DIURNAL AND TIDAL VERTICAL MIGRATION OF PRESETTLEMENT KING-GEORGE WHITING SILLAGINODES-PUNCTATA IN RELATION TO FEEDING AND VERTICAL-DISTRIBUTION OF PREY IN A TEMPERATE BAY

Vertically stratified sampling was undertaken for pre-settlement King George whiting Sillaginodes punctata at 1 site in 1995 and 4 sites in 1996, in Port Phillip Bay, Australia. In 1995, 3 depth strata were sam pled: surface, 2.5-3.0 m, and 5.0-5.5 m, in a total water depth of 7 t o 8 m. Sampling was conducted on 17 dates and encompassed all combinat ions of day and night, and ebb and flood tide. A total of 3, or in one case 4, replicate samples were taken at each depth. On 4 occasions a smaller zooplankton net was deployed at the same time as the ichthyopl ankton net. Pre-settlement S, punctata showed 'reverse' diurnal vertic al migration, with concentration near the surface during the day and d iffusion through the water column at night. A much weaker tidal migrat ion was also detected, with larvae slightly higher in the water column on flood tides. Pre-settlement S, punctata only fed in daylight and z ooplankton taxa that were eaten did not show vertical stratification d uring daytime. In 1996, 4 sites were sampled at a minimum of 10 m dept h, and an additional depth stratum, 7.5-8.0 m, was sampled. Smaller nu mbers of larvae were collected in 1996 compared with 1995. All larvae collected in daytime were in the surface stratum, whilst at night larv ae were spread through the water column. Although diurnal vertical mig ration was not a response to prey migrations, daytime ascent of larvae may have occurred to find adequate light levels for visual predation. The generality of tidal migration is unclear, and may have been a res ult of local factors. Both diurnal and tidal migration would be expect ed to strongly influence transport of pre-settlement S. punctata to se agrass beds in Port Phillip Bay.