Lower West Coast Of Australia Research Articles

Influences of microtidal regime and eutrophication on estuarine zooplankton

This study describes the ecology of mesozooplankton in the shallows of the large central basin area of the Swan-Canning Estuary on the microtidal lower west coast of Australia during the mid-1980s when, as in most other south-western Australian estuaries, it was mesotrophic. As the vast majority of species reproduce within the estuary and there is a lack of a net upstream flow of bottom water during each tidal cycle to facilitate selective tidal transport, the populations of those species are considered autochthonous. Species restricted to the estuary accounted for 63% of the total number of species and 83% of abundance. Contributions by tychoplankton to total number of species (68%) and individuals (59%) exceeded the corresponding values of 18 and 39% for holoplankton and 14 and 2% for meroplankton. Low meroplankton values result from a paucity of benthic macroinvertebrate species with planktotrophic development. Comparisons with data for the shallow basin areas of the nearby highly eutrophic Peel-Harvey Estuary were used to indicate how mesozooplankton in the microtidal estuaries respond to excessive nutrient enrichment. The mean number of species in the Swan-Canning was essentially the same as in the Peel-Harvey, whereas concentrations were only half, reflecting the lower productivity of the former estuary. However, concentrations of the four most abundant species in the Swan-Canning, the amphipod Grandidierella propendatata, the calanoids Gladioferens imparipes and Acartia cf. clausi and a harpacticoid species, exceeded those of these species in the Peel-Harvey by between 1.6 and 11.7 times. Greater concentrations in the Swan-Canning were attributable to this mesotrophic system not undergoing the reduction in oxygen concentrations and/or environmental degradation, to which these species are susceptible. In contrast, concentrations of the amphipods Corophium minor and Tethygenia elanora and large nematodes, which are tolerant of degraded environmental conditions, were greater in the Peel-Harvey. Holoplankton made a far greater contribution in the Swan-Canning than Peel-Harvey, due to substantial numbers of copepods that benefit from good environmental conditions. Mesozooplankton composition in both estuaries underwent cyclical changes during the year due to time-staggered differences in seasonal concentrations of various species. Cyclical changes and variations in composition were more closely related to measured environmental variables in the Swan-Canning than Peel-Harvey estuaries. The concentrations of many species were greater at night than during the day presumably reflecting a nycthemeral response and/or low nocturnal oxygen concentrations at the bottom of the water column.

Peak in biomass driven by larger-bodied meso-predators in demersal fish communities between shelf and slope habitats at the head of a submarine canyon in the south-eastern Indian Ocean

This study investigated variations in the composition and biomass of demersal fish assemblages over a 570-metre depth gradient on the temperate, lower west coast of Australia (32° S) in the south-eastern Indian Ocean. Fish assemblages were sampled using Baited Remote Underwater Stereo Video systems (stereo-BRUVs, n = 284 deployments) from shallow waters around a mid-shelf island (Rottnest Island) to the continental slope within a submarine canyon (Perth Canyon). A total of 9013 individual fishes (i.e. ΣMaxN) belonging to 179 species and 75 families were identified. Multivariate statistical analyses revealed three distinct fish assemblages associated with the continental shelf (5–199 m), margin (200–300 m) and upper slope (300–570 m). A distance-based linear model revealed that among environmental covariates, benthic biota (sessile invertebrates and macroalgae) accounted for the highest proportion of variation in fish assemblage composition (16.9%) followed by depth (12.5%) and seabed relief (10.5%). Generalised additive models indicated higher biomass of fish associated with habitats characterised by benthic biota. Species richness decreased with increasing depth across the continental shelf but remained constant with increasing depth on the continental slope. Average fish length was not correlated with depth but was greatest at 200–400 m depth. The continental margin and upper slope habitats revealed a distinct change in assemblage composition as well as a peak in biomass of species that was dominated by larger-bodied meso-predators at the continental margin. The trends exhibited in fish assemblage characteristics across this broad depth range can inform ecosystem based management for deepwater fisheries resources.

Ranking of length-class, seasonal and regional effects on dietary compositions of the co-occurring Pagrus auratus (Sparidae) and Pseudocaranx georgianus (Carangidae)

