Delayed timing of successful spawning of an estuarine dependent fish, black bream Acanthopagrus butcheri.

ABSTRACT

In this paper, we investigate the period of successful spawning for black bream Acanthopagrus butcheri, an obligate estuarine species in southern Australia that typically spawn in spring and early summer. However, back-calculated spawning dates of juveniles sampled in Gippsland Lakes, Victoria from February to May 2016 indicated that spawning was concentrated over a short period in the Austral mid-summer (January), with a second spawning in late summer and early autumn (late February-early March). Ichthyoplankton sampling in the tributary estuaries from October to early December collected substantial numbers of fish larvae, dominated by gobiids, eleotrids and retropinnids of freshwater origin, but no A. butcheri. The lack of A. butcheri larvae was consistent with the delayed successful spawning indicated by juvenile otolith data. Freshwater flows declined from late winter to summer, with consistent salinity stratification of the water column. Dissolved oxygen (DO) concentrations were generally very low below the halocline. These conditions may have delayed the upstream spawning migration of adults or may have been unsuitable for survival of eggs and newly-hatched larvae. Longer-term predictions for climate change in southern Victoria, including the Gippsland Lakes region, are for lower winter-spring freshwater flows, potentially benefiting the reproductive success of A. butcheri through high water-column stratification, but only if DO concentrations are not compromised by a lack of high winter flows needed to flush low DO water from the system.