東京湾におけるクラゲ類（刺胞動物及び有櫛動物）の最近１５年間の出現状況

Abstract

The occurrences of cnidarian medusae and ctenophores have been monitored monthly for 15 years (Jan. 1981-Jun. 1996) in Tokyo Bay and are compared with previous records. Four species of cnidarians, Aurelia aurita (Linnaeus), Chrysaora melanaster Brandt, Pelagia noctiluca (Forskal) and Spirocodon saltator (Tilesius), and three of ctenophores, Beroe cucumis Fabricius, Bolinopsis mikado (Moser) and Pleurobrachia pileus (O. F. Muller), were identified. Among them, only A. aurita seems to reproduce in the inner bay and often forms dense aggregations. Bolinopsis mikado appeared frequently except at the innermost part of the bay. The other species occurred rarely before 1988 but the occurrence increased after 1988. In the early reports in the 1910s, when eutrophic water occupied only in the innermost part of the bay, A. aurita occurred sporadically at much lower concentrations comparing to our observations. Bolinopsis mikado and Beroe cucumis often occurred from autumn to winter and C. melanaster in early summer. Due to the acceleration of eutrophication in the 1960s, the zooplankton community composition, including medusae and ctenophores, changed drastically. Large swarms of A. aurita began to appear contemporary wity the extreme increase of a small copepod, Oithona davisae. The hypereutrophication condition of Tokyo Bay has not been ameliorated so far, the occurrences of outer-bay species, however, tend to increase recently. The fresh water discharge into Tokyo Bay has increased, and that suppose to strengthen the density current. Another possibility of increasing water exchange is the long-term hydrographic variation caused by the change of current path of the Kuroshio in relation with the large meander. The increase of non-resident medusae after 1988 may be caused by the enhanced water exchange between the adjoining sea and the bay.