Using an effective combination of multivariate testing and ordination analyses, this study compares the extents to which the diets of two co-occurring fish species (Pagrus auratus and Pseudocaranx georgianus) are related to body size (length class), season and region and the rank order importance of those effects. Thus, volumetric dietary compositions were determined for these species on the lower west coast of Australia, where both are abundant, and for P. auratus from the mid west coast and P. georgianus from the south coast. The diet of P. auratus on the lower west coast was strongly related to body size and slightly less to season. With increasing body size, its diet shifted from predominantly ophiuroids to larger prey, such as brachyuran crabs, teleosts, echinoids and ultimately asteroids, probably reflecting a shift from foraging over soft sediments to areas over and around reefs. Seasonal changes on the lower west coast were restricted mainly to small P. auratus, while larger fish underwent seasonal changes further north. Analyses using a common size range of medium to larger P. auratus demonstrated that dietary composition differed more between regions than seasons. The relationships between diet and length class of P. georgianus on both the lower west and south coasts were less pronounced than for P. auratus and seasonal changes were restricted to the south coast, where amphipod consumption increased markedly in summer. The diet of P. georgianus was related far more to region than length class and season, with more small teleosts, small crabs, carideans and littorinids and less amphipods, isopods and small bivalves being ingested on the lower west than south coasts. Although crabs and teleosts were important typifying prey of P. auratus and P. georgianus, when co-occurring, the former predator tended to ingest greater volumes of larger and often less mobile prey. This reflects differences in dentition, jaw morphology and feeding behaviour and reduces the potential for competition for food resources. The results imply that P. auratus and P. georgianus are opportunistic feeders and that the effects of length class, season and region on dietary composition and their rank orders can vary markedly between species and for length class and season between regions for the same species.

Do the maximum sizes, ages and patterns of growth of three reef‐dwelling labrid species at two latitudes differ in a manner conforming to the metabolic theory of ecology?

The size and age data and patterns of growth of three abundant, reef-dwelling and protogynous labrid species (Coris auricularis, Notolabrus parilus and Ophthalmolepis lineolata) in waters off Perth at c. 32° S and in the warmer waters of the Jurien Bay Marine Park (JBMP) at c. 30° S on the lower west coast of Australia are compared. Using data for the top 10% of values and a randomization procedure, the maximum total length (L(T) ) and mass of each species and the maximum age of the first two species were estimated to be significantly greater off Perth than in the JBMP (all P < 0.001) and the maximum ages of O. lineolata in the two localities did not differ significantly (P > 0.05). These latitudinal trends, thus, typically conform to those frequently exhibited by fish species and the predictions of the metabolic theory of ecology (MTE). While, in terms of mass, the instantaneous growth rates of each species were similar at both latitudes during early life, they were greater at the higher latitude throughout the remainder and thus much of life, which is broadly consistent with the MTE. When expressed in terms of L(T), however, instantaneous growth rates did not exhibit consistent latitudinal trends across all three species. The above trends with mass, together with those for reproductive variables, demonstrate that a greater amount of energy is directed into somatic growth and gonadal development by each of these species at the higher latitude. The consistency of the direction of the latitudinal trends for maximum body size and age and pattern of growth across all three species implies that each species is responding in a similar manner to differences between the environmental characteristics, such as temperature, at those two latitudes. The individual maximum L(T), mass and age and pattern of growth of O. lineolata at a higher and thus cooler latitude on the eastern Australian coast are consistent with the latitudinal trends exhibited by those characteristics for this species in the two western Australian localities. The implications of using mass rather than length as the indicator variable when comparing the maximum sizes of the three species and the trends exhibited by the instantaneous growth rates of those species at different latitudes are explored. Although growth curves fitted to both the L(T) and masses at age for the males of each species lay above those for their females, this would not have influenced the conclusions drawn from common curves for both sexes.

Spawning and nursery habitat partitioning and movement patterns of Pagrus auratus (Sparidae) on the lower west coast of Australia

The ages and lengths of Pagrus auratus caught by line fishing in three marine embayments (Owen Anchorage, Cockburn Sound and Warnrbo Sound) and inshore (<80 m depth) and offshore waters (>80 m depth) on the lower west coast of Australia (31°45′–32°45′ S) were used to infer the movement patterns and habitats occupied by this species at different stages in its life cycle on this coast. These data were supplemented by results obtained by tagging individuals in spawning aggregations in the embayments. 0+ P. auratus <200 mm FL were caught exclusively in the three adjacent embayments. The ages and lengths of immature P. auratus, ranging from 1+ (ca. 200 mm FL) to 5+ years (ca. 400 mm FL), increased progressively with distance from these embayments. During the spawning period (from September to January), the relative abundances of P. auratus with either developing, developed or recently spent gonads were far greater in the three embayments (91%) than in either inshore (12%) or offshore waters (30%). Some tagged P. auratus were recaptured among spawning aggregations in the same embayment during subsequent spawning seasons, while others were recaptured in these embayments outside the spawning period. However, some other tagged individuals were recaptured up to 92 km north, 33 km west and 134 km south outside the spawning period and up to five years after tagging. The results of this study emphasise that the above three adjacent marine embayments constitute important spawning and nursery areas for P. auratus and are thus potentially critical for sustaining the stocks of this recreationally and commercially important species on the lower west coast of Australia.

