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Selection of prey that are small and in poor body condition is a widespread phenomenon in terrestrial predator–prey systems and may benefit prey populations by removing substandard individuals. Similar selection is widely assumed to operate in aquatic systems. Indeed, size‐selective predation is a longstanding and central tenet of aquatic food web theory. However, it is not known if aquatic predators select prey based on their condition or state, compared to their size. Surprisingly, no comparable information is available for marine systems because it is exceedingly difficult to make direct observations in this realm. Thus the role of body condition in regulating susceptibility to predation remains a black box in the marine environment. Here we have exploited an ideal model system to evaluate selective predation on pelagic marine fish: comparing characteristics (fork length, mass corrected for fork length) of fresh, whole, intact juvenile Pacific salmon delivered by a seabird to its single nestling with salmon collected concurrently in coastal trawl surveys. Three species of juvenile salmon (Oncorhynchus spp.) are consumed by provisioning Rhinoceros Auklets (Cerorhinca monocerata); an abundant, colonial, pursuit‐diving seabird. Samples were collected from multiple colonies and fisheries surveys in coastal British Columbia in two years. As predicted, Auklets preyed on small individuals in poor condition and consistently selected them at levels higher than their relative availability. This is the first study to provide direct evidence for both size‐ and condition‐selective predation on marine fish in the wild. We anticipate that our results will be a starting point in evaluating how selective predation may structure or influence marine fish populations and bridges a fundamental incongruity between ecological theory and application; although “bigger is better” is considered a fundamental tenet of marine food webs, marine predators are often assumed to consume indiscriminately.

Fish-associated microbiota are an integral part to the health of the host fish. The ongoing climate changes including global warming of water may disrupt the composition and diversity of host-microbiota, and subsequently, destabilize the fish homeostasis. Since the knowledge on temperature-sensitive marine fish and environmental bacteria is scarce, we investigated the effects of rearing temperatures on community structure, diversity and assembly process of bacteria on chum salmon ( Oncorhynchus keta ), which is a temperate salmon species found in the Pacific. Over the course of two weeks, laboratory-raised chum salmon were exposed to three temperatures: high (18°C), low (8°C) and, control (13°C). Their feces, cutaneous mucus, and surrounding water were sampled for community structure analysis based on 16S rRNA gene amplicon sequencing. Temperature changes from the control level triggered significant dysbiosis in the fecal and skin mucus microbiota. In particular, Vibrio and Tenacibaculum sequence variants were highly abundant at high and low temperatures, respectively, and the opportunistic growth of these pathogenic species may impede host immunity. Two temperature-specific taxonomic microbial biomarkers, the class Betaproteobacteria and the genus Flavobacterium were identified at both high and low temperatures. An analysis of bacterial community assembly processes revealed that environmental selection significantly affected the gut microbial community assembly, while the assembly process of the skin microbiota was stochastic. Our study elucidated the potential crisis of fish health when the equilibrium of the cutaneous and intestinal microbiota was disrupted by temperature changes. Our data will be a valuable tool to better understand the effects of climate change, a very pressing and important challenge now and in the future, on the fish microbiota and its homeostasis.

In adult fish, neurogenesis occurs in many areas of the brain, including the cerebellum, with the ratio of newly formed cells relative to the total number of brain cells being several orders of magnitude greater than in mammals. Our study aimed to compare the expressions of aromatase B (AroB), glutamine synthetase (GS), and cystathionine-beta-synthase (CBS) in the cerebellum of intact juvenile chum salmon, Oncorhynchus keta. To identify the dynamics that determine the involvement of AroB, GS, and CBS in the cellular mechanisms of regeneration, we performed a comprehensive assessment of the expressions of these molecular markers during a long-term primary traumatic brain injury (TBI) and after a repeated acute TBI to the cerebellum of O. keta juveniles. As a result, in intact juveniles, weak or moderate expressions of AroB, GS, and CBS were detected in four cell types, including cells of the neuroepithelial type, migrating, and differentiated cells (graphic abstract, A). At 90 days post injury, local hypercellular areas were found in the molecular layer containing moderately labeled AroB+, GS+, and CBS+ cells of the neuroepithelial type and larger AroB+, GS+, and CBS+ cells (possibly analogous to the reactive glia of mammals); patterns of cells migration and neovascularization were also observed. A repeated TBI caused the number of AroB+, GS+, and CBS+ cells to further increase; an increased intensity of immunolabeling was recorded from all cell types (graphic abstract, C). Thus, the results of this study provide a better understanding of adult neurogenesis in teleost fishes, which is expected to clarify the issue of the reactivation of adult neurogenesis in mammalian species.

