Intercomparison of zooplankton (net) sampling systems: Results from the ICES/GLOBEC sea-going workshop

An inter-comparison and evaluation of methods for sampling and determination of zooplankton distribution and biomass was conducted in a fjord environment (Storfjorden at Møre, Norway) from 2 to 13 June 1993. The inter-comparison was carried out with the German R/V " A. v. Humboldt" and the Norwegian R/V " Johan Hjort" and involved a total number of 38 scientific personnel from 8 countries. The inter-comparisons included the MOCNESS, BIONESS, MultiNet, LHPR, OPC, CPR, Gulf-V, CalCOFI 1-m Ring Net, and WP-2 net. In addition, acoustics data were collected with a Simrad EK500 echosounder operating on 4 frequencies (18, 38, 120, 200. kHz) and hydrographic, nutrient, phytoplankton, and meteorological data were collected to characterize the environment in which the comparisons were made; environmental changes were minor during the study.The results of this study corroborate the results of earlier studies. Mesh size of the net had a major influence on the biomass and species composition of the zooplankton community. There is a consistent relationship between retention (or escapement through the mesh) and the width of the organisms; about 50% of the organisms escape through the mesh at a width equal to the mesh size. The effect of towing speed on the extrusion of smaller organisms through the net can be substantial and adds to the loss due to escapement. Active avoidance of the sampler is only important for the larger macrozooplankton and not a significant problem for the mesozooplankton. Different vertical, oblique, and multiple opening/closing net systems produced similar estimates of zooplankton when operated with comparable mesh-sized nets and a sufficiently high mesh open area to mouth opening ratio. The choice of mesh size is important and 150. μm mesh is possibly an optimal mesh size for use in coastal waters with neritic zooplankton communities. Replicate tow variability was also consistent with earlier studies and reflects the need to understand the interplay between the underlying patchiness of the zooplankton distributions and the method of sampling. No single net is suitable to sample across the wide size range of zooplankton from small mesozooplankton to macrozooplankton. Recent large interdisciplinary programs to assess marine ecosystem structure and dynamics have recognized this through the use of nets designed to sample particular size fractions in combination with video and acoustic remote sensing techniques.