A regime shift is a change in the basic structure of an atmosphere, sea or marine environment that occurs on a scale of several decades. With regards to the Pacific Ocean, this can be observed as periodic fluctuations in such pelagic resources as Japanese pilchard, anchovy, pacific herring, and trout. Since the 1990s, efforts have been advanced to 10-year (or 20- of 50-year) climatic cycling. To detect a 10-year cyclic signal, we need several hundred years of records. As observational records only go back 100 years or so, this acts as a serious obstacle. Long awaited to overcome it has been some proxy for a paleoclimatological record, such as sea bottom sediment samples. Also, as the issue of global warming gains prominence in recent years, we need to better understand the relation between regime shifts and climate modes so as to predict how marine resources will fare in the future; and, for this, it is desirable to reveal the pattern of regime shifting with regards to long-term (100- to 1,000-year) climatic fluctuations. In this research, we analyze core samples taken from Beppu Bay. By this, we reconstruct water-temperature/herring-population patterns for the last 2,300 years, comparing our results against 100-/1,000-year climatic fluctuations to learn if 10-year fluctuations and regime shifting has changed over that time.