Pelagic ecology of a northern boundary current system: effects of upwelling on the production and distribution of sardine (Sardinops sagax), anchovy (Engraulis australis) and southern bluefin tuna (Thunnus maccoyii) in the Great Australian Bight

Abstract

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.

Spawning, larval abundance and growth rate of Sardinops sagax off southwestern Australia: influence of an anomalous eastern boundary current

“…Eggs and larvae are most abundant along the south and west coasts of Western Australia from June to August and from December to February (Gaughan et al 1990, Fletcher et al 1994. This is in contrast to the pattern off South Australia, where the main sardine spawning season in summer and autumn is associated with coastal upwelling, increased primary production and higher zooplankton biomass (Ward et al 2006). Favourable feeding and growth conditions for larvae therefore occur in conjunction with potentially favourable larval retention conditions.…”

“…Across the Two Rocks transect, mean larval sardine concentrations up to 1.2 m -3 were found, while at some southern stations on the winter sampling grid in 2004, larval concentrations > 2 m -3 were frequently found. Concentrations of larvae on the Two Rocks transect were comparable to or higher than those found near Albany on the south coast of Western Australia (Fletcher & Tregonning 1992, Fletcher 1999, those measured by Beckley & Hewitson (1994) off southeastern Africa, and those on the open shelf off South Australia (Ward et al 2006). Concentrations of larvae found at winter sampling grid stations in this study were comparable to those in the more protected waters of Spencer Gulf, around Kangaroo Island (Ward et al 2006), to concentrations in the Benguela Current ecosystem (Huggett et al 1998, Stevenik et al 2001, and to concentrations of Sardina pilchardus in the northwestern Mediterranean (Olivar et al 2001).…”

Genetic differentiation in bottlenose dolphins from South Australia: association with local oceanography and coastal geography

“…In SA, the distribution of sardines Sardinops sagax, an important species in the diet of coastal bottlenose dolphins in the area (Kemper & Gibbs 2002), exhibits a gap in distribution between the mouth of Spencer Gulf and eastern areas of the Great Australian Bight (Ward et al 2006). This gap, which is probably associated with cold sea surface temperatures caused by upwellings (Ward et al 2006 Table 6.…”

“…This gap, which is probably associated with cold sea surface temperatures caused by upwellings (Ward et al 2006 Table 6. Tursiops sp.…”

Open-ocean foraging ecology of southern bluefin tuna Thunnus maccoyii based on stomach contents

“…Based on previous studies, SBT of up to 4 yr old (mean FL at age for SBT are 52 cm at age 1, 78 cm at age 2, 96 cm at age 3, and 113 cm at age 4; ) foraging on the Australian continental shelf feed exclusively on teleosts (Serventy 1956, Young et al 1997, Ward et al 2006, Itoh et al 2011). In contrast, the results of the present study are quite different, with 51% of the prey weight in the open ocean accounted for by cephalopods and 46% by teleosts.…”