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The ultrastructural organization of different cell types involved in homeostatic growth in the cerebellum of juvenile chum salmon (Oncorhynchus keta) was investigated using transmission and scanning electron microscopy. The organization of astrocytes, oligodendrocytes, dark cells, adult-type glial and non-glial progenitors, stellate neurons, and eurydendroid cells (EDCs) in the molecular and granular layers and granular eminences was characterized. The organization of dendritic bouquets of Purkinje cells and climbing fibers was studied for the first time at the ultrastructural level, and the ultrastructural features of mossy fibers and the rosettes they form were characterized. Scanning electron microscopy (SEM) revealed the presence of single and paired adult-type neural stem/progenitor cells (aNSPCs) on the cerebellar surface and stromal clusters of aNSPCs outside the dorsal matrix zone (DMZ). Immunohistochemical (IHC) verification of proliferating cell nuclear antigen (PCNA) revealed five types of proliferating cells in the cerebellum of juvenile chum salmon: neuroepithelial cells (NECs), glial aNSPCs, and non-glial aNSPCs. A glial fibrillary acidic protein-positive (GFAP) complex consisting of radial glial fibers and aNSPCs was detected in the DMZ. At the same time, a complex of GFAP+ cerebellar afferents, consisting of differentiating mossy and climbing fibers, was found to develop in the cerebellum of juvenile chum salmon. Nestin+ non-glial aNSPCs and small nestin+ resident cells were detected in the dorsal, lateral, and basal areas, as well as in the granular layer (GrL) and granular eminences (GrEm). These cell types may contribute to the homeostatic growth of the cerebellum by acting as both active participants (PCNA+) and resident (silent) aNSPCs. Studying vimentin-positive systems in the cerebellum revealed a widespread presence of proliferating glial aNSPCs that actively contribute to homeostatic growth, as well as small resident immunopositive cells throughout the cerebellum of juvenile chum salmon. Immunolocalization of the neuronal RNA-binding protein marker (HuCD) was detected in numerous molecular layer (ML) cells at the early stages of neuronal differentiation in the dorsal and lateral regions of the cerebellum of juvenile chum salmon. HuCD + EDCs were detected for the first time in the dorsal (DZ) and basal (BZ) zones, forming broad axonal arborization. Immunolabeling of HuCD in combination with transmission electron microscopy (TEM) allowed EDCs to be characterized in the cerebellum of juvenile chum salmon for the first time.

Salmon represented a critical resource for prehistoric foragers along the North Pacific Rim, and continue to be economically and culturally important; however, the origins of salmon exploitation remain unresolved. Here we report 11,500-y-old salmon associated with a cooking hearth and human burials from the Upward Sun River Site, near the modern extreme edge of salmon habitat in central Alaska. This represents the earliest known human use of salmon in North America. Ancient DNA analyses establish the species asOncorhynchus keta(chum salmon), and stable isotope analyses indicate anadromy, suggesting that salmon runs were established by at least the terminal Pleistocene. The early use of this resource has important implications for Paleoindian land use, economy, and expansions into northwest North America.

The Tone River in Japan represents one of the southern limit distributions of chum salmon ( Oncorhynchus keta ) on the western side of the North Pacific, but the number of adult chum salmon observed here has declined dramatically since 2013 and reached zero in 2024. The factors behind the recent decline of the chum salmon population in the Tone River were investigated by using ocean reanalysis data and a 20-year particle-tracking simulation. Virtual chum salmon fry were released at the mouth of the Tone River in spring each year with six different swimming strategies to evaluate the effects of ocean currents on the population growth rate of salmon. None of the simulated scenarios reproduced the recent decline in the population, regardless of the swimming strategy and addition of lethal temperature thresholds. Instead, the decline in population growth rate appears to be correlated with warming water temperature and reduced zooplankton abundance caused by the northward shifts of the Kuroshio/Kuroshio Extension and Oyashio. Along the coast of northeastern Japan, the warm, nutrient-poor Kuroshio/Kuroshio Extension replaced the cold, nutrient-rich Oyashio on the migration route of chum salmon fry, increasing the water temperature and reducing zooplankton abundance. Partial correlation analysis of the water temperature and zooplankton abundance indicated that the latter was the main influencing factor coherently related to the population growth rate of salmon. The reduced zooplankton abundance would affect the growth and survival of chum salmon fry, which would result in a decline in population growth. The northward shift of the Kuroshio/Kuroshio Extension and Oyashio may continue or return southward depending on the effects of climate change, which will greatly influence the future population growth of chum salmon and whether they come back to the Tone River.

