Abstract
Spatial variation and temporal variation of spring-summer zooplankton community structure in Nemuro Bay and their relationship to environmental factors were investigated. Zooplankton community structure in Nemuro Bay was classified into three types by water temperature and depth. These community types consisted of groups emerged in 1）low temperature（＜8．8℃）and deep depth（≧10．5m in depth）offshore areas（Cluster 1）, 2）low temperature（＜8．8℃）and shallow depth（＜10．5m in depth）coastal areas（Cluster 2）, and3）high temperature（≧8．8℃）areas（Cluster 3）. Representative indicator species of Cluster 1were two medium-large-sized copepods, Acartia tumida and Calanus glacialis. Cluster1 could have been transported to Nemuro Bay with the inflow of Okhotsk surface sea water from the east side of Shiretoko Peninsula. Cluster 2,represented by Eurytemora herdmani – a brackish copepod, was considered to be a cluster dominant in low temperature and low
salinity environments（in brackish waters）. Cluster3dominated after middle June, when the development of thermocline became obvious and its representative indicator species was a cladocera, Podon leuckarti. Cluster3 could be suitable for growth and breeding under a high temperature and regenerated-production environment. Medium - and large-sized prey organisms（such as Cluster 1）, which were important for the growth of chum salmon fry emerged for a short period in Nemuro Bay. In addition,there was no replenishment of cold-water oceanic species（such as Neocalanus spp.）from outside of the bay. From the viewpoint of prey organisms, Nemuro Bay might not necessarily be favorable for the growth and survival of large chum salmon
fry compared with other coastal areas.
