Yellowfin Bream Research Articles

Release method and anatomical hook location: effects on short-term mortality of angler-caught Acanthopagrus australis and Argyrosomus japonicus

One field and 3 aquaria experiments were done to quantify the short-term mortality of yellowfin bream Acanthopagrus australis and mulloway Argyrosomus japonicus after being angled and subjected to 3 general handling treatments. Anglers were supplied with identical J-type hooks and asked to handle hooked fish by either (1) physically removing the hook or (2) cutting the line (5 cm from the mouth of the fish) and leaving the hook in. Some hooked A. japonicus were subjected to a third handling treatment where the line was cut underwater without exposing the fish to air. Technical and biological data were collected before all fish were released into sea cages and monitored for 5 d. Control fish were seined and similarly caged and monitored. Concentrations of plasma glucose and cortisol were collected from a sample of fish on the first and last day of the experiments. Significant predictors of mortality for both species involved the presence of blood at the mouth and an interaction between anatomical hook location and hook removal. A. australis and A. japonicus that had their ingested hooks removed experienced the greatest mortalities (87.5 and 72.7%, respectively). Typically, these fish suffered damage to their oesophagus, stomach wall and vital organs. Mortality rates of A. australis and A. japonicus were significantly decreased to 1.7 and 16%, respectively, when they were released with their lines cut, with some of these fish free of hooks after 5 d. In contrast, few mortalities occurred in either species when the hooks were removed or the lines cut on mouth-hooked fish or in A. japonicus when it was released with no air exposure. For A. australis, the field- and aquaria-based experiments provided comparable results in terms of identifying treatment-specific effects, but there were potential biases in rates of hook ingestion. Irrespective of the treatment of fish, all experiments caused physiological changes measured as elevations in either plasma cortisol or glucose. We concluded that anglers should cut the line from hook-ingested A. australis and A. japonicus, but remove the hook from mouth-hooked individuals to prevent subsequent ingestion. Further research is required to examine the longer-term consequences of these handling practices on fish health.

Mortality of key fish species released by recreational anglers in an Australian estuary

A field experiment was done to quantify the mortality of fish released during a recreational angling tournament in Botany Bay, Australia. Participating boat-based anglers were divided into two groups, each representing different typical catch-and-release events. The first group (termed the ‘live weigh-in group’) retained the largest two individuals of 4 species (dusky flathead, Platycephalus fuscus, yellowfin bream, Acanthopagrus australis, sand whiting, Sillago ciliata, and trevally, Pseudocaranx dentex) in onboard holding tanks and then presented these to researchers at designated weigh-in times and stations. Gear, operational and handling data were collected before 125 fish were tagged using plastic t-bar tags, returned to the anglers and then released into two sea cages. The second group (termed the ‘immediate-release group’) immediately released 224 fish into two sea cages, after they were tagged and relevant data recorded by onboard observers. This group represented those fish routinely discarded (i) as part of catch-and-immediate-release tournaments and/or (ii) due to minimum legal sizes and/or personal quotas. Appropriate species and numbers of ‘control’ fish were seined and placed into two sea cages. All fish were monitored for mortalities over 10 days. Dusky flathead, yellowfin bream, trevally and snapper, Pagrus auratus accounted for more than 85% of the total catch. Their adjusted mortalities ranged between 0% and 36.6%. Irrespective of the treatment, most yellowfin bream and snapper deaths occurred within 3 h of being hooked and released into the cages, while trevally and dusky flathead showed a delayed mortality over 4 days. Owing to confounding effects due to their confinement, dusky flathead were excluded from further analyses. Anatomical hook location and the time between capture and release were significant predictors of mortality for yellowfin bream and trevally, respectively ( p < 0.01), but none of the various gear, operational or handling factors examined were significant for snapper ( p > 0.05). The results are discussed in terms of species-specific variabilities in mortalities, their causal effects and better management of catch-and-release events.

