Seasonal variation in horizontal and vertical structure of larval fish assemblages off south-western Australia, with implications for larval transport

The coastal ocean off south-western Australia is characterised by the southward-flowing Leeuwin Current, which suppresses the upwelling typically associated with other eastern boundary currents in the southern hemisphere. This results in a unique environment for the transport and survival of planktonic fish larvae. The horizontal and vertical structure of larval fish assemblages off south-western Australia was investigated during winter (August 2003) and summer (January 2004), and related to these unusual regional oceanographic and biological processes. Larval fish were sampled along a four station transect running from the inner continental shelf to offshore waters, using depth-integrated bongo net tows and depth-stratified EZ net tows. The distribution of taxa across the shelf and offshore was strongly influenced by the current regime at the time of sampling. Larval fish assemblage structure reflected the distinctive oceanographic conditions found during each season, and vertical depth distributions of larvae affected their horizontal location. Continental shelf samples were dominated by larvae of pelagic fishes, such as clupeiform species (e.g. Sardinops sagax), whereas offshore assemblages were characterised by larvae of oceanic families, such as Myctophidae and Phosichthyidae. The winter cruise (August 2003) was completed during a time of strong, southerly Leeuwin Current flow, whereas the northward-flowing Capes Current, in combination with surface offshore Ekman transport, predominated during summer. The vertical depth preferences of larvae were particularly influential in affecting their horizontal position; especially so for surface-dwelling larval fishes found during summer. This study represents the first documentation of the vertical structure of ichthyoplankton assemblages in the oligotrophic waters off south-western Australia

Larval fishes off Western Australia: Influence of the Leeuwin Current

Although the poleward flow of the Leeuwin Current off Western Australia is unique among eastern boundary currents in the southern hemisphere, the biological oceanography of this system is comparatively poorly known. In this short review, the findings of the various studies on larval fishes completed off Western Australia are collated, synthesized and related to the influence of the Leeuwin Current. The studies range from light-trap experiments focusing on tropical fish larvae off the north-western coast to studies on larvae of commercially significant temperate clupeiod species in the Great Australian Bight. Larval fish assemblages within the Leeuwin Current appear to be composed of a mixture of oceanic, slope, tropical and temperate coastal species, with strong seasonal variation in species composition. Ichthyoplankton studies in progress off Western Australia are reported on and aim to address gaps in the knowledge about the influence of the Leeuwin Current and its meso-scale features upon larval fishes

Ichthyoplankton in two meso-scale Leeuwin Current eddies: preliminary results

In many coastal oceans, eddies and jets occur predictably under certain conditions, and are a common mechanism for entraining coastal water across the continental shelf. Eddies may therefore act as transport mechanisms for planktonic organisms and larvae. In October 2003, a cruise was undertaken on the RV Southern Surveyor to investigate the dynamics of two eddies located more than 500km off the Western Australian coast. Primary production patterns, nutrient cycling, oceanographic and biological characteristics of both eddies (one cold-core upwelling, and one warm-core downwelling) were investigated over a 23 day period, using oblique and depth stratified plankton tows, continuous physiochemical samplers (“Seasoar”), sediment traps and other methods. Larval fish were collected from plankton samples taken in the centre, body and perimeter of each eddy, during both day and nighttime, with both oblique bongo, and depth stratified EZ Net tows. Preliminary analysis shows that the larval fish assemblage was mostly composed of offshore and oceanic families, such as the Myctophidae, Phosichthydae and Gonostomatidae. The cold-core eddy showed few differences between its centre, body and perimeter in terms of larval density, and species assemblage. The warm-core eddy, however, showed much lower densities of larvae in the centre, compared to both the body and perimeter, and to the cold-core eddy. This was particularly apparent at night, suggesting that the dynamics of the centre of the warm-core eddy were affecting the presence, and/or the vertical migration of larger larvae in the eddy at night

Preliminary analysis of ichthyoplankton assemblage structure in coastal, shelf and offshore waters of southwestern Australia

As part of the SFRME Biophysical Oceanography Program, this project is examining the structure and composition of ichthyoplankton assemblages in waters off Western Australia. Larvae are sampled using replicated oblique bongo net tows at three stations (18m, 40m, 100m depth) every month, and five stations (18m, 40m, 100m, 300m, 1000m) every quarter. The samples from the 355µm mesh nets are sorted to remove all larval fish which are then identified to family and species, where possible. Larval assemblages have been characterised, inter-seasonal and inter-annual trends identified, and related to dominant oceanographic and ecological processes. Samples from 18 months of cruises have been examined thus far, and larvae from 70 teleost families and approximately 160 species have been identified. Assemblages appear distinct between seasons at the inshore station (18m), and the two offshore stations (300m and 1000m), but the shelf stations (40m and 100m) show considerable variation throughout the year. Larval densities show peaks inshore in summer, and mid-shelf in autumn, and are possibly related to variation in strength of the Leeuwin and Capes Current, or climatic events. Additionally, preliminary correlations have been made with larval fish density and fluorescence peaks, and with the depth of the mixed layer on larval composition and abundance