Mesoscale characterisation of the Pelagic Shelf ecosystem of south-eastern Australia : integrated approach using larval fish assemblages and oceanography
Creators: name: Keane, John Patrick
Publisher: name: University Of Tasmania
Year: 2023

description: This represents the first mesoscale study linking larval fish assemblages and water masses off south-eastern Australia, aimed at characterising the pelagic shelf ecosystem within the region. Data were collected during five intensive ichthyoplankton surveys carried out between 24.6¬¨‚àûS and 41.7¬¨‚àûS (-1050 nm), that included the coastlines of southern Queensland (Qld), New South Wales (NSW), eastern Victoria (Vic) and north-eastern Tasmania (Tas). Surveys were conducted in spring 2002 and 2003 (Qld-NSW), summer 2003 (NSW-Tas), summer 2004 (NSW-Vic) and winter 2004 (Qld-NSW). A total of 371 vertical plankton samples were obtained via a bongo sampler simultaneously with physical oceanographic variables to a maximum depth of 200 m. A novel multivariate approach was developed and employed to define water masses using temperature frequencies from OTD-derived profiles. While this approach departs from traditional methods, it proved to be an effective and objective method of delineating water masses, even when indistinct current structures existed. Up to three discrete water masses were identified during individual surveys, namely East Australian Current (EAO) to the north, Tasman Sea (TAS) to the south, and a mixed water mass (MIX) containing EAOTAS water in between. Surveys yielded a total of 24,135 larval fishes representing 150 taxa from 101 families. Epipelagic taxa dominated the larval fish community contributing 41-633 to the total numbers caught across each survey, with Trachurus spp. (27.73), Sardinops sagax (17.93) and Lophonectes gallus (3.83) being most abundant. Distinct larval fish assemblages identified via multivariate analyses corresponded directly to identified water masses in terms of their spatial distribution. Assemblage composition differed significantly between water masses, with EAO and TAS being most dissimilar. Such contrast was due to tropical, subtropical and temperate taxa in EAO, primarily temperate taxa in TAS, and a combination of EAO and T AS taxa within MIX, the latter being consistent with the convergence of both water masses. Discriminator taxa between the recurrent EAO and TAS assemblages included labrids and Bregmaceros spp. in EAO, as well as L. gallus and Macroramphosus spp. in TAS. Further, both water mass and assemblage boundaries were found to be dynamic, shifting spatially and temporally depending on the strength of the EAO. Analyses on individual taxa revealed larval habitats pointing to distinct water mass preferences, while water mass interfaces often acted as a barrier to larval dispersal. Moreover, habitats of the abundant small pelagic taxa, namely Etrumeus teres, S. sagax, Engraulis australis, Trachurus spp. and Scomber australasicus, were similar, and appeared closely linked to the extent, strength and timing of the seasonal EAO cycle. Overall results highlight the strength of integrating ichthyoplankton dynamics and oceanography, and set a foundation for future adaptive biophysical surveys, particularly in relation to understanding larval transport mechanisms and spawning stock habitats for key pelagic species. Ultimately, links between biological and physical factors will assist in understanding the potential effects of climate change on the distribution and abundance of fish stocks within the region.