Techniques to control the phototactic behaviour of fish have expanded with progress in LED lights. However, the phototaxis direction of fish could be reversible at some light intensities, and thus it is necessary to evaluate the light-intensity levels that will induce repulsion or attraction behaviour to understand the transition. This study determined the light intensities of white LED light required to induce repulsion or attraction behaviour from a dark place, and the degree of dark-adapted visual sensitivity in juvenile of two salmon species. Oncorhynchus keta smolts showed negative phototaxis under intense light intensity, but positive phototaxis was not observed. The range of light intensities under which O. masou exhibited positive phototaxis changed with the life stage (from pre-smolts to smolts). Notably, the light intensities that elicited positive phototaxis were relatively low and narrow for pre-smolts, whereas smolts responded to a greater range of intensities. Positive phototaxis disappeared in O. masou pre-smolts under intense light intensity, but not in O. masou smolts under the most-intense light intensity tested here. Negative phototaxis was not observed in O. masou pre-smolts or smolts. The appropriate light intensity indicated here may be used to either guide juvenile O. masou from the dam reservoir to fishways or bypass channels, or to repel O. keta smolts from the water intakes of agricultural diversion weirs or hydropower dams.

Climate change is accelerating at unprecedented rates, with disproportionate impacts on marine ecosystems. We examined the distributional responses of chum salmon (Oncorhynchus keta) , a keystone species, to the recent climatic changes in the North Pacific from 1998 to 2022. We applied a multi-model ensemble approach to examine the spatial and temporal distribution patterns of suitable habitats throughout their seasonal migratory grounds and relate them to recent declines of Japanese chum salmon stocks. Our modelling identified temporal shifts in the locations of feeding and overwintering grounds, driven by increasing ocean temperatures, deteriorating forage conditions, and strengthening wind and marine heatwave intensity. During their feeding migration, the suitable chum habitat in the Bering Sea was reduced but increased in the Arctic. Overwintering habitat patterns further captured an overall decline in the trailing edges of their distribution, accompanied by habitat shifts towards the central North Pacific. Periods of marine heatwaves further coincided with sizable habitat losses. Such habitat displacements potentially affect the Japanese chum salmon stocks, shown by substantial habitat reductions in the Okhotsk and central Bering seas. These findings highlight the exacerbating exposure and vulnerability of the chum salmon populations to recent climatic and productivity changes throughout their marine life history, with concomitant repercussions on their production dynamics and provision of ecosystem services. Thus, requiring climate-adaptive measures such as the readjustment of fishing seasons and quotas according to the changing salmon stocks and habitat conditions, and the improvement of current hatcheries practices to manage and conserve salmon resources.

To understand the ecology of juvenile chum salmon during early marine life after their downstream migration, we developed a quantitative PCR-based environmental DNA (eDNA) method specific for chum salmon and investigated the spatiotemporal distribution of eDNA in Otsuchi Bay, Iwate, Japan. Indoor aquarium experiments demonstrated the following characteristics of chum salmon eDNA: (1) the eDNA shedding and degradation were time- and water temperature-dependent and the bacterial abundance could contribute to the eDNA decay, (2) fecal discharge may not be the main source of eDNA, and (3) a strong positive Pearson correlation was found between the number of juveniles and the eDNA amounts. As we discovered strong PCR inhibition from the seawater samples of the bay, we optimized the eDNA assay protocol for natural seawater samples by adding a further purification step and modification of PCR mixture. The intensive eDNA analysis in the spring of 2017 and 2018 indicated that juvenile chum salmon initially inhabited in shallow waters in the shorefront area and then spread over the bay from January to June. The eDNA data also pointed out that outmigration of juvenile chum salmon to open ocean temporarily suspended in April, possibly being associated with the dynamics of the Oyashio Current as suggested by a previous observation. The eDNA method thus enables us large-scale and comprehensive surveys without affecting populations to understand the spatiotemporal dynamics of juvenile chum salmon.

Understanding the responses of marine organisms to environmental changes at their distribution limits is crucial for predicting climate-change associated habitat changes. This study analyzed the effect of sea surface temperature (SST) on the temporal distribution of Chum salmon ( Oncorhynchus keta ) in the eastern and southern coastal waters of Korea (ESCK) and on the southern limit of their distribution in the North Pacific. The temporal distribution of Chum in the ESCK and adjacent rivers was statistically compared based on three SST types (T1–T3). Chum were first caught in the northern and then in the southern area, with riverine migration occurring faster in the south than in the north. These migration patterns did not change with SST type. There was no significant difference in the coastal arrival timing of the Chum between T1 and T3, which respectively represented the entire region cooling either rapidly or slowly compared to an average year. In T2, in which the north cooled rapidly and the south cooled slowly, the coastal arrival timing was approximately 4 days earlier compared to T1 and T3. Moreover, as the SST type shifted from T1 to T3, the coastal residence time in the north became shorter, while in the south became longer. These findings help us to understand the adaptation strategies of Chum, and to predict changes in their distribution and resources in the North Pacific under climate change.

