Black Bream Research Articles

Influence of freshwater flows on the distribution of eggs and larvae of black bream Acanthopagrus butcheri within a drought‐affected estuary

This study tested the hypothesis that variable freshwater flow in the Gippsland Lakes, Australia, influences the location and extent of environmental conditions suitable for spawning and larval development of black bream, Acanthopagrus butcheri. Freshwater flow had a large influence on the salinity and level of stratification (difference between bottom and surface salinity) in the Gippsland Lakes. Freshwater flows throughout this study varied from no or low flows through to major flooding in 2007. Eggs of A. butcheri were found in similar concentrations throughout the lakes and rivers. More than 99% of larvae, however, were collected from within rivers, with very few larvae being collected from the lakes. A comparison of two spawning seasons revealed that the year with higher freshwater flows also had greater spawning activity and higher concentrations of larvae. Interestingly, there was a significant relationship between the distribution of eggs and larvae with the level of stratification. The highest concentrations of larvae occurred at sites with a difference in bottom and surface salinities of 15-20. This study demonstrates that despite A. butcheri spawning in the lakes and rivers, it is only locations with a halocline that function as larval nursery habitat.

The influence of freshwater inflows on spawning success and early growth of an estuarine resident fish species, Acanthopagrus butcheri

The influence of freshwater inflows and salinity on spawning success of black bream Acanthopagrus butcheri (Sparidae) was investigated over 2 years in a small estuary on the east coast of Tasmania, Australia. The individual spawning seasons experienced quite different freshwater inflows; 2004-2005 was characterized by low flows throughout the season whereas during 2005-2006 there were three relatively large discharge events in the first part of the season. Macroscopic gonad staging of adults was used to define the spawning season and daily increment analysis of otoliths from recently settled recruits was used to backcalculate spawning dates. Gonad staging indicated that adults were in spawning condition over a 3 to 4 month period during spring and summer. The timing and duration of successful spawning, however, differed markedly between years and was linked to the timing of freshwater inflows and salinity conditions, with successful spawning occurring during periods of low freshwater discharge and when salinities in the upper estuary were above c. 15. Growth rates of the recently settled recruits did not differ between years, nor did the timing of spawning within the season influence growth rates. While the latter finding was unexpected, especially given within season temperature variability, these results imply that by the onset of winter earlier spawned fish would be larger than later spawned individuals, potentially conferring advantages for survival and competition for food. Climate change predictions for eastern Tasmania indicate a decrease in river flows in spring and an increase during summer, potentially increasing environmental variability between and within years, with implications for spawning success and subsequent recruitment.

Ploidy level, morphological characteristics and performance of interploid hybrids by Megalobrama amblycephala♀× allo-tetraploids ♂

Interploid hybrids were produced by mating allo-tetraploid males, an interspecific tetraploidy induced by blunt snout bream (Megalobrama amblycephala) ♀× black bream (Megalobrama terminalis) ♂, with blunt snout bream females. The fertilization rate was high in the interploid hybrids. Although significant (P<0.05) mortalities were observed at hatching, swim-up and 30-day stages, respectively, the survival percentage of the interploid hybrids became stabilized after the 60-day stage. In different interploid hybridization batches, 64–97% of juveniles were identified as 3n and 1–36% as 2n. Less than 2% of tetraploids were obtained in these interploid hybridization batches. The daily growth rate of the interploid 3n increased 7.5%, compared with that of diploid blunt snout breams during the 180-day culture. Moreover, a method using seven morphological parameter ratios was set up with a relatively high classification accuracy of 93.8% to harmlessly discriminate interploid 3n from allo-tetraploids, black bream and blunt snout bream.

Temperature and diet affect carbon and nitrogen isotopes of fish muscle: can amino acid nitrogen isotopes explain effects?

