A comparative study on the influence of the pycnocline on the vertical distribution of fish larvae and cephalopod paralarvae in three ecoligically different areas of the Arabian Sea

Abstract

Zooplankton sampling took place during cruise 5 Leg 3 of the R.V. Meteor (March–June 1987) in three hydrographically and ecologically different areas of the Arabian Sea (Indian Ocean): an upwelling area at the coast of Oman; an oligotroph area in the central Arabian Sea; and a shelf area off the coast of Pakistan. All three areas were expected to hace similar ichthyoplankton and cephalopod components and similar light conditions. These are important prerequisites for the present comparative study, which is concerned with the importance of the structure of the water column (physical stability and prey availability), compared with the influence of the light intensity (day/night) on the vertical distribution of species and size classes of fish larvae and cephalopod paralarvae in the subtropical pelagial. First results show that the vertical structure of the water column, especially the occurrence of a pynocline and the varying mixed-layer width, either directly or indirectly had important impact on the vertical distribution patterns of both fish larvae and cephalopod paralarvae. In addition, cephalopods were influenced more consistently by the diurnal change of light intensity than fish larvae. Both taxa occurred mainly below the mixed surface layer. However, cephalopod paralarvae preferred shallower depths than fish larvae in all three areas and were closer related to the pycnocline than fish larvae in most cases. In the absence of a significant pycnocline, larvae appeared close to the surface.