Climate warming is likely to cause both indirect and direct impacts on the biophysical properties of stream ecosystems especially in regions that support societally important fish species such as Pacific salmon. We studied the seasonal variability and interaction between stream temperature and DO in a low-gradient, forested stream and a glacial-fed stream in coastal southeast Alaska to assess how these key physical parameters impact freshwater habitat quality for salmon. We also use multiple regression analysis to evaluate how discharge and air temperature influence the seasonal patterns in stream temperature and DO. Mean daily stream temperature ranged from 1.1 to 16.4°C in non-glacial Peterson Creek but only 1.0 to 8.8°C in glacial-fed Cowee Creek, reflecting the strong moderating influence glacier meltwater had on stream temperature. Peterson Creek had mean daily DO concentrations ranging from 3.8 to 14.1 mg L(-1) suggesting future climate changes could result in an even greater depletion in DO. Mean daily stream temperature strongly controlled mean daily DO in both Peterson (R2=0.82, P<0.01) and Cowee Creek (R2=0.93, P<0.01). However, DO in Peterson Creek was mildly related to stream temperature (R2=0.15, P<0.01) and strongly influenced by discharge (R2=0.46, P<0.01) on days when stream temperature exceeded 10°C. Moreover, Peterson Creek had DO values that were particularly low (<5.0 mg L(-1)) on days when discharge was low but also when spawning salmon were abundant. Our results demonstrate the complexity of stream temperature and DO regimes in coastal temperate watersheds and highlight the need for watershed managers to move towards multi-factor risk assessment of potential habitat quality for salmon rather than single factor assessments alone.

Currently, the problem of climate change on Earth is becoming increasingly urgent. These changes are the reason for the increasingly pronounced adaptive differences in different species of fish. A significant gap in ultrastructural data on the organization of the salmon cerebellum was the main motivation for this study’s microscopic and ultrastructural analyses using transmission and scanning electron microscopy of the cerebellum of juvenile chum salmon Oncorhynchus keta. The study of the interneuron composition of the cerebellum showed the presence of stellate cells in the molecular layer, projection Purkinje cells, and eurydendroid cells in the ganglion layer. Large Golgi cells and granular cells were found in the granular layer. The study of the synaptic structure of the molecular layer showed the presence of synaptic contacts of electrotonic and chemical types, which are an important link in interneuronal communications. Most synaptic endings of parallel fibers of the excitatory type in juvenile chum salmon converge onto dendrites of Purkinje cells. Transmission electron microscopy (TEM) study of neuro–glial relationships also revealed a heterogeneous population of astrocytes and microglia in the cerebellum of juvenile chum salmon. Patterns of apoptosis and phagocytosis involving protoplasmic astrocytes were detected. The presence of protoplasmic astrocytes in the cerebellum of juvenile chum salmon contrasts with data reported for zebrafish. The conducted studies allow us to conclude that the homeostatic growth of the cerebellum of juvenile chum salmon can occur according to an uncertain pattern and be mediated by the presence of adult-type neural stem/progenitor cells (aNSPCs). The presence of aNSPCs of glial and non-glial types in the cerebellum of juvenile chum salmon was demonstrated by TEM and scanning electron microscopy (SEM). The discovery of a large population of non-glial aNSPCs in the dorsal matrix zone (DMZ) and granular layer of juvenile chum salmon, as well as stromal cell clusters on the surface of the cerebellar molecular layer, suggests the activity of a neurogenic program in the brain of juvenile chum salmon that is mainly active during embryonic stages in other vertebrate species. The phenomenon of embryonization in the cerebellum of juvenile chum salmon is determined by the presence of non-glial aNSPCs, which contribute to homeostatic growth.

Korea’s northeastern coastal waters (NECWs) represent the southwestern range of chum salmon ( Oncorhynchus keta ), spanning temperate and boreal zones. However, understanding their migration to the NECWs in association with environmental changes remains challenging. Using tagging and tracking techniques, we studied salmon migration behavior and timing during spawning in ascending rivers. When sea levels rose, resulting in a decrease in sea surface temperature, salmon exhibited active vertical movement through the water column and migrated to the surface layer. The number of salmon ascending rivers between early October and early December increased when the sea surface temperature decreased below 18 °C, peaking when the water column was mixed during spring tides, and decreased when sea surface temperature dropped below 14 °C. In rivers, salmon favored gravelly riverbeds over sand/silt substrates, stayed in deep and shaded areas during the day, and advanced upstream at night. Our findings emphasize that water temperature and tidal elevation are key factors affecting salmon distribution in coastal waters and the timing of river entry. Riverbed composition, water depth, and photoperiod also influenced migration speed and timing in rivers. This research enhances our understanding of salmon behavior during spawning in the NECWs and adjacent rivers at their southern limits.