Diets of reef-dwelling labrids (Choerodon species) vary with body size, season and habitat: influence of foraging ability, specialization and opportunism.

Contemporary multivariate statistics were used to test the hypotheses that the dietary compositions of three populations of labrids on the west Australian coast are related to body size and undergo seasonal changes and to elucidate the relative extents and basis for any dietary differences within and between those populations. Gut content analyses determined the dietary compositions of Choerodon rubescens in marine waters of the outer reefs in the World Heritage Area of Shark Bay (26° S; 114° E) and of Choerodon schoenleinii in inner protected reefs of that large embayment. The dietary compositions of C. rubescens and C. schoenleinii differed significantly among length classes, progressed serially with increasing body size, both overall and almost invariably in each season and were more closely related to body size than season, whose effect was at best minimal. The size-related dietary change in C. rubescens involved, in particular, a shift from crustaceans and non-mytilid bivalves to mytilid bivalves and echinoid echinoderms. Although the diet of C. schoenleinii followed similar size-related changes, it contained a greater volume of gastropods when the fish were small and mytilids when large and only a small volume of echinoids. The dietary composition of C. rubescens in the Abrolhos Islands, 300 km to the south of Shark Bay, was related both to length class and season and differed from that of this labrid in Shark Bay with the ingestion of lesser volumes of mytilids and greater volumes of echinoids. The size-related changes in diet imply that these species shift from foraging over soft substrata to over reefs as their very well-developed jaws become sufficiently strong to remove attached and larger prey. The dietary compositions of C. rubescens and C. schoenleinii in Shark Bay and of C. rubescens at the Abrolhos Islands were related far more to habitat-locational differences than to length class and season. The above intraspecific and interspecific differences in diet are consistent with qualitative accounts of the relative abundances of the main prey in their respective environments, supporting the view that, despite specializations in their feeding apparatus, these labrids can feed opportunistically to a certain extent and could thus potentially respond to moderate changes in the composition of their prey caused by climate change and other anthropogenic effects.

Optimization of foraging and diet by the piscivorous Othos dentex (Serranidae).

The aim of this study was to determine the dietary characteristics and mouth morphology of Othos dentex and to use these data, together with in situ observations of feeding behaviour, to elucidate how foraging and diet are optimized by this piscivorous serranid. Seasonal spear and line fishing over reefs in south-western Australia yielded 426 O. dentex (total length, LT , 183-605 mm), among which the stomachs of 95 contained food. The food in the stomachs of 76 fish was sufficiently undigested to be seen to contain, almost invariably, a single fish prey, which was typically identifiable to family and often to species. The prey of O. dentex, which were measured (LT ), represented 10 families, of which the Labridae and Pempheridae constituted nearly two-thirds of the prey volume. Two-way crossed analysis of similarities of volumetric data for stomach contents showed that the dietary compositions of the different length classes of O. dentex in the various seasons were significantly related to length class of prey, but not to prey family, length class within the various prey families or season. Furthermore, an inverse (Q-mode) analysis, including one-way analysis of similarities, showed that the patterns in the prey consumed by the different length classes of O. dentex in the various seasons were related more strongly to length class than prey family. The former trend is exemplified in a shade plot, by a marked diagonality of the length classes of prey with increasing predator size. The ingestion of typically a single teleost prey, whose body size increases as that of O. dentex increases, reduces the frequency required for seeking prey, thus saving energy and reducing the potential for intraspecific competition for food. The ability of O. dentex to ingest large prey is facilitated by its possession of a very large gape, prominent recurved teeth, dorsal and independently-moveable eyes, cryptic colouration and effective ambush behaviour. Othos dentex has thus evolved very cost-effective mechanisms for optimizing its foraging and diet.

Dietary variations within a family of ambush predators (Platycephalidae) occupying different habitats and environments in the same geographical region.

