Beyond hydrography: Daily ichthyoplankton variability and short term oceanographic events on the Sydney continental shelf

Surfaceichthyoplankton concentrations along a shore-normal transact across the Sydney continental shelf and upper slope changed between three replicate nights in January and April of 1994. Over 70 families of fish were recorded, which, during January, included: Myctophidae (49% of individuals), Carangidae (14%), Gonostomatidae (11%) and Pomacentridae (8%); and during April included: Gonorhynchidae (43%), Myctophidae (10%), Berycidae (11%) and Serranidae (6%). Multidimensional scaling analysis identified inshore and offshore communities, which nightly moved between the nearshore and mid-shelf stations. During January no distinct near-surface water masses could be identified from the temperature-salinity data, although the shelf waters were under the influence of forcing by the local wind stress and the East Australian Current. Good agreement between the cross-shore transport in the near-surface layer and the temporal variability of the icthyoplankton was nevertheless found. The sampling during April was performed during a period of relatively steady oceanographic conditions, and two water masses were identified from the hydrographic data. Temporal ichthyoplankton variability at any station was correspondingly less during the April period and stable inshore and offshore communities were identified, that shifted with characteristic water masses. The results presented in this paper demonstrate that the large variance often associated with ichthyoplankton distribution within a similar water mass may be interpreted by the dynamics in cross-shelf flows, which has implications for the selection of control sites used when studying environmental impacts of coastal outfalls.