Annual, lunar and diel reproductive periodicity of a spawning aggregation of snapper Pagrus auratus (Sparidae) in a marine embayment on the lower west coast of Australia

Ichthyoplankton sampling and ovarian characteristics were used to elucidate whether the reproductive cycles of a spawning aggregation of snapper Pagrus auratus in a nearshore marine embayment were temporally and spatially specific and related with environmental conditions. The reproductive dynamics of this aggregation were studied over four consecutive years (2001-2004). Spawning occurred between September and January each year, when water temperatures ranged from 15·8 to 23·1° C. In all 4 years, the cumulative egg densities in Cockburn Sound were highest when water temperatures were between the narrow range of 19-20° C. The spawning fraction of females was monthly bimodal and peaked during new and the full moons at 96-100% and c. 75%, respectively. The backcalculated ages of P. auratus eggs collected from 16 ichthyoplankton surveys demonstrated that P. auratus in Cockburn Sound spawn at night during the 3 h following the high tide. The spatial distributions of P. auratus eggs in Cockburn Sound during the peak reproductive period in all 4 years were consistent, further implying spawning was temporally and spatially specific. High concentrations of recently spawned eggs (8-16 h old) demonstrated spawning also occurred within the adjacent marine embayments of Owen Anchorage and Warnbro Sound. Water circulation in Cockburn and Warnbro Sounds resembled an eddy that was most prominent during the period of highest egg densities, thereby facilitating the retention of eggs in these areas. The reproductive cycles of P. auratus described in this study have assisted managers with the appropriate temporal and spatial scale for a closed fishing season to protect these spawning aggregations.

Comparisons between the characteristics of ichthyofaunas in nearshore waters of five estuaries with varying degrees of connectivity with the ocean

The characteristics of the fish faunas in nearshore, shallow (<1.2 m) waters of the basins of estuaries along the same coastline, but which were open to the ocean for varying periods, have been determined and compared. The fish faunas of the permanently-open Oyster Harbour, the seasonally-open Broke, Irwin and Wilson inlets and the normally-closed Wellstead Estuary on the south coast of Western Australia were sampled by seine net seasonally for 2 years. Irrespective of the frequency and duration that the estuary mouth was open, the ichthyofauna of each estuary was numerically dominated by three atherinid species and three gobiid species (92.9–99.7%), each of which completes its life cycle within these estuaries. The ichthyofaunal compositions of each estuary differed significantly, however, from that of each other estuary. These differences were largely attributable to the relative abundances of the above six species varying between estuaries, which, in turn, reflected differences in such factors as estuary mouth status, macrophyte cover and salinity. For example, Favonigobius lateralis and Leptatherina presbyteroides, which are also represented by marine populations, were most abundant in the permanently-open estuary (Oyster Harbour), which, in terms of substrate and salinity, most closely resembled the nearshore marine environment. In contrast, Leptatherina wallacei made its greatest contribution in the only estuary to exhibit a protracted period of greatly reduced salinities, which is consistent with its distribution in permanently-open estuaries on the lower west coast of Australia, while Atherinosoma elongata and Pseudogobius olorum were particularly numerous in estuaries containing dense stands of the seagrass Ruppia megacarpa. Marine species made the greatest contribution to species richness in the permanently-open estuary and least in the normally-closed estuary. Species richness was greatest in summer and least in winter in each estuary, but differed markedly between years only in Wilson Inlet. Density of fishes was greatest in the most eutrophic estuary (Wellstead Estuary) and least in the most oligotrophic estuary (Broke Inlet) and only underwent marked seasonal variations in Wilson Inlet and Wellstead Estuary, in which densities fell to their minima in winter. Ichthyofaunal composition varied between years in the Broke and Wilson inlets and Wellstead Estuary, in which there was little or no connection with the ocean in one of those years. Species composition underwent progressive seasonal changes throughout the year in Wellstead Estuary, due to the abundance of certain species peaking at different times of the year.

