Biannual spawning periods and resultant divergent patterns of growth in the estuarine goby Pseudogobius olorum: temperature-induced?

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Samples of juveniles and adults of the goby Pseudogobius olorum were collected from seven sites in the shallows of the upper Swan Estuary, Western Australia, using a 3 mm-mesh seine net on one or two occasions in each month between September 1983 and April 1985. The mean gonadosomatic index of female fish rose from very low values in winter (June–August) to a sharp peak in mid-spring (October), reflecting the rapid maturation of ovaries over, this period. Ovaries with post-ovulatory follicles and ovaries that were undergoing degeneration were present, in November and December, but were then either rare or absent in those members of the corresponding cohort which survived into January and February. Female fish with advanced oocytes and mature ovaries were not found 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, demonstrate that P. olorum spawns in both spring and autumn and at best to only 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 ∼ 5 mo old, and that those of the autumn-spawning group frequently spawn in the following spring, when they are ∼ 7 mo old. Some representatives of these two spawning groups survive through the winter and summer, respectively, to breed in a second season. Growth of the progeny of the spring-spawning group was relatively rapid between late spring and mid-autumn, whereas that of the autumn-spawning group was negligible during winter, but then inceased markedly in spring. It is proposed that the biannual spawning periods in each year by P. olorum in the Swan Estuary developed as a result of a rise in water temperature over the last few thousand years. Such a rise would have brought forward further into spring and extended later into autumn the periods when the water temperatures lie within the range (20 to 25°C) at which P. olorum typically spawns. However, mid-summer is now characterised by water temperatures >25°C, which are considered less conducive to reproductive success.