Spatial Variability of Recent Otolith Growth and RNA Indices in Pelagic Juvenile Diaphus kapalae (Myctophidae): an Effect of Flow Disturbance near an Island?

Abstract

In February 1993, the pelagic juvenile myctophid Diaphus kapalae was sampled with a neuston net in the southern Coral Sea in a region of flow disturbance north of Cato Reef, and to the east in the free stream (northerly flow at 30 cm s-1). There was no significant difference in the size (11-16 mm SL) or age (45-74 days after hatching) between the two regions. Recent growth indices derived from the width of the peripheral daily growth increments (~10 μm each) revealed enhanced otolith growth 38 km downstream and up to 4 days before capture. No significant difference was found for times longer than a week before capture, consistent with fluctuation of the wake indicated from current-meter data. Daily increment formation was confirmed by marginal increment analysis. Recent otolith growth was correlated with the RNA content adjusted by dry weight, which also revealed significantly higher condition in the wake region. Recent growth was correlated with microzooplankton biomass, recorded with an optical plankton counter.