Seasonal variation in the long-term warming trend in water temperature off the Western Australian coast

Previous studies have demonstrated that one area of greatest increase in surface sea temperatures (SST) (0.02°C per year) in the Indian Ocean over the last 50 years occurs off the lower west coast of Australia, an area dominated by the Leeuwin Current. The present paper examines water temperature trends at several coastal sites since the early 1970s: two rock lobster puerulus monitoring sites in shallow water (&lt;5 m); four sites from a monitoring program onboard rock lobster vessels that provide bottom water temperature (&lt;36 m); and an environmental monitoring site at Rottnest (0–50 m depth). Two global SST datasets are also examined. These data show that there was a strong seasonal variation in the historic increases in temperature off the lower west coast of Australia, with most of the increases (0.02–0.035°C per year) only focussed on 4–6 months over the austral autumn–winter with little or no increase (&lt;0.01°C per year) apparent in the austral spring–summer period. These increases are also apparent after taking into account the interannual variation in the strength of the Leeuwin Current. The warming trend results in a change to the seasonal temperature cycle over the decades, with a delay in the peak in the temperature cycle during autumn between the 1950s and 2000s of ~10–20 days. A delay in the timing of the minimum temperature is also apparent at Rottnest from August–September to October. This seasonal variation in water temperature increases and its effect on the annual temperature cycle should be examined in climate models because it provides the potential to better understand the specific processes through which climate change and global warming are affecting this region of the Indian Ocean. It also provides an opportunity to further test the climate models to see whether this aspect is predicted in the future projections of how increases will be manifest. Any seasonal variation in water temperature increase has important implications for fisheries and the marine ecosystem because it may affect many aspects of the annual life cycle such as timing of growth, moulting, mating, spawning and recruitment, which have to be taken into account in the stock assessment and management of fisheries.

Nocturnally active western rock lobsters Panulirus cygnus forage close to shallow coastal reefs

The western rock lobster Panulirus cygnus is an abundant consumer along the lower west coast of Australia, sheltering in reef during the day and foraging in surrounding habitat at night. We used acoustic telemetry to determine the distances from high-relief reef where nocturnally active P. cygnus forage at 3 sites representing 3 common shallow-water habitats: meadows of the seagrasses Amphibolis spp. and Posidonia sinuosa and macroalga-dominated pavement. We focussed on among-site variation in core and total nocturnal activity area, distance of foraging from high-relief reef and total distance between position estimates. We found that P. cygnus moved over an extensive area at all sites and that 90% of lobster activity occurred within 60 m of the nearest high-relief reef at all 3 sites regardless of habitat type. Foraging activity by P. cygnus is therefore likely to be greatest within this zone around coastal reefs. Core (50% utilisation distribution (UD)) and total (95% UD) nocturnal activity areas did not differ significantly between sites and averaged 2556 ± 482 m 2 and 10 437 ± 2221 m 2 , respectively. Total distance between position estimates and average distance from reef did not differ between sites either.

Age and size compositions, growth and reproductive biology of the breaksea cod Epinephelides armatus, a gonochoristic serranid

Details of the reproductive biology, size and age compositions and growth of the breaksea cod Epinephelides armatus, the sole representative of Epinephelides, were obtained by collecting monthly samples of a wide total length (LT) range of individuals from coastal marine waters at 31–32° S on the lower west coast of Australia. Although the modal LT class of females (250–299 mm) was markedly less than that of males (400–449 mm), the modal ages of the two sexes were similar, i.e. 4 v. 5 years, respectively. The similarity in the age compositions and the histological demonstration that the gonads of all E. armatus consist solely of either ovarian or testicular tissues demonstrate that this species is gonochoristic, which is highly unusual for an anthiinine serranid. The absence of a central, membrane‐lined ‘ovarian’ lumen in the testes of juveniles would account for adult testes containing neither this ovarian remnant nor the peripherally located sperm sinuses that are found in the mature testes of almost all other serranids. The results demonstrate that E. armatus exhibits a very unusual pattern of sexual development for a serranid. The spawning period of E. armatus lasts for c. 9 months, which is long for a species in temperate Western Australian waters, but comparable with that of many other relatively small serranids elsewhere. Females grow slower than males, attaining LT at 3, 5 and 10 years of c. 200, 285 and 420 mm, respectively, compared with c. 215, 315 and 450 mm, respectively. Females, however, attain maturity at a greater LT and older age than males.

To what extent are the characteristics of nematode assemblages in nearshore sediments on the west Australian coast related to habitat type, season and zone?

This study has determined the ways in which the characteristics of the nematode assemblages in nearshore marine waters along the microtidal lower west coast of Australia are influenced by habitat type, time of year and shore-perpendicular zones. The habitat types (1, 2 and 6), which had previously been identified on the basis of a suite of enduring environmental characteristics, could be broadly described as highly sheltered from wave activity and containing dense seagrass (1), moderately sheltered from wave activity with sparse seagrass (2) and relatively exposed to wave activity with no seagrass (6). Sampling in five consecutive seasons yielded 15 751 nematodes representing 75 species, of which, at present, only three can be referred to described species. The number of species and densities in habitat type 1, and particularly those in its subtidal zone (C), were far greater than those in the other two habitat types. Both of these biotic variables underwent marked seasonal changes, declining to low levels during winter. The compositions of the assemblages differed significantly among the three habitat types, with the differences between habitat types 1 and 6 being particularly marked. Species of Paracomesoma, Dichromadora, Marylynnia, and Pomponema, which are assumed to feed primarily on benthic diatoms, were particularly abundant at the most sheltered habitat type, whereas those of Gonionchus, Theristus and Bathylaimus, which are assumed to be deposit feeders, were relatively abundant at the most highly exposed habitat type. The compositions of the assemblages differed among seasons and were most discrete in spring, due to marked increases in the densities of certain species. However, differences in the compositions in the different zones of each habitat type were relatively small, presumably reflecting the presence of only a small tide in the region.

