Diurnal variations in the vertical distribution and abundance of zooplankton in the continental shelf waters off Cochin during April 1991

The present study is based on zooplankton samples collected by vertical hauls from 0-30, 30-60 and 60-90 m depths from a 100 m depth station in the continental shelf (09° 44' N, 75° 42*8) off Cochin for two days at three hourly interval. The mean displacement volume of zooplankton biomass at these depth zones were 6.24, 6.70 and 6.07 ml/100 m water while their numerical counts were 14490,13355 and 11290 no/100 m respectively. The overall mean volume (ml) and number per 100 m in the endre water column (0-90 m) were 6.34 and 13045 respectively and were distributed as 5.89 ml and 11715 no/100 m^ in the daytime and 6.79 ml and 14376 no/100 m^ during night hours respectively. The biomass was dominated by copepods (75.6%), decapod larvae (12.2%), chaetognaths (6.5%) and appendicularians (2.4%) in April. In general, copepods, decapod larvae and appendicularians showed increasing trend in number from the bottom (60-90 m) to the surface layer (0-30 m); chaetognaths, medusae, pteropods, heteropods, salps and doliolids were distributed more in the middle zone (30-60 m); while ostracods and planktonic polychaetes were more in the bottom zone (60-90 m). Distribution of amphipods and ostracods indicated significant diurnal vertical migration. The phenomenon of mid- night sinking was noticed in the case of copepods, chaetognaths, appendicularians, medusae, siphonophores, salps and doliolids.