A statistical examination of the catches, numbers, and biomass taken by three commonly used plankton nets

Abstract

A comparison has been made between the Indian Ocean standard net (IOSN), the tropical Juday net (TJN), and the Clarke-Bumpus sampler (CBS). Various components of the zooplankton have been counted and the results submitted to analyses of variance.

The variability of the catches is greatest with the TJN, the percentage standard deviation of a single observation rangingfrom 73 to 135 %; the valuesfor the IOSN and the CBS are similar toeach other: 42-51 %. The greater variability of the TJNis ascribed to its smaller mesh with consequent clogging, and arises from variability in the volume filtered and from the difficulty of washing down such a large and fine net efficiently.

There is no evidence that the larger zooplankton avoid the smaller CBS any more successfully than they avoid the two large nets; as expected, the smaller zooplankton organisms escape more readily through the IOSN with its coarser meshes than through the other two nets. The catches for the IOSN and the CBS agree very well when expressed per volume of water filtered.

The main advantage of the larger nets--as regards the organisms considered here-is in the greater volume of water filtered; this would be of importance if a large bulk of material were required for, say, chemical analysis or if a search were being made for rare organisms.