A volumetric analysis of the catch of the Isaacs-Kidd midwater trawl and two types of plankton net in the Antarctic

Abstract

A comparison was made of plankton samples collected by different nets along the Antarctic convergence between 95 and 150°W. in October-November 1964. The nets were a 3-m Isaacs-Kidd midwater trawl (IKMT) (4-mm mesh liner, 500-µ mesh codend net), 0.5 m² Bé closing nets (202-μ mesh), and a 0.2 m² net (76-μ mesh).

IKMT catch volumes were much greater than those of the smaller plankton nets. However, when volumes of water filtered were considered, the smaller nets caught more plankton. In the upper 100 m the ratio of IKMT to closing net plankton volume per cubic metre was 1 : 10 (1 : 4 if phytoplankton-rich samples were excluded). The ratio decreased to 1 : 3 for the upper 500 m and to 1 : 2 for the 500-1000-m zone. In both the 0-250 and 0-500-m ranges the 76-μ mesh catch per cubic metre was double that of the 202-μ mesh nets.

The greatest portion of the IKMT catch was fishes, squids, scyphomedusae, and plankton larger than 4 mm. The smaller nets caught proportionately less micronekton and large plankton. The fraction of plankton greater than 4 mm increased with increasing sampling depth and diminished with decreasing mesh aperture. The IKMT in the upper 100 m caught more fishes and greater plankton volumes in twilight and dark hours than during daylight. Diurnal variations in 202-μ net catches were scarcely noticable.