Stable isotope ratios of carbon ( δ 13C) and nitrogen ( δ 15N) are widely used in food-web studies to determine trophic positioning and diet sources. However in order to accurately interpret stable isotope data the effects of environmental variability and dietary composition on isotopic discrimination factors and tissue turnover rates must be validated. We tested the effects of temperature and diet on tissue turnover rates and discrimination of carbon and nitrogen isotopes in an omnivorous fish, black bream ( Acanthopagrus butcheri). Fish were raised at 16 °C or 23 °C and fed either a fish-meal or vegetable feed to determine turnover rates in fish muscle tissue up to 42 days after exposure to experimental treatments. Temperature and diet affected bulk tissue δ 15N turnover and discrimination factors, with increased turnover and smaller discrimination factors at warmer temperatures. Fish reared on the vegetable feed showed greater bulk tissue δ 15N changes and larger discrimination factors than those reared on a fish-meal feed. Temperature and diet affected bulk tissue δ 13C values, however the direction of effects among treatments changed. Analyses of δ 15N values of individual amino acids found few significant changes over time or treatment effects, as there was large variation at the individual fish level. However glutamic acid, aspartic acid and leucine changed most over the experiment and results mirrored those of treatment effects in bulk δ 15N tissue values. The results demonstrate that trophic discrimination for δ 15N and δ 13C can be significantly different than those typically used in food-web analyses, and effects of diet composition and temperature can be significant. Precision of compound-specific isotope analyses (0.9‰) was larger than our effect size for bulk δ 15N diet effects (0.7‰), therefore future experimental work in this area will need to establish a large effect size in order to detect significant differences. Our results also suggest that compound-specific amino acid δ 15N may be useful for determining essential and non-essential amino acids for different animals.

Temporal stability of a hybrid swarm between the migratory marine and estuarine fishes Acanthopagrus australis and A. butcheri

We predict estuaries to be hotspots of hybridisation between migratory marine and estuary-restricted species, although hybridisation rates may vary in space and time, reflecting the dynamic nature of estuaries and potentially widespread but erratic dispersal of marine taxa. Within estuaries, genotype frequencies may reflect past hybridisation events, with genetically intermediate and backcrossed individuals contributing to persistent hybrid swarms. In southeastern Australia, hybridisation has occurred between estuarine black bream Acanthopagrus butcheri and marine yel- lowfin bream A. australis, but it is unclear whether this reflects a contemporary process. We recently found that, within lakes and lagoons at the southern range limit of A. australis, hybrids were abun- dant and A. butcheri extremely rare, and surprisingly, we detected hybrids within a small sample of fish from the Gippsland Lakes, an estuary 250 km further south. In the present study, we compare the genotypic composition of the contemporary Gippsland Lakes population of Acanthopagrus spp. with the historical composition revealed by analysis of museum specimens. The genetic makeup of sam- ples varied little over time, with ancestral A. butcheri virtually absent, and most introgressed individ- uals matching expectation for later-generation hybrids or A. butcheri backcrosses, suggesting that the lakes have supported persistent hybrid swarms. At each sampling time, the samples were genet- ically diverse, as measured by mean number of alleles per locus, which ranged from 8.2 to 9.2, and expected heterozygosity (He), which ranged from 0.66 to 0.70; however, we detected little allelic dif- ferentiation (FST = 0.003) across sampling times. Our data imply that introgressed populations of Acanthopagrus spp. are more widespread and persistent than previously predicted.

Highly variable recruitment in an estuarine fish is determined by salinity stratification and freshwater flow: implications of a changing climate

Estuarine-dependent fish live in a complex and dynamic environment that is primarily influenced by the salinity structure resulting from the mixing of fresh and marine waters. In response to this variable environment, the recruitment of estuarine-dependent fish can also be highly variable. Recruitment variability in black bream Acanthopagrus butcheri in Gippsland Lakes, southeastern Australia, was determined both from annual recruitment monitoring and the age structure of the pop- ulation. Recruitment was found to be episodic, with the population dominated by a few year classes. For the 2 methods of estimating recruitment and associated time periods, we found a consistent lin- ear relationship between water column stratification (difference between surface and bottom salin- ity) and recruitment. There was also a significant, non-linear (dome-shaped) relationship between freshwater flow and recruitment. This relationship suggested that the highest recruitment occurred at intermediate flows greater than ~3000 Ml d -1 in the main tributary rivers. The combination of strati- fication and freshwater flow in a multiple regression model improved the variability in recruitment explained to 71 and 79%, respectively, for the 2 methods. Analysis of age structure of bream from other Victorian estuaries showed that, as in Gippsland Lakes, recruitment was highly variable and episodic; however, the timing of strong and weak year classes varied between estuaries. This sug- gests that freshwater flow and salinity structure, and consequent recruitment variability, are unique to each estuary based on characteristics of catchment, channel topography, and entrance opening and closing. Climate change predictions for the Gippsland Lakes region include less rainfall and higher evaporation, potentially leading to higher salinities and lower stratification in Gippsland Lakes. The bream population is therefore likely to be negatively affected, and these effects may be exacerbated by any human activities in the catchment that further reduce freshwater flows or increase marine incursion.