Do the assemblages of benthic macroinvertebrates in nearshore waters of Western Australia vary among habitat types, zones and seasons?

Benthic macroinvertebrates were sampled seasonally in the subtidal and upper and lower swash zones at two sites in each of six nearshore habitat types on the lower west coast of Australia. The habitat types, which differed mainly in the extent of their exposure to wave activity and whether sea grass and/or nearshore reefs were present, had been distinguished quantitatively using values for a suite of seven statistically-selected enduring environmental characteristics (Valesini et al., 2003). The core samples yielded 121 species representing eight phyla, among which the Polychaeta, Malacostraca and Bivalvia were the most speciose classes, contributing ∼38, 23 and 10%, respectively, to the total number of individuals. The total number of species and mean density of macroinvertebrates at the most protected habitat type (1), i.e. 70 and 209·2 individuals 0·1 m−2, respectively, were far greater than in any other habitat type. Habitat type influenced species composition to a greater extent than either zone or season. Furthermore, the extents of the differences among the species compositions of the six habitat types statistically matched the extents of the differences among the values for the suite of enduring environmental characteristics that distinguished each of those habitat types. Overall, the species composition at habitat type 1 was the most distinct, containing five abundant species of polychaetes that were adapted to deposit-feeding in calm waters with high levels of organic material and which were rare in all other habitat types. In contrast, the fauna at the most exposed habitat type was characterized by four crustacean species and a species of bivalve and polychaete, whose mobility and tough external surface facilitated their survival and feeding in turbulent waters. The zonal differences in faunal compositions among habitat types were greatest in the case of the subtidal zone. The faunal compositions differed among zones and seasons only at the most protected habitat type.

Factors influencing the diets of four morphologically divergent fish species in nearshore marine waters

The dietary compositions of Atherinomorus ogilbyi (Atherinidae), Sillago schomburgkii (Sillaginidae), Lesueurina platycephala (Leptoscopidae) and Ammotretis elongatus (Pleuronectidae) in three nearshore habitats on the lower west coast of Australia, which varied in their exposure to wave energy and the extent to which they contain sea grass, have been determined. The dietary compositions of these four abundant teleosts differed, reflecting marked differences between the location in the water column, head and mouth morphology and feeding behaviour of these species. Atherinomorus ogilbyi, which has a relatively high and large mouth, fed mainly on planktonic invertebrates in the water column, while S. schomburgkii ingested predominantly benthic prey, such as polychaetes and bivalves, which it extracted from the sediment using its downward-protruding mouth. Lesueurina platycephala employed its large mouth, cryptic coloration and ambush feeding to target relatively large teleosts and invertebrates, while the small mouth and flattened body of Ammotretis elongatus facilitated the ingestion of small crustacean prey, e.g. cumaceans and amphipods, which live on the substrate surface. Atherinomorus ogilbyi consumed predominantly calanoid copepods, cladocerans and insects during the day and mainly amphipods at night, when the latter taxon became abundant in the water column. The dietary composition of each species underwent a similar pattern of size-related change, being most pronounced in L. platycephala during the day. The diets of A. ogilbyi and S. schomburgkii, the two species for which the data were most comprehensive, differed among habitats and seasons, reflecting differences in the densities of their main prey. Comparisons between the day-time diets of the above four species with those recorded previously for a further four abundant species in the same habitats during the day, show that food resources are well distributed among the main fish species in nearshore waters along the lower west coast of Australia. This feature, together with the size-related changes in the diets of the different species, reduces the potential for inter- and intraspecific competition for food by fish species in this environment.

To what extent are the fish compositions at nearshore sites along a heterogeneous coast related to habitat type?

