MEPS 433:185-194 (2011)  -  DOI: https://doi.org/10.3354/meps09179

ABSTRACT: 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.

KEY WORDS: Light levels · Marine fish larvae · Food availability