Development of an Online Multi-Frequency Profiler for Monitoring Zooplankton

The density of zooplankton within bays and in inshore areas is an important environmental parameter for determining the optimal release timing of juvenile salmon (Oncorhynchus keta) from hatchery stations. This is because the juveniles remain in these areas for 1 month, during which time they feed upon zooplankton before they begin their journey to the ocean. We monitored zooplankton, water temperature and salinity from 2013 to 2017 in Yamada Bay, Iwate Prefecture, Japan, using a multi-frequency acoustic profiler collocated with temperature-salinity sensors and a depth sensor. Our observations revealed that the large zooplankton preferred by juvenile salmon are transported into the bay by the Oyashio Current. Therefore, it is important to determine the timing of this transportation. We developed an online profiler to record these events at a hatchery station in Yamada Bay.