Mesozooplankton biomass plays a key parameter in the recruitment
processes of fish and biogeochemical processes. Four decadal
observations in the coastal Sea of Japan, the marginal sea of the North
Pacific, indicate that wet weight-based mesozooplankton biomass is
controlled by both environment-induced bottom-up and predatory-induced
top-down processes. Interannual variations in mesozooplankton biomass
using a generalized linear model approach showed a decrease in biomass
during the 1980s, followed by a rapid increase in the early 1990s, and a
gradual decrease in the 2010s. These interannual variations were the
mirror image of the small pelagic planktivorous fish biomass. The
difference in zooplankton biomass from the previous year was negatively
correlated with the difference in small pelagic planktivorous fish
biomass, which was supported by a Granger causality analysis. Therefore,
the results of this study indicate that top-down control is one of the
main causes of long-term variations of zooplankton biomass in the ocean.