マイワシ太平洋系群と対馬暖流系群の資源動態と環境要因との関係

摘要

Stock abundance of Japanese sardine (Sardinops melanostictus) widely fluctuated at multi-decadal scale. In the early 1970s, total biomass of Japanese sardine began to increase then reached 29 million tons in 1987, the highest amount of biomass since 1970. Maximum catch also amounted to 4.5 million tons in 1988. Thereafter the biomass rapidly declined. Since the early 2000s, present stock biomass remains about 100 to 200 thousand ton and the annual landings also shrunk with about 50 thousand ton. Roth are only 1% or less of the highest values in late 1980s. Such fluctuation of sardine biomass may have been driven by the decadal changes of the global-scale environmental forcing, mediated by the survival rates at the larvae and juvenile stages. The Japanese sardine is composed of two stocks, Pacific stock and the Tsushima current stock. They share a common in-phase biomass fluctuation pattern, though their magnitudes are different. In the Pacific stock, stock fluctuation correlated positively with the Pacific Decadal Oscillation (PDO) in winter, and negatively with sea surface temperature in the Kuroshio Exten-sion South Area (KESA) in winter and the Arctic Oscillation (AO) in winter. In the Tsushima current stock, stock fluctuation correlated positively with the Monsoon Index in winter (MOI), and negatively with AO in winter. Climatic Regime Shifts, also coincided with the stock dynamics of both stocks: the 1970/71 and 1976/77 regime shifts corresponded the beginnings of biomass increase and habitat expan-sion with high biomass period, respectively, and the 1988/89 shift coincided with the onset of stock collapse.