We examined seasonal, annual variation and horizontal distribution of zooplankton in the Sea of Japan from 1966 to 1990. Zooplankton was most abundant in the spring. The spring maximum appeared in February–March and in April–May in the southern and eastern parts of the study areas, respectively. In the summer and autumn, a secondary peak was most conspicuous in the eastern part. The difference between the estimated biomass at night and day was large in the spring and small in summer and autumn. The biomass in the offshore southern area peaked about every 3 years between 1966 and 1983, and increased abruptly in 1990. The density in the area north of 39°N or 40°N was high. Total biomass estimated in the upper 150 m layer in the Sea of Japan (106 km2) was 9.5 × 106 t in the daytime and 16.6 × 106 t at night.
Regional comparisons of interannual variations in springtime lower trophic-level ecosystems were made for northern subarctic regions, and for southern Tsushima Current regions of the Japan Sea, based on archival hydrographic and biological data sets collected from the mid-1960s to the early 1990s. Variations related to the Pacific Decadal Oscillation were detected for plankton biomass in both northern and southern regions, although there were regional differences with respect to mechanisms and timing. Springtime stratification increased after the late 1970s in the north, roughly coinciding with the northern Pacific regime shift in 1976/77. Stratification also increased due to warming in the south in the early 1980s, several years after the 1976/77 regime shift. Responding to the increase in stratification, springtime biomass of phytoplankton and zooplankton increased in the north and decreased in the south. Principal component analysis revealed that hydrographic conditions during spring, rather than winter, determined springtime phytoplankton biomass. In northern regions, spring phytoplankton production may be enhanced by increased light availability, due to mixed layer stabilization. In the south, where background nutrient concentration within the water column was low, increases in stratification were likely to limit nutrient supply to the surface layer, resulting in decreases in phytoplankton production. A positive relationship between phytoplankton and zooplankton biomass suggested bottom-up control of secondary production in northern regions. The nature of the links between phytoplankton and zooplankton production was not clear in southern regions, where hydrographic conditions during winter seemed to be responsible for variations in springtime secondary production.
This paper investigates the relationship between sea-surface temperature (SST) and catch fluctuations in the Pacific stock of walleye pollock Theragra chalcogramma in Japan. Incorporating time lags between years of birth and harvest, the correlation coefficients between the catch and SST in two regions off the east coast of Hokkaido were calculated. The catch in year t had a high negative correlation with the SST during January-April and November-December of the years t-2 and t-3 in the spawning area. These results coincided well with the correlation observed in the northern 'Sea of Japan' stock. Both analyses suggested that the long-term catch fluctuations of the two stocks could be explained by the same mechanism, that is, the fluctuations would be explained by the SST in their spawning area during the spawning season using 2-3 or 3-5 years time lags, which corresponded to the dominant age of the catch within these two stocks.
The aim of this paper is to elucidate the fluctuation mechanism in the catch of pink salmon Oncorhynchus gorbuscha harvested in the Maritime Province of Siberia. We used catch data on pink salmon born in odd-and even-numbered years. Monthly indices of the Arctic Oscillation and the Pacific Decadal Oscillation were used as the environmental factors. We assumed that the catch in year t, C t , and that in year t + 2, C t+2 , could be used to represent the spawning stock biomass and recruitment, respectively, and C t+2 /C t could then be used to represent the recruitment per spawning stock biomass. Under these assumptions, we adopted the equation C t+2 /C t = g (environmental factors) as the model that forecasted the trajectories of the catch. The results were as follows: 1) the trajectories of the catches of pink salmon born in oddand even-numbered years can be well reproduced by the model mentioned above. No density-dependent effect was detected in the relationship between C t+2 and C t , which corresponds to the stock-recruitment relationship (SRR), for catches in both odd-and even-numbered years. The relationship between C t+2 and C t for odd-numbered years showed a clockwise loop; however, that for even-numbered years showed an anticlockwise loop. It is believed that this difference occurs in response to the negative relationship between the catches born in odd-and even-numbered years. Pink salmon is one of the typical fish species to which a density-dependent SRR can be applied; however, this study indicates that the assumption of a density-dependent SRR is not valid.
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