The influence of tidal cycles and freshwater inflow on the distribution and movement of an estuarine resident fish Acanthopagrus butcheri

Movement patterns and habitat utilization by black bream Acanthopagrus butcheri (Sparidae), an estuarine resident species, were investigated using acoustic telemetry in a small estuary on the east coast of Tasmania, Australia. Thirty-four adult A. butcheri were tracked for periods of up to 187 days between August 2005 and January 2006. Although able to tolerate a wide range of salinities, the fish spent most of the time within the upper and middle regions of the estuary, where brackish conditions dominated. The species exhibited extensive movements linked to tidal cycles, with small-scale upstream movements during incoming tides and downstream movements during out going tides. The extent of these movements was positively correlated with the tidal height difference between consecutive tidal peaks and troughs. Freshwater inflows and resultant changes in salinity also significantly influenced distribution and movement patterns. Fish moved downstream during the periods of heavy inflows, returning upstream as salinities increased to c. >10. During the peak of spawning period (November to December) fish moved into the upper region of the estuary, where they aggregated to spawn. Periodic increases in freshwater discharge, however, resulted in fish leaving the spawning grounds and moving downstream. Towards the end of the spawning season (January), the fish became more dispersed throughout the entire estuarine system.

Identification of the adherent microbiota on the gills and skin of poly-cultured gibel carp (Carassius auratus gibelio) and bluntnose black bream (Megalobrama amblycephala Yih)

PCR-denaturing gradient gel electrophoresis (DGGE) was applied to analyse the microbial community attached to the gills and skin of poly-cultured gibel carp (Carassius auratus gibelio) and bluntnose black bream (Megalobrama amblycephala Yih) and compare these results with those detected in the rearing water. The microbiota discussed included bacteria, fungi and a specific bacterial taxa of actinomycetes was also analysed. Proteobacteria, Firmicutes, Actinobacteria, Cyanobacteria, Ascomycota, Basidiomycota and some unclassified microbiota were identified. Based on our results, we concluded that: (1) the adherent bacterial/fungal communities on the gills and skin were different from those in the rearing water, (2) the bacterial/fungal diversities on fish gills were lower than that on fish skin, (3) the adherent bacterial/fungal communities on gill and skin of gibel carp were different from that of bluntnose black bream and (4) the adherent actinomycetal community showed certain similarity between the skin of different hosts. Based on our conclusions, we suggested that the topic investigated in the present study merits further investigations.

Gamete compatibility between marine and estuarine Acanthopagrus spp. (Sparidae) and their hybrids

On Australia's south-east coast, hybridization between estuary-restricted black bream Acanthopagrus butcheri Munro and its migratory coastal congener yellowfin bream Acanthopagrus australis (Günther) has led to estuarine populations largely composed of hybrids that are most genetically similar to A. butcheri. The fertilization success achieved when ova of estuary-caught A. butcheri were fertilized with the cryogenically preserved sperm of either ocean-caught A. australis or estuary-caught A. butcheri-like was compared. The experimental crosses, which by chance included both pure parental and hybrid bream, revealed no evidence that gametic incompatibility provides a barrier to fertilization among both pure species and their hybrids.

