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Effects of spawning ecology differences between gadid fishes （Gadus macrocephalus, Gadus chalcogrammus） on stock fluctuations in the Northwestern Pacific under ocean warming

해양 온난화 환경 속 북서태평양에서 명태와 대구의 산란 생태 차이가 자원량 변화에 미치는 영향

초록/요약 도움말

Walleye pollock and Pacific cod are major commercial species, accounting for approximately 27–39% of total fishery catches by North Pacific coastal nations from 1970 to 2020. The coasts of Korea and Japan in the Northwest Pacific are the southern distribution limits of these semi-demersal cold-water species, where stocks have shown contrasting trends since the late-1980s climate regime shift （CRS）; however, the mechanism producing divergent stock variability between the two species within the same waters has not been clarified. This study examined the causes from two perspectives: long-term water-column structural change and interspecific differences in spawning ecology （pollock: pelagic eggs; cod: demersal eggs）. Hatching rates and larval survival of pollock and cod were experimentally measured, and temperature-dependent early-life-stage survival was applied to the marine environment to analyse stock variability. Cod hatching rates peaked at 5–7 °C, declined at 3 °C, decreased sharply above 10 °C, and reached zero at 13 °C. Median lethal time （M50） shortened from approximately 25 days at 3 °C to approximately 12 days at 10 °C, indicating reduced larval survival duration at higher temperatures. Pollock hatching peaked at approximately 4 °C and declined sharply above 10 °C. Hatching rate–temperature relationships were fitted with Gaussian models to quantify a habitat suitability index （HSI）, with HSI ≥ 0.6 set as the threshold for spawning- and hatching-suitable conditions. Pollock HSI was applied to spawning-season surface temperatures （reflecting pelagic-egg characteristics）, whereas cod HSI was applied to depth-specific （0–200 m） temperatures during the spawning and larval stages （reflecting demersal-egg characteristics）; for cod, the median depth of the HSI ≥ 0.6 layer was derived as the habitat suitable depth （HSD）. Along the Korean East/Japan Sea （EJS） coast, Japanese EJS coast, and Japanese Pacific coast, surface temperatures rose in common after the late-1980s CRS under intensified Kuroshio-origin warm-current transport, and pollock spawning-season surface HSI declined below 0.6 in all three regions. Along the Korean and Japanese EJS coasts, spawning-ground temperatures remained below 10 °C from 1982 through the 2020s, but nursery-ground temperatures rose gradually after the late-1980s CRS, with values exceeding 10 °C （HSI 〈 0.6） persisting from the 2000s; pollock catches in the EJS have accordingly been negligible since the 2000s. Along the Japanese Pacific coast, nursery-ground temperatures decreased to approximately ≤ 8 °C （HSI ≥ 0.6） in the late 1990s, sustaining low but persistent pollock catches; surface temperatures have risen gradually since the late 2010s under intensified warming. Along the Korean EJS coast, bottom temperatures decreased from the mid-1990s, HSD was maintained through the 2020s, and cod stocks increased gradually from the 2000s. Along the western coast of Korea, expansion and contraction of the Yellow Sea Bottom Cold Water （YSBCW） towards the Korean coast governed cod larval habitat extent and stock variability. Along the southeastern coast of the South Sea of Korea, unfavourable conditions for cod early life stages （HSI 〈 0.6） prevailed broadly, but favourable conditions of 6–8 °C （HSI ≥ 0.8） persisted around Jinhae Bay （the principal spawning ground）, sustaining cod stocks. Along the Japanese EJS coast, cod HSD deepened from the 2000s and stocks increased similarly to the Korean EJS coast; HSD has contracted since 2015, suggesting potential stock variability. Along the Japanese Pacific coast, cod HSI generally remained within the suitable range （HSI ≥ 0.6）, but HSI and catch variability were greater than in other regions; in the principal spawning ground of the Japanese Pacific stock, the unique thermal circulation of the semi-enclosed bay topography stably maintained HSD at approximately 25 m, whereas in the southern Pacific waters the northward shift of the Kuroshio axis since 2015 deepened HSD and progressively reduced cod catches. Surface and bottom water-column structural changes along the Korean and Japanese coasts reflect a common surface mechanism but region-specific bottom mechanisms. Surface layers in all three regions lie within the Kuroshio-origin warm-current system, with surface temperatures rising in common under intensification of the East Korea Warm Current （EKWC; Korean EJS）, Tsushima Warm Current （TWC; Japanese EJS）, and Kuroshio Extension （KE; Japanese Pacific）. By contrast, bottom layers are governed independently under suppressed downward heat transfer from enhanced surface stratification: by the bottom persistence of East/Japan Sea Intermediate Water （EJSIW） supplied by the North Korean Cold Current （NKCC） along the Korean EJS; by EJSIW residence reflecting subregional differences in the Coastal Branch of the Tsushima Warm Current （CBTWC） trapping effect along the Japanese EJS; and by shifts in the Kuroshio–Oyashio balance and bay-scale thermal circulation along the Japanese Pacific coast. Although walleye pollock and Pacific cod are cold-water semi-demersal species with similar life histories at the southern distribution limits along Korea and Japan, their differing spawning ecologies （pelagic vs. demersal eggs） expose them to distinct water-column environments, producing contrasting species- and region-specific stock variability under common warming forcing. These findings can support region-specific projections of cold-water fish stocks under climate-change scenarios and inform species- and region-specific adaptive stock-management strategies.

