Ecological studies on the deep-water pelagic community of Korsfjorden, Western Norway. Comparison of the catch of the more numerous species by two different nets

Abstract

The sampling characteristics of two zooplankton nets, a Longhurst Frame, 0.5 mm mesh and 0.09 m² mouth opening, and a Beyer Midwater Trawl, 1.0 mm mesh and 1.0 m² mouth opening, are compared. Volumes of water filtered per haul have been determined using data from a time-depth recorder. This method of volume determination is compared favorably with that of using a flow meter.

Estimates of numbers per 690 m³ water (volume under 1 m² surface water) per size group per species have been calculated for the two nets. The Longhurst Frame is shown to sample most groups as well as or better than the Beyer Trawl with the notable exception of Meganyctiphanes norvegica. No reduction in the variance of the density estimates can be shown using individual volumes per haul instead of an average volume from all hauls in the calculations.