Abstract

The reproduction of Australian bass from the Sydney Basin was studied between 1977 and 1982. The gonadal cycle was investigated using a four-stage gonad maturity classification, the gonado-somatic index, oocyte measurements, and histological examination of representative gonads. An ichthyoplankton survey was conducted, fecundity was estimated, and sperm viability in different salinities was also measured.

Spermatogenesis and vitellogenesis commenced between February and April each year. Bass spawned between late June and early September in 1978 and 1981. Final ovarian maturation, and the catadromous migration of females from freshwater to the brackish spawning zone were dependent on flooding. Ovarian involution ensued if river levels did not rise before September. Reproductive success was high after floods in1978. Bass did not reproduce during the drought in 1979 and 1980, and reproductive success was low in the low river flows of the 1981 winter. The results suggest that bass are cyclical spawners, shedding eggs that are demersal in the spawning salinity. Fecundity is high (mean = 440 000). The location of spawning and recently spent bass, the distribution of larvae in the estuary, and results of the sperm viability experiments indicated that bass spawn in the estuary in salinities between 8 and 14 g kg^-1. Most males were sexually mature by age 3 + years, and females by 5 + or 6 + years.