BIANNUAL SPAWNING PERIODS AND RESULTANT DIVERGENT PATTERNS OF GROWTH IN THE ESTUARINE GOBY PSEUDOGOBIUS-OLORUM - TEMPERATURE-INDUCED

Samples of juveniles and adults of the goby Pseudogobius olorum were c ollected from seven sites in the shallows of the upper Swan Estuary, W estern Australia, using a 3 mm-mesh seine net on one or two occasions in each month between September 1983 and April 1985. The mean gonadoso matic index of female fish rose from very low values in winter (June-A ugust) to a sharp peak in mid-spring (October), reflecting the rapid m aturation of ovaries over this period. Ovaries with post-ovulatory fol licles and ovaries that were undergoing degeneration were present in N ovember and December, but were then either rare or absent in those mem bers of the corresponding cohort which survived into January and Febru ary. Female fish with advanced oocytes and mature ovaries were not fou nd in December and January, but were present in February to April. The above trends exhibited by ovarian maturity indices, together with the appearance of larvae and small fish in both spring and autumn, demons trate that P. olorum spawns in both spring and autumn and at best to o nly a limited extent in summer. Length-frequency and gonadal data show that the progeny of the spring-spawning group frequently spawn in the following autumn, when they are similar to 5 mo old, and that those o f the autumn-spawning group frequently spawn in the following spring, when they are similar to 7 mo old. Some representatives of these two s pawning groups survive through the winter and summer, respectively, to breed in a second season. Growth of the progeny of the spring-spawnin g group was relatively rapid between late spring and mid-autumn, where as that of the autumn-spawning group was negligible during winter, but then increased markedly in spring. It is proposed that the biannual s pawning periods in each year by P. olorum in the Swan Estuary develope d as a result of a rise in water temperature over the last few thousan d years. Such a rise would have brought forward further into spring an d extended later into autumn the periods when the water temperatures l ie within the range (20 to 25 degrees C) at which P. olorum typically spawns. However, mid-summer is now characterised by water temperatures > 25 degrees C, which are considered less conducive to reproductive s uccess.