We examined the fate of larval fish assemblages after the East Australian Current (EAC) had separated from the coast and larval fish were advected eastward along the Tasman Front. There was no difference in the assemblages at four stations as the EAC meandered from the continental shelf to 220 km eastward. At a fifth station, we sampled a submesoscale, frontal eddy that had formed at the EAC separation zone 11 d earlier and had entrained shelf water. Zooplankton biomass was greater within the eddy compared to the adjacent shelf. The larval fish assemblage in the eddy was significantly different from all other stations. There was an order of magnitude greater abundance of three species characteristic of the shelf: sardine (Sardinops sagax; Clupeidae), blue mackerel (Scomber australasicus; Scombridae), and yellowtail scad (Trachurus novaezelandiae; Carangidae), which were also significantly larger than larvae from a station on the adjacent shelf. In particular, S. sagax in the eddy were , 5 mm longer and , 10 d older, although growth rates were similar. Larval retention in the eddy was inferred from the co-occurrence of small and large larvae of all three species compared to the adjacent shelf. The EAC is only 20-30 km from the inner-shelf water, where frontal eddies may facilitate three stages of successful recruitment: entrainment, enrichment, and retention. Frontal eddies off southeastern Australia entrain preconditioned shelf water, move slower than the mean flow of the EAC, decreasing transport rates, and may sustain planktonic communities through eddy uplift. These eddies are frequent and short-lived (2 to 4 weeks), and we suspect they are of fisheries importance as their duration is sufficient for fish larvae to complete their early life history and, presumably, recruit back to the coast.

No abstract

Entrainment and transport of larval fish assemblages by the East Australian Current (EAC) were examined from the coastal waters of northern New South Wales (NSW) to the western Tasman Front, via the separation of the EAC from the coast, during the austral spring of 2004. Shore-normal transects from the coast to the EAC off northern NSW revealed an inner shelf assemblage of near-shore families (Clupeidae, Engraulidae, Platycephalidae and Triglidae), an EAC assemblage dominated by Myctophidae and Gonostomatidae, and a broadly distributed assemblage over the continental shelf dominated by Scombridae and Carangidae. Further south and after the EAC had separated from the coast, we observed a western Tasman Front assemblage of inner shelf and shelf families (Clupeidae, Engraulidae, Serranidae, Scombridae, Carangidae, Bothidae and Macroramphosidae). The abundance of these families declined with distance from the coast. Surprisingly, there was no distinctive or abundant larval fish assemblage in the chlorophylland zooplankton-enriched waters of the Tasman Sea. Water type properties (temperature-salinity, T-S), the larval fish assemblages and family-specific T-S signatures revealed the western Tasman Front to be an entrained mix of EAC and coastal water types. We found an abundance of commercially important species including larval sardine (Sardinops sagax, Clupeidae), blue mackerel (Scomber australasicus, Scombridae) and anchovy (Engraulis australis, Engraulidae). The entrainment and transport of larval fish from the northern inner shelf to the western Tasman Front by the EAC reflects similar processes with the Gulf Stream Front and the Kuroshio Extension.

The poleward flowing East Australian Current (EAC) drives sporadic upwelling, entrains coastal water and forms the western Tasman Front (wTF), creating a mosaic of water types and larval transport routes along south eastern Australia. The spatial distribution, otolith chemistry and growth rates of larval sardine (Sardinops sagax) were examined to infer spawning location and larval transport. A gradient of increasing larval size from north to south along the shelf was not detected but was evident between the shelf and offshore in the wTF. Here larvae were larger and older. Based on the occurrence of newly hatched larvae, spawning by S. sagax between southern Queensland and mid New South Wales (NSW) was more extensive than previously reported. The otolith chemistry from two wTF larval size classes differed, implying different origins. The otolith chemistry of wTF post-flexion larvae was similar to larvae from northern NSW, whereas wTF flexion larvae were similar to larvae observed nearby from mid-NSW. Two possible larval transport routes, direct and indirect, are inferred from otolith chemistry, current velocities and a previously published particle tracking study. Either larvae from northern NSW were advected south and entrained with younger larvae directly into the wTF, or larvae from a range of shelf regions were advected around the southern edge of an anticyclonic eddy, to join younger larvae directly entrained into the wTF. Based on the co-occurrence of larval ages and sizes in the wTF and their advection routes, the wTF appears to be an important larval retention zone.

No abstract