Journal of the Marine Biological Association of India

Volume 30 Issue 1&2

Net avoidance behaviour among larvel, juvenile and adult euphausids

K. J. Mathew

During the present investigations an attempt was made to assess the degree of net avoidance capability of euphausiids at the larval, juvenile and adult levels. The study was facilitated by a series of zooplankton samples collected during both day and night from 5 m above bottom to the surface. In spite of sampling from' almost' bottom, striking difference in the availability was observed especially for the adults of several species being, in most of the cases, more in the night samples. The results obtained have led to derive some conclusions on the behaviour of euphausiids.

The present studies indicated that as far as the larvae were concerned, the degree of variation in abundance was least between day and night and this speaks for their inability to avoid an approaching net in the day light. However, marked increase in number was observed with the juveniles in the night samples. A night time increase by 57 times was observed with the juveniles of Euphausia diomedeae, a surface shoaling species, whereas Stylocheiron affine, a mesopelagic species showed an increase by 0.21 times only in the night samples. The adults also showed marked difference of occurrence between day and night. An increase in number by 65.7 times was noticed in the night abundance of Euphausia sibogae, again a surface swarming species, over the day time occurrence. However, in the case of S. affine the night time increase was of the magnitude of 2.3 times only.

Date : 31-12-1988