Regimes and stock-recruitment relationships in Japanese sardine (Sardinops melanostictus), 1951-1995

We used reproductive success, rather than abundance or catch, to identify regimes because reproductive success responds faster to environmental changes. Peak abundance of Japanese sardine during 1951-1995 was about 1000 times higher than minimum abundance. A regime shift occurred in the early 1970s when carrying capacity (measured using spawner-recruit models) increased by about 75 times. We hypothesize that this was due to large-scale changes in the Kuroshio and Oyashio Current systems. Long-term environmental variation (regimes), interannual variability in recruitment success, and density-dependent recruitment and growth rates affected dynamics of Japanese sardine. We hypothesize that density-dependent effects on recruitment of Sardinops spp. are common but usually obscured in short data sets by environmental variability and measurement error. Virtual population analysis and forward-simulation modeling approaches gave similar biomass and recruitment estimates. The relationship between sardine biomass and catch per unit search time was nonlinear. Mass-at-age and biomass were correlated, and it may be possible to use mass-at-age as an abundance index. Current abundance is low, and we believe that the environment has shifted to a regime that is unfavorable for Japanese sardine.