To standardize the netplankton datasets in terms of sampling gear and sampling depth, previous plankton data obtained from the oceanographic A-line surveys in the western subarctic Pacific were analyzed. Comparable netplankton sampling was carried out using two plankton sampling gears, the mini-Bongo net and the NORPAC (North Pacific Standard) net, installed with 335 µm mesh. Considering the two sampling depths, 0–50 m and 0–150 m, the mean ratio of dry weight (DW) of plankton biomass in unit volume was 2.22±1.19. Using the data of plankton population, the ratio of population at the sampling depths was close to DW, suggesting that the plankton biomass at the sampling depths was comparable. On the other hand, individuals of species/taxa in most samples did not differ significantly in terms of sampling depth and sampling gear. However, plankton populations of some species, mainly distributed below the surface mixing layer, was higher in the layer of 0–150 m than in the layer of 0–50 m. This result indicated the need for careful biomass evaluation for some species at different sampling depths.

Mass occurrences of scyphozoan jellyfish, such as Aurelia coerulea, have been reported in various waters; however, little is known about the hydrozoans and ctenophores in Tokyo Bay. In this study, the abundance of these jellyfishes was monitored for four years(April 2015–November 2016, April 2018–July 2019)in Tokyo Bay. Thirteen hydrozoan species and three ctenophores were observed. The dominant hydrozoan species identified were Cytaeis uchidae, Podocoryne minima, Rathkea octopunctata, Diphyes chamissonis, and Liriope tetraphylla, while Bolinopsis mikado was the most dominant among the ctenophores. Cytaeis uchidae, P. minima, and R. octopunctata appeared for a short period with extremely high abundance. Cytaeis uchidae occurred when the salinity was low, but P. minima occurred under high water temperature and high salinity conditions during summer. The abundance of R. octopunctata increased with decreasing water temperature during winter. On the other hand, D. chamissonis and L. tetraphylla presented prolonged abundance during most of the studied period. Diphyes chamissonis occurred in summer when the water temperature was high, and L. tetraphylla was observed at low temperatures and high salinities in autumn. It was suggested that these two species, without polyp stages, had entered from the adjacent waters in Sagami Bay. Diversity for the species composition of hydrozoans and ctenophores is very low, with some species dominant in Tokyo Bay; the occurrence of these dominant species is greatly affected by water temperature and salinity in the ambient seawater. These species play an important role in the gelatinous plankton ecosystem in Tokyo Bay.