This study has tested the following main hypotheses. (1) The composition of fish assemblages in nearshore waters along the lower west coast of Australia will differ significantly among four habitat types (1–4), which could be distinguished quantitatively from each other by differences in the values for a statistically selected suite of enduring environmental variables. (2) The extents of the differences in ichthyofaunal composition among the four habitat types will statistically match those in the suite of environmental variables that distinguish those habitat types. A 60.5 m long seine net, with 9 mm mesh in the bunt, was used seasonally for 2 years to sample fish from sites representing each of the four nearshore habitat types, which differed broadly in their exposure to wave activity and the extent and location of seagrass beds. The compositions of the fish faunas in each habitat type were significantly different from each other in all but one case. Ichthyofaunal composition did not differ significantly between years in any habitat type and differed significantly among seasons in only one habitat type. The arrangement of the rank orders of similarity in matrices constructed from the abundances of the fish species at each site also did not differ between years but did differ among seasons. Moreover, the arrangement of the ranks in the similarity matrices produced from the ichthyofaunal data recorded at the various sites in each season were significantly correlated with that in the distance matrix constructed from the environmental data for those sites, with the correlation ( ρ) ranging from 0.476 in winter to 0.696 in autumn. These results thus demonstrate that the extents of the differences in the fish compositions among the various habitat types parallel those in the environmental data for the corresponding habitat types. The subset of species that provided the best correlation with the environmental distance matrix, and which was thus most responsible for distinguishing among the fish compositions at the four habitat types, was also determined for each season.

Size-related movements of Rhabdosargus sarba in three different environments and their influence on estimates of von Bertalanffy growth parameters

In temperate, marine waters on the lower west coast of Australia, Rhabdosargus sarba settles in unvegetated nearshore areas and then, as it increases in size, moves progressively to nearby seagrass beds and then to exposed unvegetated nearshore areas and finally to areas around reefs where spawning occurs. The von Bertalanffy growth curves fitted to the lengths at age of R. sarba collected from the different habitats mentioned above were significantly different from each other (P<0.001). These significant differences were at least partly attributable to the fact that the size-related movement of R. sarba between habitats affects the composition of the lengths at age of this species in each habitat. The problem of producing a sound quantitative description of the overall growth of the individuals in populations such, as that of R. sarba in coastal marine waters, was overcome by incorporating logistic functions within the von Bertalanffy growth model that accommodated the probability of a fish of a given length occurring in a particular habitat. R. sarba also underwent size-related changes in habitats in two other environments, namely a temperate estuary and a subtropical embayment. Thus, as this species increases in size, it moves from nearshore, shallow waters to offshore, deeper waters in the former environment and from mangroves to areas around reefs in the latter environment. Although descriptions of the growth of individuals in the populations of R. sarba in these latter two environments were improved statistically by using the adjusted von Bertalanffy growth equation, these improvements were so small that they were considered not to be of biological significance. This was because, in contrast to the situation in temperate coastal marine waters, the samples collected from the different habitats in the estuary and subtropical embayment provided, as a whole, better representations of the compositions of the lengths at age of those two populations. The growth of R. sarba was slightly faster in the lower reaches of the temperate estuary than in either temperate coastal marine waters or the subtropical marine embayment, where food was almost certainly less abundant.

Do the age compositions and growth of the crab Portunus pelagicus in marine embayments and estuaries differ?

Portunus pelagicus (Crustacea: Portunidae) was collected monthly for a minimum of two years from two marine embayments (Cockburn Sound and Koombana Bay) and two estuaries (Peel-Harvey and Leschenault) that are located between 32° and 33°S on the lower west coast of Australia. A birth date of 1 December, derived from estimates of the peak time of spawning, was used to determine the average age of crabs in the main size cohort(s) in carapace width–frequency histograms for each month in each water body. The resultant carapace width-at-age data for each sex in Cockburn Sound and the Peel-Harvey Estuary, in which recruitment occurs at an early age, were then described using a seasonal von Bertalanffy growth curve. The results demonstrated that the growth rates of P. pelagicus differed significantly neither between females and males nor between crabs in the two water bodies and that growth was highly seasonal, with little or no increase in size occurring during the cold winter and early spring months. They also showed that relatively few P. pelagicus live beyond 18 months and indicate that, as a result of legal restrictions against retaining ovigerous crabs and the poor quality of recently-moulted females, fishing mortality is far lower amongst female than male crabs. The fact that the carapace widths attained by P. pelagicus at the end of its first year of life in Cockburn Sound (105 mm) and the Peel-Harvey Estuary (109 mm) were very similar to those in the Leschenault Estuary (105 mm) and Koombana Bay (107 mm) into which that estuary discharges emphasizes that the growth of P. pelagicus in each of these systems was similar. Data are also presented which strongly indicate that the second moult undergone by mature females is accompanied by an increase in the relative size of the abdominal flap.

