Relationship between spawning and egg and larval stages of a unique estuarine-resident species and environmental variables and prey

Abstract

This study has elucidated the environmental factors associated with spawning and the ecology of early life cycle stages of the sparid Acanthopagrus butcheri, which is atypically long-lived and large for a species confined to its natal estuary. The prevalence of A. butcheri eggs in monthly plankton samples from the Blackwood River Estuary, south-western Australia, in 2014/15, allied with long-term gonadal data, imply that spawning in this estuary commences in mid-spring (October) as water temperatures increase to ~17 °C and peak at ~22 °C in early summer (December). Eggs were initially found only in deeper, downstream areas, where a salt wedge had started to penetrate following precipitous seasonal decline in freshwater discharge. Spawning occurred progressively further upstream during November/December as the salt wedge continued its intrusion of the upper estuary. The concentrations of eggs and yolk-sac and preflexion larvae, which were related positively to salinity and temperature and less so to oxygen concentration, were located predominantly below the halocline, implying that successful spawning typically requires salinities ≥15. Concentrations of eggs and yolk-sac and preflexion larvae peaked in December and of flexion larvae in January. As the concentrations of copepod nauplii exceeded by orders of magnitude those of larval A. butcheri, with which they co-occur and constitute their main prey, it is unlikely that restricted food resources accounted for the poor recruitment of the 2014/15 year class. In this context, the presence of many yolk-sac larvae and preflexion larvae in the low oxygen concentrations, which have become increasingly prevalent in microtidal estuaries of south-western Australia due to climate change, is likely to have led to considerable mortality. In most populations throughout its range in southern Australia, A. butcheri spawns in a restricted period between the austral mid-spring and mid-summer, with the synchrony achieved through differences in the temperatures that cue spawning at different latitudes, i.e. higher temperature cues at lower and thus warmer latitudes. Spawning occurs earlier, however, in atypical estuaries in which salinities and temperatures are high in late winter/spring. Despite experiencing episodic recruitment, A. butcheri sustains substantial populations through maturing early relative to its maximum age, and therefore producing numerous offspring in those few years when spawning conditions are optimal. Episodic recruitment could thus be related to subtle inter-annual differences in environmental conditions during the short spawning periods, with oxygen levels greater than usual a potential key factor. The adaptability of A. butcheri makes this sparid useful in assessing the degree, direction and scale of climate change effects.