Undersized Acanthopagrus butcheri Caught and Released from Commercial Gill Nets Show High Survival Rates in the Gippsland Lakes, Southeastern Australia

AbstractA substantial decline in the commercial catches and catch rates of black bream Acanthopagrus butcheri in the Gippsland Lakes, Australia, signified a depletion of the stock and prompted management to increase the legal minimum length (LML) from 26 cm to 28 cm (total length) in December 2003. The effectiveness of this increase depends on the postrelease survival rate of undersized fish. The present study estimates the rates of release and initial survival (1 h after capture) of undersized A. butcheri during commercial fishing by gill nets in the Gippsland Lakes. Field capture and holding trials were also used to estimate initial and delayed survival (72 h after capture). Almost 6,000 A. butcheri from 347 gill net shots were caught during 2005–2006. The average total length increased by 1 cm from 2005 to 2006, increasing the proportion of the catch that was retained in 2006 (75%) compared with 2005 (50%). The best estimates of initial and delayed survival were 97.2% (SE, 0.3%) and 94.4% (SE, 3.8%), respectively. The total survival rate, which combines the initial survival rate from observer work with the delayed survival rates from the trials, was estimated as 90.8% (SE, 3.8%). The increase in the LML from 26 to 28 cm afforded significant protection to fish between these size limits, but the level of protection will be short term (1 or at most 2 years) for each year‐class because of the growth rate of the species. Released, retained, and postrelease survival rates are discussed for various LMLs and with regard to their implications for fishery management.

Interactive effects of food quality, temperature and rearing time on condition of juvenile black bream Acanthopagrus butcheri

A laboratory experiment was conducted to determine the interactive effects of temperature and diet on condition indices of juvenile black bream Acanthopagrus butcheri, reared for time periods ranging from 2 to 42 days. After fish were reared for varying periods, growth, morphometric (Fulton's K) and biochemical [RNA:DNA (R:D) ratios] indices were measured. Fulton's K responded primarily to temperature, with progressive decrease in condition over time for fish reared at high temperatures. In contrast, R:D ratios were primarily affected by diet composition, with the highest values observed for fish reared on fish-based diets as opposed to vegetable-based diets. Significant effects of rearing time were also observed for Fulton's K and R:D ratios, as were some interactive treatment effects. In addition, Fulton's K and R:D ratios were not significantly correlated, perhaps due to the different periods of time integrated by each index or their relative sensitivity to lipid and protein deposition. These results highlight the complex responses of these condition indices to environmental variables and nutritional status.

Implications of Age, Size and Region on Mercury Contamination in Estuarine Fish Species

This study investigated the effects of age and length on mercury contamination in four fish species; yellow-eye mullet (Aldrichetta forsteri), black bream (Acanthopagrus butcheri), sand flathead (Platycephalus bassensis) and sea-run brown trout (Salmo trutta) from the Derwent Estuary, Tasmania, Australia, and examined the implications of these findings for public health monitoring. Mean mercury levels exceeded the Food Standards Australia and New Zealand maximum permitted level (0.5 mg kg �1 ) for all species except yellow-eye mullet. Mean levels in black bream were significantly higher (p<0.05) than other species and consequently are a particular concern for human health. Regional differences (p< 0.05) in mercury levels in sand flathead were not obviously correlated with metal levels in the sedi- ments. However, age and length significantly (p< 0.05) influenced mercury levels in brown trout and sand flathead, with age being more strongly related to intraspecies differences. In addition, movement and distribution within the estuary and trophic status appeared to be important factors in contribution to interspecific variation. Consequently, a sound under- standing of fish life history and biology is important in identifying species which may be susceptible to accumulating mercury and hence pose a potential threat to human health.

Marine genetic swamping: hybrids replace an obligately estuarine fish

Populations of obligately estuarine taxa are potentially small and isolated and may lack genetic variation and display regional differentiation as a result of drift and inbreeding. Hybridization with a wide-ranging marine congener should introduce genetic variation and reduce the effects of inbreeding depression and genetic drift. However, high levels of hybridization can cause demographic and genetic swamping. In southeastern Australia hybridization occurs between obligately estuarine Black bream (Acanthopagrus butcheri) and migratory marine Yellowfin bream (Acanthopagrus australis). Here, we surveyed genetic variation at eight microsatellite loci and the mitochondrial control region of juvenile fish from five coastal lagoons (including temporal replication in two lagoons) (total n = 970) to determine the frequency and persistence of hybridization, and its likely consequence for the estuarine restricted A. butcheri. Of 688 juvenile fish genotyped 95% were either A. australis (347) or hybrids (309); only 5% (32) were A. butcheri. Most hybrids were later generation hybrids or A. butcheri backcrosses, which are likely multi-generational residents within lagoons. Far greater proportions of hybrid juveniles were found within two lagoons that are generally closed to the ocean (>90% hybrid fish within generally closed lagoons vs. 12-27% in permanently or intermittently open lagoons). In both lagoons, this was consistent across multiple cohorts of fish [79-97% hybrid fish (n = 282)]. Hybridization and introgression represent a major threat to the persistence of A. butcheri and have yet to be investigated for large numbers of estuarine taxa.