Patterns in the abundance and size-distribution of syngnathid fishes among habitats in a seagrass-dominated marine environment

Syngnathid fishes were sampled using a 1 m wide beam trawl during the day and night in each season from summer 1996/1997 to summer 1997/1998 in five habitat types in an approximately 90 km 2 area located on the lower west coast of Australia. The seagrasses Amphibolis griffithii, Posidonia sinuosa and Posidonia coriacea and shallow unvegetated sand were all in depths of 4–9 m, while deep habitat, which comprised mainly bare sand with isolated patches of the seagrasses Heterozostera tasmanica and Halophila ovalis, occurred at depths of 12–16 m. While A. griffithii and P. sinuosa each formed dense monospecific meadows, P. coriacea occurred in sparse clumps surrounded by areas of bare sand and patches of H. tasmanica. While catches of spotted pipefish Stigmatopora argus occurred mainly in P. sinuosa and P. coriacea, individuals of this species that exceeded ca. 55 mm in snout–vent length were far more abundant in the former habitat whereas smaller fish occurred mostly in the latter. Densities of S. argus were similar in P. sinuosa and P. coriacea, but differed between seasons, and a season/habitat interaction was present. In contrast, wide-bodied pipefish Stigmatopora nigra were collected mainly in P. coriacea and deep habitat and, although the densities were greater in P. coriacea than in deep habitat, the size-distributions of this species in these habitats were similar. Notably, S. nigra was never collected in P. sinuosa. Although less abundant than the above pipefish, the long-snouted pipefish Vanacampus poecilolaemus was collected almost exclusively in P. sinuosa. Few syngnathids were collected from shallow unvegetated sand or from A. griffithii, which differed markedly in plant structure from both Posidonia species. It is suggested that these syngnathid species occupy habitats that best enable them to remain inconspicuous to predators. Both S. argus and S. nigra have green or brown colouration and mimic strap-like seagrass leaves which they grasp with prehensile tails, whereas V. poecilolaemus, which is dark coloured and lacks a prehensile tail, most likely lies among the rich detrital material that accumulates beneath the dense P. sinuosa canopy. The movement of larger-sized S. argus from the narrow-leaved P. coriacea to the relatively broad-leaved P. sinuosa may enable the adults of this species to be better camouflaged than if they had remained in the former habitat. As they grow to only approximately half the size of S. argus, S. nigra of all sizes could remain concealed in narrow-leaved seagrasses like P. coriacea and H. tasmanica. The presence of more S. nigra in P. coriacea compared with deep habitat probably reflected the greater expanse of seagrass canopy available in the former habitat, while the inability of either Stigmatopora species to mimic the short leaves of A. griffithii could help to explain their scarcity in this seagrass. The almost total absence of S. nigra and small S. argus from P. sinuosa could be due to their inability to grasp the relatively broad leaves of this seagrass or the presence in this habitat of predators that selectively predate these smaller pipefish. That very few small-sized S. argus were present in P. coriacea during winter 1997, despite the fact that males of both Stigmatopora species carried broods of young in all seasons, suggests that the reproductive output of this species and/or the survival of recruits varied during the study period.

A user-friendly quantitative approach to classifying nearshore marine habitats along a heterogeneous coast

A scheme, which can be readily used by fisheries and environmental managers and ecologists, has been developed for quantitatively classifying the different habitats found in nearshore marine waters along the heterogeneous lower west coast of Australia. Initially, 25 beach sites, representing a wide range of nearshore environments, were separated into six a priori habitat types on the basis of characteristics that could readily be observed and were likely to influence the extent to which a particular (fish) species occupies a particular habitat. Focus was thus placed on such features as the degree of exposure to wave activity and whether or not seagrass and/or reefs were present in the nearshore vicinity. Subsequently, quantitative data for 27 environmental variables, considered likely to characterise the six habitat types, were obtained for each of the 25 sites from readily accessible sources. When the latter data were subjected to multidimensional scaling (MDS) ordination, the points for the sites representing only three of those six habitat types formed discrete groups. The bvstep routine in the Primer v5.0 statistical package (Clarke & Gorley, Primer v5.0: User Manual/Tutorial, Primer-E Ltd, Plymouth, 2001) was thus used to select a subset of the 27 environmental variables that would provide a better resolution of the six a priori habitat types. This process involved matching the distance matrix constructed from the quantitative environmental data with a matrix constructed from scored data that reflected the criteria for the initial a priori classification scheme. A subset of seven environmental variables gave the best correlation between the two matrices ( ρ=0.823), and thus provided the optimal set of quantitative data for discriminating between the six a priori habitat types. These variables comprised both the direct and north-westerly fetches, the minimum distance from the shoreline to the 2 m depth contour, the distance from the shoreline to the first offshore reef chain along a south-westerly transect, and the relative contributions of bare sand, subtidal reef and seagrass. Data for these characteristics at any nearshore site along the coastline can readily be recorded by managers and ecologists and subjected to the ‘nearest-replicate’ classification procedure developed in this study to ascertain the habitat type to which that site should be assigned. Current work is using MDS ordination, in conjunction with associated statistical tests and the bvstep routine, to elucidate the extent to which the compositions of assemblages of fish, benthic macroinvertebrates, meiofauna and zooplankton in nearshore waters along the lower west coast of Australia are related to habitat type(s).

