Spatio-temporal variations of ichthyoplankton over a macrotidal, tropical continental shelf off the Kimberley, NW Australia

The Kimberley region of north-western Australia is characterised by huge tides (>10 m), a wide continental shelf and complex coastal topography. Fish diversity is high (>800 neritic species) although little is known about their larvae in the region. We examined spatio-temporal variations in ichthyoplankton assemblages and key environmental drivers, particularly tidal forcing, by surveying four cross-shelf (coastal-oceanic) transects at different temporal scales (hours, days, weeks). We sampled ~7,000 larvae representing 94 neritic and 21 meso-pelagic teleost families. Distinct ichthyoplankton assemblages characterised coastal (Engraulidae, Gobiidae, Leiognathidae), outer shelf / slope (Scombridae, Lutjanidae) and oceanic (Myctophidae, Gonostomatidae, Phosichthyidae) waters, reflecting changes in biophysical properties (fluorescence, temperature, turbidity). Coastal waters were further delineated by distinct assemblages that reflected the complex coastal topography and modified physical regimes of waters around the Lacepede Islands and within King Sound. Extreme tidal forcing was hypothesised to be the principal physical driver of pelagic variability however the spring-neap tidal cycle had no significant influence on larval fish assemblages. There was no significant difference between assemblages on subsequent tides (same station sampled 12 h apart). However, sampling over 24 h at a shelf-edge station showed shallowing of the mixed layer in the water column during high tides and a concomitant significant increase in larval fish concentrations but with no significant diel effect. This suggests the use of vertical migration to minimize offshore dispersal of neritic larvae. If so, it represents an important behavioural adaptation in the extreme physical conditions of the Kimberley continental shelf ecosystem