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

Abstract

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.