Comparisons between the influence of habitat type, season and body size on the dietary compositions of fish species in nearshore marine waters

Fish were collected from over bare sand in nearshore shallow waters at three sites that varied in the extent to which they were exposed to wave activity and which were located within a 45-km stretch on the lower west coast of Australia. Sampling was undertaken within a 4–6-week period in each season. The volumetric contributions of different prey to the stomach contents and the mouth characteristics of four species, i.e. Sillago bassensis and Sillago vittata (Sillaginidae), Spratelloides robustus (Clupeidae) and Pseudorhombus jenynsii (Bothidae), were determined. Overall, the dietary compositions of the four species differed significantly from each other and those of fish at both the three different sites and in four consecutive seasons were also significantly different. In comparison with S. bassensis, the morphologically similar S. vittata fed to a relatively greater extent on polychaetes than zooplankters, presumably reflecting in part its greater ability to extend its upper jaw downwards towards the benthos. S. robustus typically targeted calanoid copepods in the plankton, whereas P. jenynsii fed on larger benthic prey taxa, reflecting the large differences in mouth morphology and feeding behaviour of these species. Although the diets of S. bassensis and S. vittata were strongly influenced by habitat type and season, the former variable was slightly more important for both of these species. However, the reverse applied with S. robustus. Season strongly influenced the dietary composition of P. jenynsii at the one site at which it was regularly caught. The diets of the two Sillago species and P. jenynsii underwent pronounced size-related changes, which would help distribute the food resources among the individuals of the different size classes of each of these species. In contrast, all size classes of S. robustus fed predominantly or exclusively on calanoid copepods at all sites and in all seasons, except at the most sheltered site in winter when these zooplankters were not found in samples taken from the water column. The seasonal and habitat variations recorded in the diets of the fish species in this study imply that these species are able to feed opportunistically, a characteristic that would be of particular value to fish that live in nearshore waters where the relative abundance of the different prey types varies with habitat type and season.

Reproductive biology and growth during pre- and postnatal life of Trygonoptera personata and T. mucosa (Batoidea: Urolophidae)

This study demonstrates that the females and males of two species of ray (Trygonoptera personata and T. mucosa) attain between ∼33 and ∼49% of their asymptotic sizes by the time of parturition, and that their growth throughout the whole period of pre- and postnatal growth can be described well by a single and smooth growth curve. The individuals used for this study were caught at regular intervals by trawling along the lower west coast of Australia. Conception takes place at a similar time in both Trygonoptera species (i.e. between late autumn and mid-winter), and parturition occurs 10-12 months later. The eggs found in the left uterus of female T. personata were enclosed in a membranous egg case and remained in embryonic diapause for ∼5 months between mid-winter and late spring, which is the first recorded instance of such diapause in a urolophid. Embryonic diapause in T. personata was followed by a period of particularly rapid embryonic development, during which the mean disc width increased from 11 mm in December to 113 mm in April, when parturition occurred. Although no females of T. mucosa were caught with either eggs or embryos in certain months, the data still showed that their eggs are enclosed in a case and that their embryos grow less rapidly than those of T. personata. Trygonoptera mucosa attained greater maximum disc widths than T. personata both in females (369 vs 311 mm) and males (283 vs 269 mm). The maximum ages attained, after parturition, by the females of T. personata (16 years) and T. mucosa (17 years) were greater than those of their males (10 and 12 years, respectively). Maturity is typically attained by the females of T. personata and T. mucosa at ∼230 and 250 mm, respectively, and by the males of both species at ∼220 mm. Maturity was reached by at least 50% of the females of T. personata and T. mucosa by the end of their fourth and fifth years of life after parturition, respectively, and by > 50% of the males of these two species by the completion of their fourth and second years of life after parturition, respectively. The von Bertalanffy and Schnute growth equations for the females and males of both species provided a good fit for the disc widths-at-age of postnatal fish, with r2 values for these equations ranging from 0.874 to 0.938. Although the von Bertalanffy growth curve also provided a good fit for the disc widths-at-age of both sexes of both species throughout much of the total growth period (i.e. including embryonic growth), it did not accommodate well the disc widths-at-age of the oldest and largest fish. This latter deficiency was entirely or largely eliminated by the use of the Schnute growth equation (r2 values ranged from 0.919 to 0.938), which incorporates four rather than three parameters as in the von Bertalanffy growth equation.