This study has determined the extents and basis for variations in the composition of the prey ingested by the abundant species of a family highly adapted for ambush predation, i.e. Platycephalidae, in a region (south-western Australia) where that family is found in different habitats and environments. Dietary data were thus collected for Leviprora inops and Platycephalus laevigatus from seagrass in marine embayments and for Platycephalus westraliae from over sand in an estuary. These were then collated with those recorded previously for Platycephalus speculator from over sand and in seagrass in an estuary and for Platycephalus longispinis from over sand in coastal marine waters. While crustaceans and teleosts together dominated the diet of all five species, their percentage volumetric dietary contributions varied greatly, with those of crustaceans ranging from 7% for L. inops to 65% for P. speculator and those of teleosts ranging from 29% for P. longispinis to 91% for L. inops. For analyses, the data were separated into two sets. The first comprised the 17 dietary categories of invertebrates and all identified and unidentified teleosts collectively, while the second consisted of the 23 identified teleost families, both of which were subjected to permutational analysis of variance (PERMANOVA), analysis of similarities (ANOSIM) and a new (two-way) version of the RELATE procedure. The diets of three species changed seasonally, when using invertebrate dietary categories and teleosts collectively, but with only one species, when employing identified teleost families, probably reflecting a greater tendency for invertebrate than teleost prey abundance to change during the year. On the basis of dietary data for invertebrate taxa + teleosts collectively, the diets of three of the five species changed serially with body size, with a fourth species feeding, throughout life, predominantly on the carid Palaemonetes australis. Based on identified teleost families, the diets of the three species that fed predominantly on teleosts underwent serial size-related changes. Although L. inops and the co-occurring P. laevigatus both consume large volumes of teleosts, the former ingests larger, less demersal and more mobile prey, e.g. the labrids Haletta semifasciata and Neoodax balteatus, than the latter, e.g. the scorpaenid Gymnapistes marmoratus, reflecting the possession by L. inops of a far longer head and larger buccal cavity. Circumstantial evidence suggests that the large differences in the volumes of crustaceans and teleosts consumed by each platycephalid species are related to differences in the relative availability of these prey in the different habitats or environments of each species.

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

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

To what extent are the dietary compositions of three abundant, co-occurring labrid species different and related to latitude, habitat, body size and season?

This study demonstrated that the dietary composition of each of three abundant reef-associated labrid species in temperate Western Australia differed significantly with latitude and changed with increasing body size and almost invariably differed among those species when they co-occurred. These results were derived from comparisons and multivariate analyses of volumetric dietary data, obtained from the foregut contents of Coris auricularis, Notolabrus parilus and Ophthalmolepis lineolatus from the Jurien Bay Marine Park (JBMP) and waters off Perth, 250 km to the south. Latitudinal differences in the dietary compositions of each species in exposed reefs typically reflected greater contributions by large crustaceans, bivalve molluscs, echinoids and annelids to the diets in the waters off Perth than in the JBMP, whereas the reverse was true for gastropods and small crustaceans. The diet of each species exhibited similar, but not identical, quantitative changes with increasing body size, with the contributions of small crustaceans declining and those of large crustaceans and echinoids increasing, while that of gastropods underwent little change. Within the JBMP, the dietary compositions of both C. auricularis and N. parilus were similar in exposed and sheltered reefs and the same was true for N. parilus in the sheltered reefs and interspersed areas of seagrass. The latter similarity demonstrated that, in both of those divergent habitat types, N. parilus feeds on prey associated with either the sand or the macrophytes that cover and lie between the reefs. Although the main dietary components of each species were the same, i.e. gastropods, small crustaceans (mainly amphipods and isopods), large crustaceans (particularly penaeids and brachyuran crabs) and echinoids, their contributions varied among those species, which accounts for the significant interspecific differences in diet. Coris auricularis had the most distinct diet, due mainly to an ingestion of greater volumes of small crustaceans, e.g. amphipods and isopods, and lesser volumes of large crustaceans, e.g. brachyuran crabs, which was associated with a relatively narrower mouth and smaller teeth and the absence of prominent canines at the rear of the jaw. The above intra and interspecific differences in dietary composition would reduce, on the south-west coast of Australia, the potential for competition for food among and within these three abundant labrids, each of which belongs to different genera within the Julidine clade.

Influence of selected factors on the dietary compositions of three targeted and co-occurring temperate species of reef fishes: implications for food partitioning.

