Diel vertical migration related to foraging success in snapper Chrysophrys auratus larvae

The vertical distributions of marine fish larvae can change markedly over time due to changes in diel vertical migration (DVM). DVM is thought to be influenced by a number of factors including light levels and prey availability. In Port Phillip Bay, Australia, the DVM of snapper Chrysophrys auratus (Sparidae) larvae and their prey were investigated using depth-stratified sampling (four depths: surface, 4, 8, 11 m) over 4 sampling periods of 24 h. We sampled ichthyoplankton at the same location twice in 2 spawning seasons (austral summers of 2008/09 and 2009/10). Sufficient snapper larvae for analysis of DVM behaviour occurred once in each season. At both 24 h sampling times, snapper larvae displayed the same DVM behaviour of nocturnal diffusion and diurnal aggregation at ~4 m depth. The water column was homogenous for temperature, salinity, dissolved oxygen and fluorescence during the two 24 h periods. Two out of 6 important zooplankton prey of snapper larvae were also aggregated at ~4 m depth during the day. Gut analyses indicated that larvae only fed during daylight hours and had an average digestion time of 3 to 5 h. Snapper larvae had highest foraging success at 4 m depth, which was supported by minimal digestion of prey at time of capture. This suggests that the observed DVM was related to feeding success, and that 4 m depth provided optimal foraging conditions. These results have important implications for developing individual- based biophysical models of larval transport that include interaction with prey fields and larval foraging success.