exactly symmetric between rising and falling phases. This behaviour is surprisingly consistent with a simple one-dimensional model of inverse estuarine salt dynamics. Assuming that desalination, as it affects the mean salinity of the whole of NSG, is equivalent to evaporation in its removal of freshwater, estimates of the changes of NSG mean salinity based on proposed desalination rates, and based on projections of local climate change effects in 2030 and 2070, are provided. Finally, each of the principal oceanographic mechanisms that contributes to the efflux of salt from northern Spencer Gulf is examined to show that anticipated perturbations (both natural and anthropogenic) are not expected to perturb the dynamics beyond the Gulf’s ability to respond in the manner described.

Temperature dependent larval duration and survival of the western king prawn, Penaeus (Melicertus) latisulcatus Kishinouye, from Spencer Gulf, South Australia

The effects of temperature on growth, development and survival of penaeid prawn larvae ultimately influence recruitment to fisheries. This is of particular importance for temperate prawn fisheries in South Australia that target the semi-tropical western king prawn Penaeus (Melicertus) latisulcatus. P. latisulcatus resides at its lower temperature limit in South Australia, where it is restricted to the warmer waters of the gulf and embayment systems. Duration and survival were determined for three larval stages of P. latisulcatus reared at constant temperatures: 17, 20, 22.5 and 25°C. Data were subsequently used to construct a larval development model to generate predictions of larval duration for varying hatch dates when water temperature was within spawning range (November–May) at two different spawning grounds within Spencer Gulf (north vs. south). Results demonstrated that water temperature had affected larval development, duration and subsequently total survival. A power relationship was found between temperature and larval duration such that lower temperatures exerted the greatest influence. Under a constant laboratory temperature regime, total larval development ranged from 12.7days (at 24.4°C) to 31.3days (at 17.1°C). Stage-specific duration was generally greatest for protozoea, while developmental variability among individuals was greatest at later stages and at higher temperatures. Daily survival (~98% day−1) was similar between 17°C (total end survival: 36%) and 25°C (total end survival: 74%) treatments, suggesting the upper thermal tolerance limit is greater than mean water temperature maxima in temperate Spencer Gulf. A seasonal larval duration model predicted total duration to be shorter at the beginning of the spawning season (26.8days: 9 November) due to increasing daily water temperatures, compared to later in the season (35.4days: 29 May). Furthermore, larval duration was predicted to be significantly shorter in warmer northern (minimum 12.7days) compared to southern (minimum 17.2days) spawning grounds. These results improve the understanding of the factors that affect larval dispersal, settlement and recruitment of P. latisulcatus.