The significance of stressors on Black Bream in the Gippsland Lakes
Creators: name: Ronalds, Trevor
Publisher: name: RMIT University
Year: 2024

description: Black bream (Acanthopagrus butcheri) occupy both the Gippsland Lakes and Lake Tyers, which are two adjacent estuarine systems in south-eastern Australia. The Gippsland Lakes have a more developed catchment than Lake Tyers and are permanently open to the sea. The aims of the project were to investigate the potential of black bream as an ecological indicator for chronic stress that may impact on it and, if found, to investigate some possible causes.<br><br>A behavioural test to identify acute stress response to 30 s air exposures such as experienced by catch-and-release fishing practices showed that both response time and subsequent mortality in Lake Tyers fish increased with number of exposures and time after exposure. The short-term behavioural test thus accurately predicted long-term mortality. The air-exposure test was used to compare responses in Gippsland Lakes and Lake Tyers fish in conjunction with confinement at different densities. The Gippsland Lakes fish had a significantly lesser proportion of fish with a response time of &lt;1 min and a greater proportion of larger fish with enlarged spleens liver and gonads than Lake Tyers fish. There was no difference with confinement density. Inadequate replication made conclusions tentative.<br><br>As a measure of chronic stress, Gippsland Lakes fish showed greater interrenal cell hyperplasia (but not cellular or nuclear hypertrophy) in the head kidney of wild-caught fish than those from Lake Tyers. Gippsland Lakes fish also had a lesser condition factor than Lake Tyers fish and green bile colour, suggesting lack of recent feeding, unlike the yellow bile colour of Lake Tyers fish. Taken together, this suggests greater chronic stress in Gippsland Lakes than Lake Tyers fish.<br><br>Otolith composition was investigated as a measure of environmental differences between fish from different estuaries using eelectron-dispersive X-ray spectroscopy Multivariate analysis of atom% for all elements and their ratios clustered otoliths from both estuaries together but atom% Cl, Sr, Cl:Ca and Sr:Ca were greater in otoliths from Gippsland Lakes fish than in those from Lake Tyers. This was as expected from the Gippsland Lakes being permanently open to the ocean (hence more Cl) and having a more developed catchment (hence more Sr)<br><br>The genetic composition of black bream was compared using microsatellite and mitochondrial primers. Gippsland Lakes and Lake Tyers samples were grouped separately by inter-simple sequence repeats, suggesting genetic differences. Restriction fragment length polymorphism of the mitochondrial D-loop suggested that only 9/47 samples from Gippsland Lakes fish and 4/47 samples from Lake Tyers were maternally A. butcheri, while the others were maternally yellowfin bream (A. australis) or hybrids.<br><br>Major outcomes from this study were (1) development of a short-term stimulus-response response test that predicted longer-term mortality, (2) evidence for catch-and-release fishing rendering undersized black bream more likely to die after repeated air exposure and (3) evidence for greater chronic stress in Gippsland Lakes than in Lake Tyers black bream, which could be due to differences in either or both of environment and genetics.