Phyllosoma, prey fields and patchiness in the pelagic ecosystem of the SE Indian Ocean

Abstract

The dramatic decline in settlement of puerulus of the valuable western rock lobster (Panulirus cygnus) along the coast of Western Australia has triggered several investigations including a study on the biological oceanography of the phyllosoma stage. During this planktonic period of almost a year in the pelagic ecosystem of the SE Indian Ocean, the phyllosoma are subject to a wide range of oceanographic conditions. Feeding experiments have shown that the phyllosoma are fussy feeders and, in this presentation, we report on the results of an extensive survey of the prey fields found from the shelf out to 400 km offshore between 28°and 34°S and down to 200 m depth. The water masses in this area comprise both Leeuwin Current Water (LCW) and Subtropical Surface Water (STSW) and they showed clear distinction with respect to nutrients and chlorophyll a concentration. Depth-stratified sampling revealed bio-volume of zooplankton to be consistently higher in the upper 50 m of the water column and also significantly higher in LCW than STSW. Chaetognaths, a favoured prey item, generally occurred in higher densities in the LCW. Consistent with earlier studies, phyllosoma larvae were patchy in distribution. Nevertheless, they were more frequently encountered in STSW, particularly in an area between 30°- 31°S associated with strong eastward transport but relatively low zooplankton biomass and chlorophyll a concentration. We analyse the importance of links between oceanographically-driven prey fields and nutritional state of phyllosoma in the oligotrophic SE Indian Ocean