Factors affecting mark quality of alizarin complexone‐stained otoliths in juvenile black bream Acanthopagrus butcheri and a prescription for dosage

The relationships between otolith score, a function of increasing stain visibility, and the ratio of alizarin complexone (mg) to fish biomass (g) for 0.1, 0.6 and 7 g black bream Acanthopagrus butcheri show that acceptable marks are produced in all size classes using a stain-to-biomass ratio of 3.3 mg g(-1) following an immersion period of 24 h.

Of Droughts and Flooding Rains: Local and Institutional Perceptions of Environmental Change in an Australian Estuary

In 2003 when the commercial catch of Acanthopagrus butcheri (black bream) dropped to its lowest point since the 1970s, the commercial fishers of the Gippsland Lakes in Australia's southeast maintained that it was too soon to tell if the stock was in trouble. The fishers did not consider the decline fundamentally different from other changes that had occurred over the last 100 years. It seemed to the representatives of the fisheries department, however, that fishers were denying that a change had occurred. The divergent interpretations were, in part, a result of divergent frameworks through which the passage of time was conceptualized. I show that the temporal frames of reference that commercial fishers use to judge the state of the environment are out of sync with institutional orientations that focus on smaller temporal intervals.

A novel approach to controlling dissolved oxygen levels in laboratory experiments

Hypoxia is a widespread environmental stressor that affects marine, estuarine and freshwater systems worldwide. Investigating the effects of hypoxia on aquatic animals in the natural environment is difficult and expensive. Laboratory experiments provide an alternative that allows manipulation of environmental variables and simulation of altered conditions that are expected in the future. However, controlling dissolved oxygen (DO) levels precisely in laboratory test situations is challenging and generally costly and time intensive. In this paper, we describe a novel chamber design that is capable of maintaining low DO levels precisely for a period of at least 4 days with minimal re-adjustment and nitrogen gas requirement. The system is simple, inexpensive and easy to use, and offers the additional benefit of being portable. The utility of this design is demonstrated with an experiment to test the effects of hypoxia and high salinity on hatch rates and yolk-sac larvae survival in the estuarine fish black bream Acanthopagrus butcheri (Munro, 1949). The results show that early life-stage black bream are sensitive to hypoxia and increased salinity, indicating that deterioration in estuarine environmental conditions may have negative effects on spawning success and recruitment to fish populations. This new laboratory technique will be useful for testing future scenarios for the estuaries of southern Australia as predicted by climate change models. Other potential applications and modifications for further development of this new technique are discussed.

The value of stress protein 70 as an environmental biomarker of fish health under field conditions

Hsp70 protein in three tissue types (gill, liver, and muscle) from black bream (Acanthopagrus butcheri) collected in a highly variable estuarine environment was investigated to determine which tissue provides better intersite discrimination. The usefulness of hsp70 expression to identify anthropogenic stress under field conditions was evaluated. Intersite differences were detected in hsp70 levels in gill and white muscle of black bream while liver showed no spatial difference. There was high interfish variability in hsp70 levels in each tissue group. A post hoc power analysis of the datasets for each tissue found that in black bream, white muscle provided the best discriminatory power to elucidate spatial variability. Only 11 fish per site are required to identify significant intersite differences in white muscle whereas for gill and liver tissues 14 and 21 fish per site, respectively, would be required. Because of high intertissue and interindividual variability, field measurement of hsp70 should be complemented by evidence of changes in other biomarkers of fish health.