Monitoring PAHs in the Brisbane River and Moreton Bay, Australia, using semipermeable membrane devices and EROD activity in yellowfin bream, Acanthopagrus australis

Two water quality monitoring strategies designed to sample hydrophobic organic contaminants have been applied and evaluated across an expected concentration gradient in PAHs in the Moreton region. Semipermeable membrane devices (SPMDs) that sequester contaminants via passive diffusion across a membrane were used to evaluate the concentration of PAHs at four and five sites in spring and summer 2001/2002, respectively. In addition, induction of hepatic cytochrome P4501, EROD activity, in yellowfin bream, Acanthopagrus australis, captured in the vicinity of SPMD sampling sites following deployment in summer was used as a biomarker of exposure to PAHs and related chemicals. SPMDs identified a clear and reproducible gradient in PAH contamination with levels increasing from east to west in Moreton Bay and upstream in the Brisbane River. The highest PAH concentrations expressed as B(a)P-toxicity equivalents (TEQs) were found in urban areas, which were also furthest upstream and experienced the least flushing. Cytochrome P4501 induction in A. australis was similar at all sites. The absence of clear trends in EROD activity may be attributable to factors not measured in this study or variable residency time of A. australis in contaminated areas. It is also possible that fish in the Moreton region are displaying enzymatic adaptation, which has been reported previously for fish subjected to chronic exposure to organic contaminants. These potential interferences complicate interpretation of EROD activity from feral biota. It is, therefore, suggested that future monitoring combine the two methods by applying passive sampler extracts to in vitro EROD assays.

Differences in trophodynamics of commercially important fish between artificial waterways and natural coastal wetlands

Extensive artificial waterways have replaced natural wetlands and created new estuarine habitats on the southern Queensland coast, Australia. Economically important fish species found in adjacent natural wetlands of mangrove, saltmarsh and seagrass also occur in the artificial waterways. Stable isotope analyses ( δ 13C, δ 15N) were used to test whether the relative importance of basal sources of energy varied for foodwebs found in artificial (canals and tidal lakes) and natural waterways. None of the fish species differed in their isotope values between artificial waterways. In contrast, isotopic signatures of snub-nosed garfish ( Arrhamphus sclerolepis; Hemiramphidae) varied greatly between natural and artificial waterways, having highly enriched δ 13C values (−10.5‰) in natural wetlands, demonstrating reliance on seagrass (−11.4‰), and significantly less enriched values (−19.0‰) in artificial waterways, consistent with either local algal sources (−19.8 to −20.4‰) or a mixture of seagrass and other less enriched autotrophs from adjacent natural wetlands. Isotopic signatures of sand whiting ( Sillago ciliata; Sillaginidae) were also significantly more enriched in natural (−18.2‰) than artificial (−21.0‰) habitats, but means were not far enough apart to distinguish between different sources of nutrition. δ 13C values of yellowfin bream ( Acanthopagrus australis; Sparidae) did not differ between artificial and natural habitats (about −20‰ in both). δ 15N values of fish varied among habitats only for A. sclerolepis, which in artificial waterways had values enriched by 2‰ over those in natural waterways. This was consistent with a shift from seagrass (relatively depleted δ 15N) as a source in natural habitat to algal sources (relatively enriched δ 15N) in artificial habitats. This study provides some of the first evidence that at least some fish species rely on different autotrophs in artificial waterways than in adjacent natural wetlands.

Recruitment and growth of juvenile yellowfin bream, Acanthopagrus australis Gunther (Sparidae), in an Australian intermittently open estuary

Recruitment and growth of juvenile yellowfin bream, Acanthopagrus australis Gunther (Sparidae), an economically important species, were examined in an intermittently open Australian estuary for 13 months. Fish had a mean fork length of 77.9 (± SE 9.5) mm after 1 year. Growth rates were faster than those found in previous studies in permanently open estuaries and appeared to be dependent upon the water temperature, which ranged from 0.04 mm/day in winter to 0.67 mm/day during summer. However, periods of slower growth may also reflect the emigration of larger fish from the cohort when the estuary was open and which coincided with lower water temperatures. Growth and length-at-age of fish using length-frequency data may be estimated more accurately in intermittently open estuaries because the time of recruitment can be determined, especially when the estuary opening period is short.

Allozyme variation and stock structure in the black bream, Acanthopagrus butcheri (Munro) (Sparidae) in southern Australia: implications for fisheries management, aquaculture and taxonomic relationship with Acanthopagrus australis (Günther)

An investigation of allozyme variation at 32 loci in the black bream, Acanthopagrus butcheri (Munro), and the yellowfin bream, Acanthopagrus australis (Günther), revealed heterozygosities (H) equal to 0.035 and 0.065, respectively, and polymorphism (P0.95) equal to 0.067 and 0.200, respectively. The genetic data confirm the very high level of genetic similarity previously found between the two species, but nevertheless also support their recognition as separate species. The finding of only limited allozyme differentiation at three polymorphic loci amongst six black bream samples from south‐eastern Australia is consistent with the existence of a single panmictic population in this region. This would require that adult black bream have a greater capacity to survive in near‐shore marine environments than previously realised and that dispersal between estuaries is more extensive than was shown by tagging studies. In contrast, significant differences were found at two loci between black bream from south‐eastern Australia and a sample from south‐western Australia. The degree of genetic divergence between stocks in south‐eastern and south‐western Australia suggests that only local brood stock should be used for aquaculture or re‐stocking as a precaution until further investigations with DNA‐based techniques are conducted. The genetic divergence detected supports separate management for populations in Western Australia and Victoria.

