Climate-change associated changes in oceanographic conditions, particularly sea surface temperatures (SSTs), have systematically affected many marine fisheries resources around Japan. The Pacific Decadal Oscillation (PDO) index is considered one of the most important climate indices for describing basin-scale SST variations in the North Pacific, which are closely related to decadal variations in fisheries resources. Time series of the PDO index has been used to conventionally classify decadal conditions of ocean and fisheries resources around Japan as either warming or cooling regimes. It is now clear that for the 2000s to the mid-2010s, mostly during the "global surface warming slowdown," the SST regime around Japan was unconventional; despite the PDO index being in a negative phase, which normally corresponds to a warming regime around Japan, SSTs exhibited decadal cooling in some waters and seasons. SSTs in the western part of the North Pacific subtropical gyre gradually decreased, particularly in autumn-spring, whereas SSTs in the western subarctic gyre and the Sea of Okhotsk increased. Moreover, SSTs between the subtropical and subarctic waters around northern Japan exhibited seasonal contrasts that were decadally intensified through the combined effects of cooling in winter-spring and warming in summer-autumn. Some major marine fisheries resources around Japan showed decadal increases or decreases beginning in the mid-2000s, and appeared to respond to the unconventional SST changes in their early life stages. In this paper, we review atmosphere and ocean conditions around Japan in the 2000s-2010s in terms of global climate, present an overview of potential impacts of decadal SST trends on five major commercial fisheries resources (walleye pollock, chum salmon, Japanese sardine, Japanese anchovy, and Japanese flying squid), and raise awareness that an unconventional regime can appear transiently during a state of global warming.

To apply otolith microstructure to examination of age and growth of juvenile chum salmon Oncorhynchus keta inhabiting coastal waters, formation of otolith increments was investigated for juveniles reared in a seawater aquarium and in net pens. In all otoliths examined, a distinctive check was formed at the time of sea entry of the fish. The deposition of otolith increments after the check was daily for rearing both in the aquarium (57 days) and in the net pens (26 days). Check formation associated with sea entry was also observed in otoliths of juvenile salmon collected 1 km off the coast of Shari, Hokkaido, Japan. Transmitted light observation of otoliths of those fish revealed a transition in otolith increment appearance from dark to light. Otolith Sr:Ca ratio remarkably changed from a low to a high level, coinciding with the transition in otolith appearance. It is suggested that the transition was associated with individual sea entry. This study demonstrated that the check and/or transition associated with sea entry are applicable to a benchmark for otolith increment counts of juvenile chum salmon inhabiting coastal waters.KEY WORDS: chum salmon, daily growth increment, otolith check, Sr:Ca ratio.

To assess effects of intra-and inter-specific interactions on chum salmon in the central Bering Sea, chum salmon lipid content was analyzed as a proxy for body condition. We measured the lipid contents of 466 immature individuals collected during summer from 2002 to 2007. Individual variation in log-transformed lipid content was tested using multiple regression analysis with biological and environmental variables. A regression model that included chum salmon fork length and pink salmon CPUE (number of fish caught per 1500 m of gillnet) was the most effective in describing variation in lipid content. Path analysis showed that the negative effect of pink salmon CPUE was stronger than the effect of chum salmon CPUE on chum salmon lipid content. Stomach content analysis of 283 chum salmon indicated non-crustacean zooplankton (appendicularian, chaetognath, cnidarian, ctenophore, polychaete, and pteropod) was higher under conditions of high pink salmon CPUE. Increased consumption of non-crustacean zooplankton containing a low lipid level could lower the lipid content of chum salmon. Thus, chum salmon lipid content could be affected directly by their shift in prey items and indirectly by interspecific competition with pink salmon.

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Regime shifts of climatic and environmental conditions potentially affect the productivity of fishery resources, posing challenges in stock management. The stocks of the Japanese flying squid (Todarodes pacificus) are suspected to suffer from regime shifts, but detecting the occurrence of regime shifts in this species is generally difficult and unreliable because the short‐lived nature of this species inherently confounds the effect of regime shifts with observation and process errors. Here we developed a new state‐space assessment model to evaluate the influence of regime shifts on the spawner‐recruit relationship of the Japanese flying squid. The model simultaneously estimates the population dynamics of multiple stocks that could share some life history parameters, thereby stabilizing parameter inference. We demonstrate that two regime shifts in productivity around 1991 and 2015 caused two‐ to threefold changes of maximum sustainable yields. The model with regime shifts clarifies the relationship between fishing pressure and spawner abundance that is difficult to detect in a model with no regime shift. The state‐space approach is a promising tool for accurately assessing stock status by separating the recruitment process from observation errors and is expected to contribute to the effective management of marine biological resources sensitive to regime shifts.