Habitat utilisation and movement of black bream Acanthopagrus butcheri (Sparidae) in an Australian estuary

Acoustic telemetry was used to document patterns of movement by black bream Acan- thopagrus butcheri (Munro, 1949) throughout Australia's largest estuary, the Gippsland Lakes. Forty- four fish were surgically implanted with acoustic transmitters and monitored over 12 mo (November 2005 to October 2006). Fish moved throughout the Gippsland Lakes at average rates of 8.7 km d -1 over 12 mo, with some fish moving distances of up to 2600 km. Fish frequently moved among the major estuarine rivers (Tambo, Mitchell and Nicholson rivers), sometimes moving up to 30 km in a day. Fish use of the rivers, river entrances and lakes varied strongly with the time of year. Fish spent more time in the lakes than rivers in late summer and early autumn, but began to use the rivers more than the lakes at the end of autumn. River use was greatest in early to mid-winter, then gradually decreased through spring. Fish also spent more time in some rivers than others, with use of their respective entrances peaking during transition phases when fish were moving from the rivers to the lakes and vice versa. Time of day was a weak predictor of regional patterns of fish use, but during the transitional phases (March through May) fish use of lakes was greater at night, while use of rivers was greater during the day. Monthly variation in time spent by fish in particular rivers varied posi- tively with the discharge of freshwater (with a concomitant negative relationship between lake use and overall river discharge).

Population growth and mass mortality of an estuarine fish, Acanthopagrus butcheri, unlawfully introduced into an inland lake

Abstract In 2006, two periods of hypoxia resulted in the death of approximately 35 tonnes of black bream (Acanthopagrus butcheri) in Lake Indoon, a small inland lake in Western Australia. Acanthopagrus butcheri was the first fish species to be recorded in this lake, along with the mosquitofish (Gambusia holbrooki) which was also observed during sampling in 2006. Acanthopagrus butcheri appears to have been introduced to Lake Indoon between 1998 and 2003 and formed a self‐sustaining population. It is believed to have been deliberately introduced for the purpose of creating a recreational fishery, despite the existence of substantial penalties for illegal translocation of fish in Western Australia. Recent human‐induced environmental changes, including rising groundwater and salinization, have probably aided the establishment of both species in Lake Indoon. The importance of salinity to recruitment success by A. butcheri was indicated by the presence of only two age classes in 2006, with estimated recruitment dates coinciding with the years of highest recorded salinity in the lake. The ‘fish kills’ provided an opportunity to examine aspects of A. butcheri biology in a relatively low salinity environment which is atypical for this estuarine species. In particular, the recruitment period in Lake Indoon was delayed until autumn/winter, rather than spring/summer as seen in other populations. Biological responses in Lake Indoon have implications for natural populations living in estuaries with modified salinity regimes. The ecological, social and economic impacts potentially arising from the introduction of fish to Lake Indoon, which is an important migratory bird habitat and a recreational amenity for local residents and tourists, illustrate the complexities of fish translocation and the need for rigorous assessment before stocking to identify potential costs and benefits. Copyright © 2008 John Wiley &amp; Sons, Ltd.

The influence of ontogeny and light environment on the expression of visual pigment opsins in the retina of the black bream,Acanthopagrus butcheri

The correlation between ontogenetic changes in the spectral absorption characteristics of retinal photoreceptors and expression of visual pigment opsins was investigated in the black bream, Acanthopagrus butcheri. To establish whether the spectral qualities of environmental light affected the complement of visual pigments during ontogeny, comparisons were made between fishes reared in: (1) broad spectrum aquarium conditions; (2) short wavelength-reduced conditions similar to the natural environment; or (3) the natural environment (wild-caught). Microspectrophotometry was used to determine the wavelengths of spectral sensitivity of the photoreceptors at four developmental stages: larval, post-settlement, juvenile and adult. The molecular sequences of the rod (Rh1) and six cone (SWS1, SWS2A and B, Rh2Aalpha and beta, and LWS) opsins were obtained and their expression levels in larval and adult stages examined using quantitative RT-PCR. The changes in spectral sensitivity of the cones were related to the differing levels of opsin expression during ontogeny. During the larval stage the predominantly expressed opsin classes were SWS1, SWS2B and Rh2Aalpha, contrasting with SWS2A, Rh2Abeta and LWS in the adult. An increased proportion of long wavelength-sensitive double cones was found in fishes reared in the short wavelength-reduced conditions and in wild-caught animals, indicating that the expression of cone opsin genes is also regulated by environmental light.