Ecology and physiology of forage fish species in the Murray Estuary and Coorong, South Australia

Author: Md Afzal Hossain

Hossain, Md Afzal, 2017 Ecology and physiology of forage fish species in the Murray Estuary and Coorong, South Australia, Flinders University, School of Biological Sciences

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Estuaries and coastal lagoons are the dynamic transition between marine, freshwater and terrestrial environments. As a result of drought and low freshwater input in the last decade the Murray Estuary and Coorong have become the largest degraded hyper-saline lagoon in Australia. This thesis investigates the changes in life history and ecology of three key forage fish species associated with environmental deterioration in the Coorong. Four studies were performed to investigate (1) the variation in fish assemblage structure, (2) fish growth performance, (3) fish feeding ecology and diet selection, and (4) physiological response of forage fish to changes in salinity under laboratory conditions. In study 1, the forage fish assemblages were characterised by greater abundance and dominance of small-mouthed hardyhead (Atherinosoma microstoma) in the South Lagoon; low abundance of sandy sprat (Hyperlophus vittatus) and Tamar goby (Afurcagobius tamarensis) in the North Lagoon, but complete absence of sandy sprat and Tamar goby in the South Lagoon. The spatial variation in distribution of forage fish is attributed to elevated salinity levels (Murray Estuary 2–30; North Lagoon 11–75 and South Lagoon 40–85) in the Coorong. This study indicates that the change of forage fish assemblage is mainly driven by salinity variation in the Coorong. In study 2, the estimated growth rates were 0.019 cm day-1 for small-mouthed hardyhead, 0.038 cm day-1 for Tamar goby and 0. 016 cm day-1 for sandy sprat. The length-weight relationship showed the variation of regression slope in small-mouthed hardyhead (b = 2.96), Tamar goby (b = 3.06) and sandy sprat (b = 3.1). Growth performance was mainly influenced by chlorophyll a, water transparency and salinity but to less extent by other environmental factors. This study indicates that environmental factors can significantly impact growth parameters of forage fish. Study 3 addresses food selectivity among forage fishes and reveals the dominance of crustaceans (amphipods, ostracods and harpacticoids) and the presence of nematodes and acanthocephalans in the gut of all forage fishes. Sandy sprat and Tamar goby showed high dietary overlap in the Murray Estuary while the diets of all three forage fishes were potentially overlapped in the North Lagoon. Prey abundance was temporally variable and predominantly regulated by salinity, pH, dissolved oxygen, water transparency and chlorophyll a in the Coorong. Study 4 detects the induction of stress as measured by reactive oxygen species using a range of salinity gradients overtime in forage fish. Salinity was found to affect superoxide dismutase activity in Tamar goby but not in small-mouthed hardyhead. Conversely, salinity altered catalase activity in small-mouthed hardyhead but not in Tamar goby. The study reveals that salinity stress occurs in Tamar goby but not in small-mouthed hardyhead. The current study indicates that the antioxidant response to stress varies with fish species. The overall findings of this thesis add new knowledge to improve our understanding of the impact of environmental variation on the population distribution, feeding ecology, growth performance and adaptation to physiological stress of small-bodied forage fish in the Coorong. This thesis contributes to the strategy of ecosystem management in an estuarine system of gradual environmental deterioration due to low freshwater input and protracted drought in southern Australia.

Keywords: estuary, forage fish, salinity, Murray Estuary, North Lagoon, zooplankton, gut content, Stress, antioxidants

Subject: Biological Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2017
School: School of Biological Sciences
Supervisor: Professor Jian Qin