Catchability Of Three Types Of Commonly Used Plankton Nets: A Comparison Study

By analyzing the zooplankton samples collected in central Bohai Sea (China) in autumn of 2012, three types of commonly used plankton nets (the 505μm-meshed coarse net, the 200 μm-meshed WP-2 net and the 77μm-meshed fine net) were compared in terms of catchability for copepods and a chaetognath species Sagitta crassa. Mainly the abundance was compared in connection with organism body sizes to reveal the differences in catchability of the three nets.1. Catchability for copepodsThe coarse net benefited from its large mouth area and captured 2 rare species. However, it substantially under-sampled small copepods due to its coarse mesh size, leading to the lowest species number. The fine net was so fine-meshed that it collected all small copepod species, but failed to collect any rare species because of its small mouth area. The WP-2 net appeared to be a compromise of the other 2 nets, collecting all small copepod species and 4 rare species.Significant differences among the nets were detected in terms of total copepod abundance. The abundance of the coarse net was 1-2 orders of magnitude lower than the WP-2 net, while the fine net was 2-3 times higher than the WP-2 net. Mesh size proved to be the key factor affecting catchability:the smaller the mesh size, the higher the abundance. The lowest body width which the coarse net and the WP-2 net can collect was close to 1/2 the mesh diagonal and 1/3 the mesh width. This threshold value for the fine net was about 70～90μm. Where copepod body width exceeded 1.33-1.41 times of the mesh width, organisms could be fully collected.2. Catchability for Sagitta crassaIn respect of body length frequency distribution, the result obtained by the coarse net was wrongly pictured. The 1-4 mm length interval was the most severely underestimated fraction. The results for the WP-2 net and the fine net were in good agreement except for 1-2 mm interval.The fine net collected the most individuals, reaching 2.9 and 1.3 times the abundance of the coarse net and the WP-2 net respectively. The coarse net significantly under-sampled the 1-9 mm length interval. The WP-2 net to some extent under-sampled the 1-4 mm length interval. Such underestimation was clearly due to relatively large mesh size. The fine net somehow was less efficient for the 9+mm interval, which might be ascribed to the avoidance of motile individuals.To sum up, mesh size act as the critical factor affecting the functioning of the nets. The fine net apparently outperformed the coarse net in terms of collecting copepods. The catchability for Sagitta crassa varied with mesh sizes. The differences in abundance appeared to be the biggest in the 1-2 mm interval and gradually reduced for longer individuals. This thesis aims at contributing to elevating sampling strategies for a better understanding of zooplankton communities.