The dietary compositions of three medium to large targeted fish species, which co-occur over reefs in temperate waters of south-western Australia, were determined. These data were then used to ascertain statistically the extent to which body size, season and habitat influence the diets of these species and the degree to which food resources were partitioned among and within those species, and thus reduced the potential of interspecific and intraspecific competition. On the west coast, Bodianus frenchii (Labridae) and Epinephelides armatus (Serranidae) spent their whole life over prominent limestone reefs, as did Glaucosoma hebraicum (Glaucosomatidae) in all but juvenile life, when it lived over low-relief, limestone substrata. The dietary composition of each species changed with increasing body size, which, in G. hebraicum, was particularly pronounced at c. 300 mm total length (L(T)) and therefore at the size when this species shifts habitat. When the three species co-occurred over the same reefs, their dietary compositions were significantly different, with that of B. frenchii being by far the most discrete, reflecting a far greater contribution by sedentary taxa. Thus, the diet of B. frenchii was distinguished from those of the other two species in containing substantial volumes of bivalve and gastropod molluscs and echinoid echinoderms and essentially no teleosts. Although the diets of G. hebraicum and particularly E. armatus were dominated by teleosts, and especially for larger individuals, the former species ingested greater volumes of cephalopods and small crustaceans. The pointed jaws of B. frenchii, with their forwardly directed and interlocking anterior incisors, are ideally adapted for biting and retaining their invertebrate prey, which are attached to or reside within reef crevices. In contrast, the mouths of G. hebraicum and E. armatus are broader and rounder and contain numerous small, slender and inward-pointing teeth. These teeth, in conjunction with prominent backward-curved canines in E. armatus, facilitate the capture and retention of fish prey. Observations in situ indicate that G. hebraicum is a suction feeder, while E. armatus is predominantly a ram feeder. Although reef environments on the west and south coasts differ, the diet of B. frenchii on these coasts differed only slightly. Interspecific differences in diet, combined with size-related changes in dietary compositions and the occupation of different habitats by juvenile and adult G. hebraicum, reduce the potential for competition for food resources among and within B. frenchii, G. hebraicum and E. armatus and thus helps facilitate the coexistence of these species which historically have been abundant over reefs in south-western Australia.

Partitioning of food resources amongst 18 abundant benthic carnivorous fish species in marine waters on the lower west coast of Australia.

The volumetric contributions made by prey and plant material to the diets of 4 elasmobranch and 14 teleost species, collected seasonally by trawling from waters along ca. 200 km of the lower west coast of Australia, have been compared. These benthic carnivores, which were all abundant and collectively contributed 83% to the total number of fish caught, represented nine families (Urolophidae, Scorpaenidae, Triglidae, Platycephalidae, Sillaginidae, Carangidae, Gerreidae, Mullidae and Pempherididae). Some species were numerous in both shallow (5-15 m) and deeper (20-35 m) waters and in both northern and southern regions, whereas others were largely confined to one of these water depths or regions. Comparisons between the diets of the different species, which utilised data collected from individuals throughout the study area, demonstrated that the dietary composition of any given species was almost invariably significantly different from that of every other species. This partly reflected the fact that, while errant polychaetes, gammarid amphipods and tanaids were ingested by all species, their contributions to the diets of the different species varied. Furthermore, echinoderms contributed to the diets of just nine species, and this was substantial only in the case of two sillaginid species, while teleosts were never consumed by six species and only made a marked contribution to the diets of the single species of platycephalid. The diet of each species underwent size-related changes, reflecting a shift from the consumption by smaller fish of prey such as amphipods, mysids and copepods, to the ingestion by larger fish of prey such as polychaetes, carid decapods, isopods and small teleosts. The interspecific and intraspecific differences in dietary compositions would spread the food resources amongst and within species, thereby reducing the potential for competition for those resources within the fish community. Non-metric multi-dimensional scaling (MDS) ordination plots emphasised that the dietary compositions of species within each family possessed some obvious similarities, reflecting similarities in body and mouth morphology and feeding behaviour. However, the extent to which the dietary compositions of the different families were similar or different was often not related to the phylogenetic relationships amongst those families. Furthermore, while differences in mouth size and morphology could sometimes be used to account for differences amongst the diets of the full suite of species, this was not always the case. Thus, the ways in which species feed and use their ancillary feeding structures were also employed to help elucidate the basis for variations in diets.