MEPS 170:239-248 (1998)  -  doi:10.3354/meps170239

Diurnal and tidal vertical migration of pre-settlement King George whiting Sillaginodes punctata in relation to feeding and vertical distribution of prey in a temperate bay

Gregory P. Jenkins1,*, Dirk C. Welsford2, Michael J. Keough2, Paul A. Hamer1

1Marine and Freshwater Resources Institute, PO Box 114, Queenscliff, Victoria 3225, Australia
2Department of Zoology, University of Melbourne, Parkville, Victoria 3052, Australia

ABSTRACT: 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 sampled: surface, 2.5-3.0 m, and 5.0-5.5 m, in a total water depth of 7 to 8 m. Sampling was conducted on 17 dates and encompassed all combinations 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 ichthyoplankton net. Pre-settlement S. punctata showed 'reverse' diurnal vertical migration, with concentration near the surface during the day and diffusion through the water column at night. A much weaker tidal migration was also detected, with larvae slightly higher in the water column on flood tides. Pre-settlement S. punctata only fed in daylight and zooplankton taxa that were eaten did not show vertical stratification during daytime. In 1996, 4 sites were sampled at a minimum of 10 m depth, and an additional depth stratum, 7.5-8.0 m, was sampled. Smaller numbers of larvae were collected in 1996 compared with 1995. All larvae collected in daytime were in the surface stratum, whilst at night larvae were spread through the water column. Although diurnal vertical migration 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 result of local factors. Both diurnal and tidal migration would be expected to strongly influence transport of pre-settlement S. punctata to seagrass beds in Port Phillip Bay.

KEY WORDS: Vertical distribution · Diurnal vertical migration · Pre-settlement fish · Selective tidal transport · Sillaginodes punctata

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