Effects of environmental stressors on the black bream (Acanthopagrus butcheri)
Creators: name: Hassell, Kathryn
Publisher: name: RMIT University
Year: 2024

description: This PhD study investigated the effects of selected environmental stressors on aspects of reproductive biology in the native estuarine fish, black bream (Acanthopagrus butcheri). Through a series of laboratory-based experiments, and a field study, the effects of selected environmental stressors have been described and new techniques have been developed. Firstly, in order to distinguish normal development from abnormal development, the different stages of black bream embryo development were described and 20 developmental stages were identified. A series of experiments were then conducted to observe the effects of different environmental conditions on embryo and larval survival. Severe hypoxia (30%S) reduced one day survival by 60%, and caused abnormal embryo development that always resulted in mortality and thus hatch rates of 0%. Exposure to moderate hypoxia (45%S) delayed hatching by 7-8 h, reduced hatch rates by 25% and always produced deformed larvae that died within 24 h of hatching. In addition, other stressors such as altered salinity and temperature regimes were shown to have significant (p &lt; 0.005) interactive effects with dissolved oxygen. When embryos were exposed to altered salinity (hypo salinity or hyper salinity) and temperature in combination with moderate hypoxia, hatch rates were reduced by 5-50% and the rates of deformities increased. Exposure to low temperature (16oC) delayed hatching by 20 h compared to embryos maintained at 20oC whilst exposure to elevated temperature (26oC) resulted in hatching 12 h earlier than embryos maintained at 22oC. Time to hatching was more variable (spread over 1-3 days) in embryos exposed to a combination of stressors, and exposure to hypoxia also reduced survival of newly hatched larvae. A novel chamber system, the Controlled Atmosphere Chamber, was developed to control dissolved oxygen levels to enable the testing of hypoxia scenarios. In order to measure reproductive condition in mature female black bream, an enzyme-linked immunosorbent assay (ELISA) and Western blot assay were developed to measure the egg yolk precursor protein vitellogenin (Vtg), which circulates in the blood plasma. The techniques were then validated by measuring Vtg levels in blood plasma of black bream from a range of Victorian estuaries that are subject to different environmental stressors. There was one reference site, two sites in close vicinity to a large sewage treatment plant, and two urban sites subject to both industrial effluents and storm water runoff. The highest concentration of Vtg detected (measured as rainbow trout Vtg equivalent concentrations) was 59.5 mg/ml RT-VtgEq, in a 245 mm, 302 g, 3 year old fish (Stage 5 ovaries) from the Werribee River. The smallest female with detectable levels of Vtg (0.73 mg/ml RT-VtgEq) was a 170 mm, 86 g, 2 year old fish (Stage 3 ovaries) from the Yarra River. Environmental conditions, both manipulated in the laboratory and measured in different field locations were shown to affect both spawning success (early life stage survival) and reproductive condition in black bream. These findings indicate that black bream are sensitive to environmental stressors, and that wild populations are likely to be affected by environmental degradation, pollution and climate change.