Physical environmental conditions, spawning and early-life stages of an estuarine fish: climate change implications for recruitment in intermittently open estuaries

Significant variation in the egg and larval survival and juvenile recruitment of estuarine fishes has been linked to fluctuating environmental conditions. This present study compared the distribution and abundance of black bream (Acanthopagrus butcheri) eggs and yolk-sac larvae between two microtidal estuaries of different flow regimes, where the riverine flow into the Glenelg estuary was around eight times the flow volume into the Hopkins estuary. Samples were collected monthly from September to November at sites along each estuary where vertical profiles of temperature, salinity and dissolved oxygen (DO) were measured, and vertically stratified sampling of black bream eggs and yolk-sac larvae was conducted using a Schindler sampler. Salt wedge formation was apparent in both estuaries, with significant de-oxygenation of deeper, saline waters. Eggs occurred in a wide range of DO levels but yolk-sac larvae were less common at the lowest levels. Most eggs and yolk-sac larvae were collected in salinities greater than 10. Results suggested that egg mortality was higher in the Hopkins than the Glenelg estuary, which may be associated with the hypoxic conditions characteristic of low-flow conditions. The results have significant implications in terms of climate change that is predicted to lead to warmer, drier conditions in south-eastern Australia, potentially increasing stratification and subsequent hypoxic zones.