Levels of selected chlorinated hydrocarbons in edible fish tissues from polluted areas in the Georges/Cooks Rivers and Sydney Harbour, New South Wales, Australia

During the period from February to March 1995 edible fish were sampled from various locations known to be polluted with chlorinated hydrocarbons. Samples were collected from five locations in the Georges/Cooks Rivers and four locations in Sydney Harbour. The muscle tissue of a range of fish species from each location was analysed for selected chlorinated hydrocarbons (i.e. polychlorinated biphenyls (PCBs) and organochlorine pesticides). The Georges/Cooks Rivers locations were in Prospect Creek. Chipping Norton, Saltpan Creek, Cooks River and Alexandra Canal. The Sydney Harbour locations were in Duck River, Parramatta River, Homebush Bay, Iron Cove and Long Bay. Chlorinated hydrocarbons were found in the tissues of fish sampled from every location. The major contaminants were PCBs and the Group A organochlorine pesticides, which include chlordane, dieldrin and heptachlor epoxide. At some locations, the mean concentration of one or more of these contaminants was significantly greater than the National Food Authority Maximum Residue Limit (MRL) in certain fish species. DDT and/or its metabolites were not found above their MRL at any location, but were detected at all locations. The highest mean concentrations of PCBs were detected in fish from Prospect Creek, Chipping Norton, Duck River, Alexandra Canal and Homebush Bay. The highest mean concentrations of Group A pesticides, particularly chlordane and dieldrin, were detected in fish from Alexandra Canal, Cooks River and Duck River. The highest mean concentrations of DDT were found in the tissues of fish from Prospect Creek. The fish species which accumulated contaminants to concentrations exceeding the relevant MRLs were yellow-fin bream (Alexandra Canal, Cooks River, Saltpan Creek, Duck River, Homebush Bay and Parramatta River), sea mullet (Prospect Creek, Chipping Norton, Cooks River, Alexandra Canal, Saltpan Creek, Duck River, Homebush Bay and Iron Cove), pink-eye mullet (Prospect Creek) and silver biddy (Prospect Creek, Chipping Norton, Alexandra Canal and Iron Cove), whereas the luderick, fan-tail mullet and sand whiting had mean concentrations of contaminants below the relevant MRLs.

Udonella myliobati n. comb. (Platyhelminthes: Udonellidae) and Its Occurrence in Eastern Australia

A total of 2,571 of the copepod Caligus epidemicus Hewitt from the yellow-fin bream, Acanthopagrus australis Günther, was examined for udonellids. Calinella myliobati Guberlet, 1936 was found on 3.3% of male copepods and 10.1% of female copepods. A new combination, Udonella myliobati (Guberlet, 1936) is proposed and its description emended. It differs from the type species Udonella caligorum Johnston, 1835 (as described by Price [1938]) in having a short egg filament and a median genital aperture. Only 1 of 102 chalimus larvae was infected, by a single adult worm. Juvenile udonellids common on mature copepods, were not found on any of the chalimus larvae. The highest number of U. myliobati (Guberlet, 1936), including hatching and unhatched eggs on 1 copepod, was 159. Adult and juvenile worms were attached mainly along the margins of the carapace and to the genital complex, whereas the majority of the eggs were attached to the ventral surface of the genital complex. The transfer of an adult worm from a male copepod to a female copepod was observed while the copepods were copulating. The large number of udonellids on a small population of mostly female copepods from the Brisbane River suggested that this was an aging population of copepods compared to the copepods collected from a semi-enclosed pond.

