Abstract
Understanding challenges posed by climate change to estuaries and their faunas remains a high priority for managing these systems and their communities. Freshwater discharge into a range of estuary types in south‐western Australia between 1990 and 2015 is shown to be related to rainfall. This largely accounts for decreases in discharge in this microtidal region being more pronounced on the west coast than south coast, where rainfall decline was less. Results of an oxygen‐balance model imply that, as demonstrated by empirical data for the Swan River Estuary, declines in discharge into a range of estuary types would be accompanied by increases in the extent of hypoxia. In 2013–15, growth and body condition of the teleost Acanthopagrus butcheri varied markedly among three permanently open, one intermittently‐open, one seasonally‐closed and one normally‐closed estuary, with average time taken by females to reach the minimum legal length (MLL) of 250 mm ranging from 3.6 to 17.7 years. It is proposed that, in a given restricted period, these inter‐estuary variations in biological characteristics are related more to differences in factors, such as food resources and density, than to temperature and salinity. The biological characteristics of A. butcheri in the four estuaries, for which there are historical data, changed markedly between 1993–96 and 2013–15. Growth of both sexes, and also body condition in all but the normally‐closed estuary, declined, with females taking between 1.7 and 2.9 times longer to attain the MLL. Irrespective of period, body condition, and growth are positively related. Age at maturity typically increased between periods, but length at maturity declined only in the estuary in which growth was greatest. The plasticity of the biological characteristics of A. butcheri, allied with confinement to its natal estuary and ability to tolerate a wide range of environmental conditions, makes this sparid and comparable species excellent subjects for assessing estuarine “health.”
Highlights
Estuaries are among the most productive of aquatic ecosystems and provide an abundant source of food for fishes and other fauna (Blaber & Blaber, 1980; Elliott & Whitfield, 2011; Whittaker & Likens, 1975)
As estuaries are often surrounded by areas of agricultural, industrial and urban developments and frequently receive substantial amounts of nutrients and organic material through runoff from surrounding land and by direct input, these systems are prone to developing these detrimental effects
The increase in the extent of bottom water hypoxia in the Swan River Estuary since the 1990s was accompanied in the demersal Black Bream Acanthopagrus butcheri by reductions in growth and body condition and a strong tendency for aggregating in shallow, better-oxygenated areas, which led to a marked increase in density in those waters (Cottingham, Hall, Hesp, & Potter, 2018; Cottingham et al, 2014)
Summary
Estuaries are among the most productive of aquatic ecosystems and provide an abundant source of food for fishes and other fauna (Blaber & Blaber, 1980; Elliott & Whitfield, 2011; Whittaker & Likens, 1975). The increase in the extent of bottom water hypoxia in the Swan River Estuary since the 1990s was accompanied in the demersal Black Bream Acanthopagrus butcheri by reductions in growth and body condition and a strong tendency for aggregating in shallow, better-oxygenated areas, which led to a marked increase in density in those waters (Cottingham, Hall, Hesp, & Potter, 2018; Cottingham et al, 2014) These trends parallel those recorded for fish species in North America when exposed to hypoxia (e.g., Campbell & Rice, 2014; Eby & Crowder, 2002). These comparisons were used to explore the hypothesis that reductions in freshwater discharge between the two periods were accompanied in A. butcheri by declines in growth, body condition, and length at maturity and increases in the age at maturity
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