Comparative analysis of ichthyoplankton samping gear for early life stages of fish in the Yangtze River estuary and adjacent areas

Although it is common sense that the accurate sampling and unbiased data acquired are essential bases for further evaluation, during the field work of ichthyoplankton study, samples from conventional single gear are highly possible to deviate from the population greatly. To choose the more efficient sampling gear, comparative gear tows were conducted from April to July of 2014 in the Yangtze River estuary and adjacent areas with focus on species composition, density estimates (ind./100 m³), developmental stage composition of larval fish assemblages. Larval fish were collected by the plankton net (0.5 mm mesh) and frame net (1.0 mm mesh), respectively. Results indicated that the species richness was higher in the catches of frame net (51 species) than that in plankton net (37 species), along with 34 mutual species between them, while the composition of dominant species showed no significant difference. There was an unpredicted bias in average density of catches between two gears. In April, the average density was higher in catches of plankton net, due to the abundant occurrence of pre-flexion larvae which were too tiny to be efficiently caught by large mesh. In contrast, the average density was higher in frame net than those in plankton net from May to July. The plankton net samples were dominated by pre-flexion larvae, and the average density of pre-flexion larvae of anchovy (Engraulis japonicus) and gobies (Gobiidae spp.) was higher in catches of plankton net. While post-flexion larvae were dominated in frame net, and the average density of postflexion larvae of the dominant species, which were small yellow croaker (Larimichthys polyactis), anchovy and gobies, was higher in frame net samples. These differences suggested that there was significant catch efficiency between the two nets. In conclusion, frame net is more efficient. However, when it comes to specifically collect fishes of elongated type (e.g. anchovy), combination of multiple types of sampling gears should be in consideration. Conversely, an unique frame net is highly adequate for broad type fishes (e.g. croakers).