Marine, Brackish, and Freshwater Fish Communities in the Vegetated and Bare Shallows of an Australian Coastal River

Fish communities found in the marine, brackish, and freshwater shallows of the Clarence River (New South Wales, Australia) were sampled over a 12-mo period to compare communities inhabiting bare and vegetated substrata. A total of 26,107 fish representing 57 species and 36 families was collected biomonthly suing a 6-mm mesh siene. Permanent residents (i.e., species found throughout the year) were primarily gobies (Gobiidae) and gudgeons (Eleotridae). Juveniles of many species found as adults in other parts of the river were seasonally abundant in the shallows. Most numerous was a small ambassid, the glassy perchlet (Ambassis jacksoniensis), the majority of which was collected from marine and brackish regions of the river. Firetail gudgeons (Hypseleotris compressus) were caught in highest numbers at the freshwater sites. Cluster analysis suggested that fish communities in widely separated vegetated sites were more similar to each other than to those in adjacent bare sites. Shallow vegetated (Zostera capricorni) habitats in the marine region of the river had greatest diversity and highest abundances of fish, particularly during recruitment periods. At these times, juveniles of many commercially important species were captured, including yellowfin bream (Acanthopagrus australis), sea mullet (Mugil cephalus), flat-tail mullet (Liza agentea), tarwhine (Rhabdosargus sarba), luderick (Girella tricuspidata), silver biddy (Gerres subfasciatus), and sand whiting (Sillago ciliata). The vegetated (Vallisneria gigantea) sites in the brackish region also had significantly more species and individuals during recruitment periods (spring) than bare sites. Although freshwater vegetated sites consistently had more individuals than freshwater bare sites, there were no significant differences in species richness between vegetated and bare habitats. The need to conservatively manage shallow-water fish habitats is stressed. *** DIRECT SUPPORT *** A01BY073 00004

Changes in monogenean and copepod infestation on captive Acanthopagrus australis (Sparidae)

Infection levels by 17 species of ectoparasite on 491 yellowfin bream collected throughout 1990 from Moreton Bay, S.E. Queensland were compared to infections on 82 bream collected during the same period from a captive population in a large pond at Sea World, Gold Coast, Queensland. There was a significant increase in prevalence and/or intensity of monogeneans Lamellodiscus acanthopagri and Polylabroides multispinosus, but a decrease in the monogenean Anoplodiscus australis and the copepods Ergasilus australiensis, Lernanthropus atrox, Bomolochus stocki and Alella macrotrachelus on captive fish. Twenty‐eight bream collected from the pond during autumn were placed in an experimental 1 m3 cage within the pond for 4–6 weeks. Compared to the baseline data for the pond, the caged fish showed increased prevalences of the monogeneans Lamellodiscus acanthopagri, L. squamosus, L. major and Haliotrema spariensis, and increased intensities of L. squamosus, Allomurraytrema robustum and P. multispinosus. The copepod Caligus epidemicus dropped off in preservative, but its abundance (average number per fish) was significantly higher on caged fish than on pond or wild fish. Increased infection levels by L. acanthopagri and A. robustum were due in part to autoinfection. The large skin area promoted large populations of C. epidemicus.

Commercial and recreational harvest of fish from two Australian coastal rivers

Commercial and recreational harvests of fish from two eastern Australian coastal rivers, the Richmond and the Clarence, have been compared for the period from March 1988 to May 1989, using commercial fisheries statistics and a roving creel census of daytime recreational anglers. For both rivers, sea mullet (Mugil cephalus) made up about 70% of the commercial catch by weight but was not caught in significant numbers by anglers. Fish species harvested in large numbers by both sectors were yellowfin bream (Acanthopagrus australis), dusky flathead (Platycephalus fiscus), luderick (Girella tricuspidata), sand whiting (Sillago ciliata), mulloway (Argyrosomus hololepidotus) and tailor (Pomatomus saltatrix). For the Richmond River, yellowfin bream, dusky flathead and tailor were harvested principally by recreational anglers; sand whiting were equally shared between fishing sectors, and harvests of luderick and mulloway were dominated by commercial fishers. For the Clarence River, which supports one of the largest estuarine-based commercial fisheries in Australia, tailor were harvested principally by recreational anglers; yellowfin bream, dusky flathead and mulloway were equally shared between fishing sectors, and sand whiting and luderick were taken predominantly by commercial fishers. Although data on recreational fishing throughout Australia are limited, anglers now appear to be the dominant harvesters of several estuarine fish species. As in other parts of the world, the numbers of recreational anglers in Australian waters appear to be increasing, and this situation is likely to lead to both heightened conflict between the user groups and increased exploitation of a limited resource. The need for sustained and cost-effective monitoring of recreational angler effort and harvests is emphasized.