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목차 도움말

1. GENERAL INTRODUCTION 1
Chapter 2. Influence of Spawning and Nursery Ground Environmental Changes on Walleye Pollock Catches Along the Eastern Coasts of Korea and Japan After the Late-1980s Climate Regime Shift 5
2.1 Abstract 6
2.2 Introduction 7
2.3 Materials and Methods 10
2.3.1 Study Area and Summary of Abbreviations 10
2.3.2 Sea Surface Temperature and Climatic Factors 12
2.3.3 Habitat Suitability and Catch Data for Pollock 13
2.4 Results 15
2.4.1 Time Series Changes in the Pollock Catch 15
2.4.2 Environmental Changes in the Pollock Habitats along the ECK and the ECJ 16
2.4.3 Habitat Suitability Index （HSI） of Pollock along the ECK and the ECJ 19
2.5 Discussion 23
2.6 Conclusions 27
2.7 Reference 28
Chapter 3. Linking temperature-dependent survival of Pacific cod （Gadus macrocephalus） early life stages to habitat suitability and catch variability 37
3.1 Abstract 38
3.2 Introduction 39
3.3 Materials and Methods 43
3.3.1 Experimental design for early life stages of Pacific cod 43
3.3.2 Pacific cod catches and the marine environment 44
3.3.3 Statistical analysis 47
3.3.4 Statistical analyses of lagged HSI–catch relationships 47
3.4 Results 48
3.4.1 Hatching period, hatching rate, and density of fertilised eggs 48
3.4.2 Growth rate, yolk absorption rate, median lethal day, and larval density 49
3.4.3 Changes in Pacific cod catch and habitat conditions for early life stages of Pacific cod by region 54
3.4.4 Lagged catch responses to egg-stage habitat suitability and hatch timing 65
3.5 Discussion 66
3.5.1 Survival of Pacific cod eggs and larvae at different temperatures 66
3.5.2 Regional effects of early life stage habitat changes on catch fluctuations 67
3.5.2.1 Eastern coast of Korea 67
3.5.2.2 Western coast of Korea 68
3.5.2.3 Southern coast of Korea 69
3.6 Conclusions 71
3.7 Reference 72
3.8 Appendix 83
Chapter 4. Spawning strategy drives divergent stock trends of walleye pollock and Pacific cod under ocean warming in the northwestern Pacific 94
4.1 Abstract 95
4.2 Introduction 96
4.3 Materials and Methods 99
4.3.1 Study area and catch 99
4.3.2 Marine environmental analysis 101
4.3.3 Habitat suitability index （Habitat suitability depth） 103
4.3.4 Statistical analysis 104
4.4 Results 105
4.4.1 Water column environment 105
4.4.2 Catch trends 106
4.4.3 Regional changes in habitat suitability （habitat suitability depth） of Pacific cod and Walleye pollock 108
4.4.4 Cross-correlation analysis between habitat suitability index （habitat suitability depth） and fish catch by region 111
4.5 Discussion 113
4.5.1 Physical Mechanisms of Water Column Structure Changes by Region 113
4.5.2 Effects of Regional Changes in Early Life Stage Habitat on Catch Fluctuations 114
4.6 Conclusions 119
4.7 Reference 120
4.8 Appendix 132
5. GENERAL DISCUSSION 137
6. GENERAL CONCLUSIONS 141
7. REFERENCES 143
8. KOREAN ABSTRACT 152

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