Shelf waters of southern Australia support the world's only northern boundary current ecosystem. Although there are some indications of intense nitrate enrichment in the eastern Great Australian Bight (GAB) arising from upwelling of the Flinders Current, the biological consequences of these processes are poorly understood. We show that productivity in the eastern GAB is low during winter, but that coastal upwelling at several locations during the austral summer-autumn results in localized increases in surface chlorophyll a concentrations and downstream enhancement of zooplankton biomass. Sardine (Sardinops sagax) and anchovy (Engraulis australis) eggs and larvae are abundant and widely distributed in shelf waters of the eastern and central GAB during summer-autumn, with high densities of sardine eggs and larvae occurring in areas with high zooplankton biomass. Egg densities and distributions support previous evidence suggesting that the spawning biomass of sardine in the waters off South Australia is an order of magnitude higher than elsewhere in southern Australia. Sardine comprised >50% of the identified prey species of juvenile southern bluefin tuna (SBT, Thunnus maccoyii) collected during this study. Other studies have shown that the lipid content of sardine from the GAB is relatively high during summer and autumn. We suggest that juvenile SBT migrate into the eastern and central GAB during each summer-autumn to access the high densities of lipid-rich sardines that are available in the region during the upwelling period. Levels of primary, secondary and fish production in the eastern GAB during summer-autumn are higher than those recorded in other parts of Australia, and within the lower portion of ranges observed during upwelling events in the productive eastern boundary current systems off California, Peru and southern Africa.

This paper examines the hypotheses (1) that Sardinops sagax and Engraulis australis are spatially segregated and do not interact directly, and (2) that recent mass mortalities of S. sagax have facilitated an expansion in the distribution and abundance of E. australis. In South Australian waters, S. sagax and E. australis both spawn during summer and autumn. Eggs and larvae of both species occur over the continental shelf, and are abundant in areas where upwelling occurs (e.g. off the Coffin Bay Peninsula and the western tip of Kangaroo Island) and frontal systems form (e.g. in Investigator Strait and the entrance of Spencer Gulf). After the mass mortality events in 1995 and 1998, eggs and larvae of S. sagax were confined mainly to these areas, and estimates of the total abundance of S. sagax eggs and larvae in South Australian waters fell by between 48 and 83% respectively. Between 1996 and 1999, densities of E. australis eggs and larvae increased in both key spawning areas and the central and eastern Great Australian Bight, and total abundance of eggs and larvae increased by over 215 and 285% respectively. These results indicate that (1) S. sagax and E. australis are not spatially segregated and may interact directly, and (2) the mass mortalities of S. sagax may have facilitated an expansion in the distribution and abundance of E. australis. Hence, fluctuations in the relative abundance of S. sagax and Engraulis spp. observed in the world's productive boundary current systems may also be possible in Australian waters.

Arripis georgiana was collected from along the Western Australian and South Australian coasts at regular intervals between October 1996 and December 1998. The trends exhibited during the year by gonadosomatic indices, gonadal maturity stages and oocyte stages demonstrate that spawning is restricted to south-western Australian waters from at least as far north as ~28˚43′S to as far east as ~119˚24′E, and that it occurs mainly during late May and early June. The simultaneous presence of post-ovulatory follicles and hydrated and yolk-granule oocytes in some ovaries during the spawning period indicates that A. georgiana is a multiple spawner, i.e. females spawn more than once in a breeding season. In south-western Australia, where all life-cycle stages are found, the overall sex ratio in catches collected by netting was close to parity, whereas females predominated in those obtained by anglers. The length at which 50% of fish reached maturity was 197 mm for females and 179 mm for males, and just over 50% of females and ~80% of males attained maturity at the end of their second year of life. The implications of these data for management of the fishery for A. georgiana are discussed.

Gonosomatic indices and egg and larval densities observed from 1986 to 2001 suggest that the peak spawning season of the Australian anchovy (Engraulis australis) in South Australia occurs during January to March (summer and autumn). This coincides with the spawning season of sardine (Sardinops sagax) and the period when productivity in shelf waters is enhanced by upwelling. Anchovy eggs were abundant throughout gulf and shelf waters, but the highest densities occurred in the northern parts of Spencer Gulf and Gulf St Vincent where sea surface temperatures (SST) were 24-26°C. In contrast, larvae >10 mm total length (TL) were found mainly in shelf waters near upwelling zones where SSTs were relatively low (<20°C) and levels of chlorophyll a (chl a) relatively high. Larvae >15 mm TL were collected only from shelf waters near upwelling zones. The high levels of larval abundance in the upwelling zones may reflect higher levels of recruitment to later stages in these areas compared with the gulfs. The sardine spawns mainly in shelf waters; few eggs and no larvae were collected from the northern gulfs. The abundance of anchovy eggs and larvae in shelf waters increased when sardine abundance was reduced by large-scale mortality events, and decreased as the sardine numbers subsequently recovered. We hypothesize that the upwelling zones provide optimal conditions for the survival of larval anchovy in South Australia, but that anchovy can only utilize these zones effectively when the sardine population is low. At other times, northern gulf waters of South Australia may provide a refuge for the anchovy that the sardine cannot utilize.