In recent years, sea ice has been rapidly decreasing in the polar regions due to global warming. In the northern Bering Sea, which is a seasonal sea ice area, a remarkable sea ice decline was observed in 2018, with sea ice melting about one month earlier than usual. As a result, various changes have been observed in the marine ecosystem. For example, a delay in phytoplankton blooms*1 , a northward shift in the distribution range of fish, a decrease in seabird populations, and a deterioration in the nutritional status of seals are known. Since the adverse effects observed in each organism could not be explained by the increase in water temperature alone, it was speculated that the deterioration of the feeding environment was the cause, but the details remained unclear.
Zooplankton is the most important food for higher-order predators in the ocean. Zooplankton feed on phytoplankton and are themselves eaten by fish and other predators, thereby transferring energy from lower-order organisms to higher-order predators. Among zooplankton, the research group paid particular attention to copepods, which are dominant in terms of biomass. By studying the abundance and composition of copepods, they attempted to elucidate the processes that affect marine ecosystems, starting with changes in the timing of sea ice melting.