Observations on the pigmentation of the monogeneans, Anoplodiscus spp. (family Anoplodiscidae) in different microhabitats on their sparid teleost hosts

The monogeneans Anoplodiscus australis and A. cirrusspiralis infect the fins of their respective hosts, the yellowfin bream, Acanthopagrus australis, and the snapper, Pagrus auratus. In these habitats the adult parasite contains melanin-like pigment deposits within the body. Very young parasites lack the pigment but deposits of pigment appear and coalesce into dense deposits within the parasite as it grows. The evidence suggests that the pigment is synthesized within the body of the parasite rather than acquired through feeding on the host epidermis. However, adults of Anoplodiscus cirrusspiralis from the nares of snapper lack this pigment, which suggests that environmental cues may promote the production of pigment as a possible aid to camouflage.

Cortisol content of eggs and larvae of teleosts

The whole-animal content of the cortisol was measured in embryos and larvae of tilapia ( Oreochromis mossambicus), rainbow trout ( Oncorhynchus mykiss), ayu ( Plecoglossus altivelis), milkfish ( Chanos chanos), and yellowfin bream ( Acanthropagrus latus) by radioimmunoassay following the validation of an extraction method. The total cortisol content in tilapia was 50.3 ± 19.1 pg immediately following fertilization, then decreased abruptly and maintained a lower level of 10–17 pg until larval hatching; after hatching the cortisol content increased to 47.2 ± 11.9 pg by the seventh day. Newly hatched rainbow trout had 60.3 ± 6.4 pg cortisol and then increased their cortisol level slowly to 83.0 ± 7.2 pg by the fifth day after hatching. Ayu larvae contained 5.2 pg cortisol immediately following hatching. On the other hand, pelagic milkfish revealed a much lower cortisol level, being undetectable from hatching until the second day and ranging from 0.4 to 3.7 pg from the third to seventh day after hatching. Yellowfin bream, demonstrating a similarity to milkfish, were not found to have any detectable cortisol from hatching until the third day, but presented 1.6–7.7 pg from the fifth to seventh day after hatching. The presence and clearance of cortisol during early development of fertilized eggs of tilapia suggest a maternal origin of the hormone. The amount of cortisol deposited in the larval body of tilapia increased after hatching from 25% to nearly 100% of the total cortisol in whole larvae, while that in the larval yolk sac decreased to an undetectable level, implying that the increased cortisol may be produced or secreted by the larva. The possible role of cortisol in larval development is discussed.

Factors affecting the activation, motility and cryopreservation of the spermatozoa of the yellowfin bream, Acanthopagrus australis (Günther)

Abstract. The effects of varying temperature, salinity and pH on the activation and subsequent motility of sperm of the yellowfin bream, Acanthopagrus australis (Gunther), were assessed using a linear scale based on the overall activity of the sperm over time. Motility half‐life was calculated using log transformation. Conditions reflecting the natural habitat of the fish, oceanic salinity (1200 mOsM) and slight alkalinity (pH 8‐8‐5), were shown to produce both maximum activation and subsequent motility duration. The half‐life of activated sperm was shown to be greater at 4°C than at 20‐23°C (5·13 min:22·8 min). Storage of freshly stripped semen was shown to be most successful at 4°C with a half‐life of 98·9 min. The cryopreservation of semen was tested using the cryoprotectants glycerol and dimethyl sulphoxide at concentrations ranging from 0·5M to 2·0M, and pre‐freezing equilibration times of 5 and 15 min. Glycerol at 20 M was shown to give significantly superior results. There was no significant difference between sperm activation or sperm half‐life for fresh‐stripped semen and frozen semen, using glycerol at 2·0M as the cryoprotectant. Copyright

Aphanomyces species associated with red spot disease: an ulcerative disease of estuarine fish from eastern Australia

Abstract. Broad, non‐septate, fungal hyphae are a constant finding in the necrotising dermatitis lesions and dermal ulcers characteristic of red spot disease. During a one‐year period, fungi with hyphae morphologically consistent with those seen in histological sections of lesions were recovered from 27 out of 28 lesions on sea mullet, Mugil cephalus L., yellowfin bream, Acanthopagrus australis (Owen), and sand whiting, Sillago ciliata Cuvier, collected from three widely separated river systems in eastern Australia. Cultural and morphological characteristics, including spore formation, of 24 of these isolates were examined. In addition, tolerances to different salinities, growth on different media and growth at different temperatures were studied for three representative isolates, one from each species of fish. These studies suggest that all isolates belonged to a single species within the genus Aphanomyces. Their role in the pathogenesis of red spot disease, and their relationship with other Aphanomyces spp. affecting fish, is discussed.