Over the last 5 decades, natural populations of pink (Oncorhynchus gorbuscha) and chum (O. keta) salmon were the most abundant salmon species returning to Washington, USA. Pink salmon predominantly returned in odd years, and chum salmon stocks that interacted with pink salmon exhibited strong even- and odd-year variations in abundance, size, age-at-maturity, and productivity (recruits-per-spawner). We investigated the effects of competition between pink and chum salmon originating from Washington during different life-history phases. Overall, chum salmon returns were 34% lower in pink salmon (odd) years compared to non-pink salmon (even) years. Chum salmon productivity tended to be below average for odd broods, especially along the Washington coast where there are no pink salmon populations, suggesting that competition during overlapping marine periods established their distinct even- and odd-year patterns. We evaluated long-term trends in chum (and pink) salmon productivity using correlation and time series analysis and examined the influence of ocean indicators on those trends. In general, chum salmon productivity increased between brood years 1968 and 1989 and was positively related to summertime Pacific Decadal Oscillation (PDO), but declined between brood years 1990 and 2012 and was negatively related to the PDO. Pink salmon productivity had no directional trend, but exhibited declines over the last decade and was negatively related to the PDO over the entire time series. Chum salmon productivity was also negatively related to pink salmon abundance, supporting the conclusion that ocean conditions and competition accounted for brood-year differences in chum salmon age-at-maturity and lower returns in odd versus even years.

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.

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.

This study examines the relationship between salmon lice Lepeophtheirus salmonis infestation pressure from Atlantic salmon farms and the prevalence of L. salmonis on wild juvenile Pacific salmon in British Columbia, Canada. We estimated weekly copepodids released from salmon farms and analysed infestation data from wild juvenile chum salmon Oncorhynchus keta and pink salmon O. gorbuscha across 4 regions from 2016 to 2023. A mixed-effects regression model revealed a significant positive relationship between farm-derived infestation pressure and L. salmonis prevalence on wild salmon at the sampling event level, with notable spatial and temporal variations. Chum salmon exhibited higher infestation rates than pink salmon, and fish length was positively correlated with infestation likelihood at the individual level. Our findings highlight the importance of accounting for fish length, environmental variables, and larval dispersal patterns to refine infestation pressure estimates. The complexities and uncertainties in quantifying infestation pressure underscore the need for further research. Future studies should validate underlying assumptions and incorporate additional variables and methodologies to improve assessments of infestation pressure on wild salmon populations.

The considerable post-traumatic brain recovery in fishes makes them a useful model for studying the mechanisms that provide reparative neurogenesis, which is poorly represented in mammals. After a mechanical injury to the telencephalon in adult fish, lost neurons are actively replaced due to the proliferative activity of neuroepithelial cells and radial glia in the neurogenic periventricular zone. However, it is not enough clear which signaling mechanisms are involved in the activation of adult neural stem cells (aNSC) after the injury (reactive proliferation) and in the production of new neurons (regenerative neurogenesis) from progenitor cells (NPC). In juvenile Pacific salmon, the predominant type of NSCs in the telencephalon are neuroepithelial cells corresponding to embryonic NSCs. Expression of glutamine synthetase (GS), a NSC molecular marker, was detected in the neuroepithelial cells of the pallium and subpallium of juvenile chum salmon, Oncorhynchus keta. At 3 days after a traumatic brain injury (TBI) in juvenile chum salmon, the GS expression was detected in the radial glia corresponding to aNSC in the pallium and subpallium. The maximum density of distribution of GS+ radial glia was found in the dorsal pallial region. Hydrogen sulfide (H2S) is a proneurogenic factor that reduces oxidative stress and excitotoxicity effects, along with the increased GS production in the brain cells of juvenile chum salmon. In the fish brain, H2S producing by cystathionine β-synthase in neurogenic zones may be involved in maintaining the microenvironment that provides optimal conditions for the functioning of neurogenic niches during constitutive neurogenesis. After injury, H2S can determine cell survivability, providing a neuroprotective effect in the area of injury and reducing the process of glutamate excitotoxicity, acting as a signaling molecule involved in changing the neurogenic environment, which leads to the reactivation of neurogenic niches and cell regeneration programs. The results of studies on the control of the expression of regulatory Sonic Hedgehog genes (Shh) and the transcription factors Paired Box2 (Pax2) regulated by them are still insufficient. A comparative analysis of Pax2 expression in the telencephalon of intact chum salmon showed the presence of constitutive patterns of Pax2 expression in neurogenic areas and non-neurogenic parenchymal zones of the pallium and subpallium. After mechanical injury, the patterns of Pax2 expression changed, and the amount of Pax2+ decreased (p < 0.05) in lateral (Dl), medial (Dm) zones of the pallium, and the lateral zone (Vl) of the subpallium compared to the control. We believe that the decrease in the expression of Pax2 may be caused by the inhibitory effect of the Pax6 transcription factor, whose expression in the juvenile salmon brain increases upon injury.

In Japan, chum salmon Oncorhynchus keta resources sustained by hatchery programs have been experiencing a decline in return rates since the 2000s, prompting the need to understand the underlying cause. On the Japan Sea side of Honshu, this study estimated the return rates of chum salmon using mass mark-recapture surveys. It examined the factors affecting return rates using statistical models. Groups of chum fry from 2011–2016 brood years tagged with otolith thermal marks were released at different release times (late January to late March) and sizes in two rivers. Their return rates into natal rivers as age 3–5 in 2014–2020 were estimated at 0–0.45%. The best model showed that the return rates increased with lower coastal water temperature at release (9.7–12.1℃). The recent increase in coastal water temperature during spring, associated with global warming, may have diminished the return rate. Notably, the model indicated a positive correlation between return rates and larger release size (0.34–1.16 g), although these effects were weak in the river where relatively large fry were released. This suggests a threshold size of approximately 1 g that may improve return rates. Based on these results, release strategies to enhance the return rates of the two rivers are discussed.