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.

Objective: We examined differences in salient beliefs about physical activity (PA) and their influence on PA intention and behavior among South Korean university students before and during the COVID-19 pandemic. Methods: We used the Theory of Planned Behavior as a basis for the study. We used a repeated cross-sectional design. We conducted elicitation studies, and main and followup surveys with South Korean university students at two time points during September-October 2018 (n = 234) and September-December 2021 (n = 199). We used content analysis to identify salient beliefs about PA; we used structural equation modeling to analyze relationships among salient beliefs, PA intention, and behavior. Results: Prior to the pandemic, salient beliefs such as ‘builds social relationships,’ ‘takes too much time,’ and ‘friends’ significantly predicted PA intention and behavior. During the pandemic, ‘improves health status,’ ‘family members,’ and ‘laziness’ were significant predictors of PA intention and behavior although students’ average weekly PA time was decreased by half during the pandemic. Conclusions: The findings highlight how salient beliefs can shift substantially across contexts. Identifying such changes may improve predictions of PA engagement among university students, not only in the post-pandemic era but also in preparation for potential future public health crises.

Stratification is a fundamental process influencing nutrient availability and biological productivity in coastal marine ecosystems. In this study, we examine multi-decadal variability in winter stratification and nutrient distribution within Western margin of the East Sea (WES), using observational data collected between 1990 and 2023. Stratification characteristics were quantified using the Brunt-Väisälä frequency (N²), with particular focus on changes in intensity and the depth of maximum stratification. These variables were then analyzed in relation to environmental factors such as upper-layer temperature, surface wind speed, and East Korea Warm Current (EKWC) transport. Over the study period, stratification intensity increased at an average rate of 1.1% per year, while the maximum stratification depth shoaled by approximately 0.6% annual trends that became more pronounced after 2015. These shifts were closely linked to enhanced thermal stratification and increased EKWC volume transport. Enhanced stability of the water column was accompanied by reduced vertical mixing, which in turn intensified the nitrate gradient between surface and subsurface waters. The most marked nitrate accumulation was observed near 100 m, where stratification was strongest. This study highlights how physical oceanographic changes during winter can regulate nutrient supply prior to the spring bloom, potentially influencing seasonal productivity in marginal seas. The results emphasize the importance of long-term monitoring of vertical structure in assessing the ecological impacts of ocean variability.

This study aimed to determine how spatial and seasonal variations influence the trophic structure of fish assemblages in the eastern Yellow Sea during 2023. Carbon and nitrogen stable isotope analyses revealed significant but spatially variable differences in δ¹³C and δ¹⁵N values among seasons and sites, without consistent overarching trends. These results suggest that phytoplankton-derived organic matter predominantly supports the regional food web, with isotopic niche shifts reflecting local differences in prey availability and hydrographic conditions. The findings contribute to the development of adaptive, ecosystem-based fishery management strategies amid increasing anthropogenic and climatic pressures.