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Biological characteristics of Acanthopagrus butcheri. Changes following environmental degradation and comparisons of restocked and wild fish

Cottingham, AlanORCID: 0000-0002-4157-1972 (2015) Biological characteristics of Acanthopagrus butcheri. Changes following environmental degradation and comparisons of restocked and wild fish. PhD thesis, Murdoch University.

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The Black Bream Acanthopagrus butcheri, which is the subject of this thesis, completes its life cycle within estuaries and has particularly plastic biological characteristics. The overarching aims of the thesis were 1) to determine the ways in which A. butcheri has responded to marked environmental changes in the Swan River Estuary and 2) to track the biological performance of restocked A. butcheri in the Blackwood River Estuary in which the stock of this species had become depleted.

Between the early 1990s and mid-2000s, the deeper waters of the upper Swan River Estuary became increasingly hypoxic, due to a reduction in the flushing of nutrients and organic material from the estuary as a result of declining rainfall and thus freshwater discharge. Over that same time period, the condition, growth and length at maturity of female and male A. butcheri declined and the age at maturity increased. Furthermore, catch data implied that, during that period, the larger A. butcheri showed a marked tendency to move from the deeper hypoxic waters into the shallows, occupied by smaller A. butcheri. This habitat compression led to greatly increased densities in the shallows. It is therefore proposed that the detrimental effects of hypoxia and high densities led to the above retrograde changes in the biological characteristics of A. butcheri.

The development of a year-effect growth model demonstrated that the growth of A. butcheri in the Swan River Estuary also differed substantially among years between 2007/08 and 2013/14. Annual growth of one year old A. butcheri in those seven years were positively correlated with temperature during the main growing period, but not with freshwater discharges in the preceding wet ‘winter’ months. That positive correlation with temperature, which is consistent with the metabolic theory of ecology, contrasts with the decline in growth between the early 1990s and mid-2000s, when temperatures were increasing. This implies that the influence of temperature on growth between those two 2 periods was overridden by other factors, i.e. hypoxia and increased densities, but then, in 2007-14, became the most important factor, when fish became concentrated in relatively similar densities in nearshore waters. Although there was no evidence that the overall abundance of A. butcheri in the Swan River Estuary had changed markedly between the early 1990s and early 2010s, there is circumstantial evidence that the stock of this species declined during the 1980s and 1990s in the Blackwood River Estuary, due to fishing pressure and/or environmental changes.

Concern for the status of the stock of A. butcheri in the Blackwood River Estuary led to a study aimed at determining the efficacy of using restocking to replenish the population of this species in this system. The growth and maturity schedules of the 2001 and 2002 year classes of A. butcheri, which had been cultured and introduced into the Blackwood River in south-western Australia at seven and four months old, respectively, were determined from samples collected regularly between 2002 and 2014. Restocked A. butcheri could always be distinguished from its wild stock because their otoliths retained the pink coloration of the alizarin complexone with which they had been stained prior to release. The growth and maturity schedules of restocked fish were only slightly inferior to those of its wild stock and the mean gonad weights of these two groups did not differ significantly in any month. As increasing numbers of restocked A. butcheri attained the minimum legal length (MLL) of 250 mm for retention, their contribution to the commercial fishery increased from 6% in 2005 to 74% in 2010. That contribution subsequently declined to 39% in 2012 and 10% in 2014, due predominantly to the introduction of the very strong 2008 year class in the commercial catches, the first substantial recruitment into the population since 1999. As restocked fish were estimated as contributing ~55% to the eggs produced in 2008, substantial numbers of the 2008 year class were derived from spawning by restocked fish. The results of this and a previous genetic study imply that restocking is an effective and appropriate way for replenishing stocks of an estuarine species such as A. butcheri, especially as its recruitment is highly episodic.

Protection of riverine discharge into the estuaries of south-western Australia is likely to be the most effective management strategy to maintain the ‘health’ of estuaries and their fisheries. It follows that a thorough understanding of the hydrology of estuaries will become increasingly important to managing these systems as they become subjected further to the effects of climate change and increases in the demand for water resources by human populations. As A. butcheri completes its life cycle within its natal estuary and has plastic biological characteristics, it is an ideal candidate for use as an indicator of the health of an estuary and hypothesising on the effects of climate change.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Veterinary and Life Sciences
Supervisor(s): Potter, Ian, Hall, Norman and Hesp, Alex
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