Differing importance of salinity stratification and freshwater flow for the recruitment of apex species of estuarine fish

Estuarine fish live in a highly dynamic environment where recruitment variability is a key determinant of population trajectory. Environmental requirements for successful recruitment may differ between co-occurring species, and therefore species may be advantaged or disadvantaged under climate change. Recruitment variability in black bream Acanthopagrus butcheri and estuary perch Macquaria colonorum in estuaries along the coast of western Victoria, south-eastern Australia, was determined from the age structure of the populations. Recruitment was found to be episodic in both species, with the populations dominated by a few year classes; however, abundant year classes differed between species. Historical freshwater flows were found to be similar across estuaries, reflecting broad-scale rainfall. In contrast, water column stratification (difference between surface and bottom salinity) over a 7 yr period varied widely amongst estuaries. In general, recruitment of black bream was negatively correlated with freshwater flow and positively correlated with the level of stratification. In contrast to black bream, significant correlations for recruitment of estuary perch were negatively related to stratification and positively related to flows. Thus, although both species spawn in the spring and early summer, they have different environmental requirements in terms of flow and stratification for successful recruitment. This means that high inter-annual variability in flow is required for both species to experience favourable environmental conditions over time. Furthermore, the climate change scenario of significantly reduced flows in the Victorian region may mean that in many estuaries the conditions will be more favourable for the recruitment of black bream than for estuary perch.