Why did the copepod Calanus sinicus increase during the 1990s in the Yellow Sea?

Recent field and retrospective data were combined to investigate variations from 1984 to 1998 in the spring abundance of dominant copepods in the Yellow Sea. A calanoid copepod, Calanus sinicus, was chosen to assess the long-term changes in abundance associated with temperature, salinity, chlorophyll-a and predator abundance. Average anomalies of sea surface temperature (SST), salinity, and the abundance of C. sinicus were positive in the 1990s and negative in the 1980s. The average abundances of C. sinicus in the study area in the 1990s were also significantly higher than those in the 1980s (p < 0.01). Catches of the anchovy Engraulis japonicus, a predator of C. sinicus, showed a decreasing trend during the study period. The higher abundances of C. sinicus in the 1990s may have been affected by an increase in water temperature and a decrease in predators, without distinctive changes in chlorophyll-a concentrations during the study period.