Factors influencing growth of Acanthopagrus butcheri (Sparidae) in a eutrophic estuary have changed over time

• Factors influencing growth of Black Bream changed between early 1990s and mid-2000s.
• Hypoxia in deep waters of Swan Estuary increased between early 1990s and mid-2000s.
• Growth declined between 1993–95 and 2007–14 as bream congregated in shallow waters.
• A year-effect model demonstrated that growth varied among years in 2007–14.
• Annual growth in 2007–14 was related to temperature but not freshwater discharge.

The overall growth of Acanthopagrus butcheri in the eutrophic Swan River Estuary has previously been shown to decline between 1993–95 and 2007–11. This was attributed to the effects of an increase in hypoxia on A. butcheri in deeper water, brought about by reductions in freshwater flushing, and to density-dependent effects as this species became concentrated in the better-oxygenated, nearshore shallow waters. In the present study, a year-effect model was developed that provided a good fit to the lengths at age and could thus be used to explore the following: 1) The extent to which somatic growth of A. butcheri differed between years in the above two periods and within the later period, which was extended to include data for 2012 to 2014, and 2) whether annual growth in 2007–14 was related to temperature and/or freshwater discharge. Annual length increments for females and males during the second year of life, predicted from the model and when using a common initial length, were almost invariably less in each year in 2007–14 than in 1993–95. In 2007–14, these predicted increments varied by ∼2 times for both females and males and were positively correlated with average temperature during the main growth phase of A. butcheri. They were not significantly correlated, however, with freshwater discharge in the preceding cool wet ‘winter’ months, when the vast majority of rainfall occurs. The demonstration that the growth of A. butcheri was positively correlated with temperature for years in the latter period is consistent with the metabolic theory of ecology and contrasts with growth declining between 1993–95 and 2007–14 when temperatures were increasing. Thus, any influence of temperature on growth between those periods was overridden by the effects of other factors, i.e. hypoxia and related changes in density. 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 for hypothesising on the effects of climate change on fish species.

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