Histopathology of leech, Austrobdella bilobata Ingram, infestation on the yellowfin bream, Acanthopagrus australis (Günther), in northern New South Wales

Abstract. The histopathology of leech, Austrobdella bilobata, infestation on the upper palate and pelvic fin of the yellowfin bream, Acanthopagrus australis (Günther), is described. The feeding site was often haemorrhagic and swelling surrounded the attachment site. Histological examination revealed massive cellular infiltration (predominantly lymphocyte and macrophage) in the submucosa beneath the feeding and attachment sites. The latter also had a complete erosion of the integument beneath the parasite. Localized proliferation of connective tissue fibres and zones of necrosis and oedema were present in some cases.

The reproductive cycle of yellowfin bream, Acanthopagms australis (Günther), with particular reference to protandrous sex inversion

Yellowfin bream, Acanthopagrus australis, of all age classes were collected from Moreton Bay, Australia. The species possessed typical sparid ovotestes in which the testis and ovary occur in separate zones. During the spawning period (June‐August) juveniles, functional males and functional females could be distinguished by the macroscopic appearance of the gonad. The sex ratio of males to females decreases with age, indicating protandrous sex inversion.Histological and structural study of the ovotestis showed all fish have previtellogenic cells in the ovarian zone but only juvenile and male fish have developing spermatogenic cells in the testis. Most juveniles become functional males by the age of two years but a small proportion of juveniles develop directly into functional females (primary females). Protandrous sex inversion commences after the spawning period when male fish appear with spermatozoa and no other spermatogenic cells in the testis. During the period November‐January male fish with no spermatogenic cells are common and a reduction in size of the testis occurs so that by March‐April the ovotestis becomes structurally and histologically similar to the female ovotestis. Some fish remain functional males during their whole life‐history (primary males). In functional females vitellogenic cells are present in the ovary only during the spawning period and the testis remains very small in size.

Relations between migration, reproduction and nutrition in yellowfin bream Acanthopagrus australis

A.a. est un poisson d'estuaire; une large proportion de la population adulte ne participe pas a la migration reproductrice annuelle vers la mer. On etablit un rapport entre le cycle de developpement des gonades et l'activite alimentaire d'une part, l'activite migratoire d'autre part (Moreton Bay, Australie, mars-oct. 1982)

Hybridization between the estuarine fishes yellowfin bream, Acanthopagrus australis (Gunther), and black bream, A. butcheri (Munro) (Pisces : Sparidae)

The status of bream, from estuaries near Narooma on the south coast of New South Wales, which display morphological characteristics intermediate to those distinguishing the species A. australis and A. butcheri was investigated. The two species were separated electrophoretically at a malate dehydrogenase locus (Mdh-1) where they are fixed for alternative alleles, and by a significant difference in allele frequency at the lactate dehydrogenase locus Ldh-A. Because four individual bream, which displayed intermediate morphometric characteristics, have inherited both species-specific Mdh-1 alleles and are heterozygous at the Ldh-A locus, their status as hybrids is determined. The presence of three bream with allelic characteristics of A. australis at the Mdh-1 locus but allelic characteristics of A. butcheri at the Ldh-A locus is evidence for the presence of later generation or backcross hybrids. No evidence of introgression was found. The reproduction of A. australis and A. butcheri is temporally and spatially isolated under normal circumstances but in some landlocked, coastal lakes there is a breakdown of these isolating mechanisms. Because A. australis and A. butcheri differ morphometrically and ecologically over their sampled ranges, and only appear to hybridize in the unusual environmental conditions of closed lakes, it is suggested that their specific status be retained. Using electrophoretic data, a high level of genic similarity (I = 0.91) was found to exist between the two species. This, together with the close relationship indicated by the production of fertile F1 hybrids, suggests a recent speciation.

The seasonal occurrence of postlarval stages of yellowfin bream, Acanthopagrus australis (Gunther), and some factors affecting their movement into an estuary

Planktonic postlarvae of yellow fin bream Acanthopagrus australis were found at surf bar entrances to Moreton Bay, a large subtropical estuarine system on the east coast of Australia. The postlarvae occurred from April to September with peak abundance during July to September. Most postlarvae entered the estuary at night during the full moon. They were more abundant on the flood and full tides than on the ebb tide. The transition from planktonic to littoral existence occurs from August to November when the fish are 13‐14 mm in length. Young yellow fin bream in littoral areas reach a modal length in excess of 100 mm after 1 year.