Kuroshio Current E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Serie: Geophysical Monograph Series
• Sprache: Englisch

Seitenzahl: 927

Veröffentlichungsjahr: 2019

Beschreibung

An interdisciplinary study of the Kuroshio nutrient stream The surface water of the Kuroshio, a western boundary current in the North Pacific Ocean, is nutrient-depleted and has relatively low primary productivity, yet abundant fish populations are supported in the region. This is called the "Kuroshio Paradox". Kuroshio Current: Physical, Biogeochemical and Ecosystem Dynamics presents research from a multidisciplinary team that conducted observational and modeling studies to investigate this contradiction. This timely and important contribution to the ocean sciences literature provides a comprehensive analysis of the Kuroshio. Volume highlights include: * New insights into the role of the Kuroshio as a nutrient stream * The first interdisciplinary examination of the Kuroshio Paradox * Reflections on the influence of the Kuroshio on Japanese culture * Research results on both the lower and higher trophic levels in the Kuroshio ecosystem * Comparisons of nutrient dynamics in the Kuroshio and Gulf Stream * Predictions of ecosystem responses to future climate variability

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Geophysical Monograph 243

Kuroshio Current

Physical, Biogeochemical, and Ecosystem Dynamics

Takeyoshi NagaiHiroaki SaitoKoji SuzukiMotomitsu TakahashiEditors

This Work is a co‐publication of the American Geophysical Union and John Wiley and Sons, Inc.

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CONTRIBUTORS

Sophie ClaytonOcean, Earth & Atmospheric Sciences Department, Old Dominion University, Norfolk, VA, USA

Hisashi EndoFaculty of Environmental Earth Science, Hokkaido University, Hokkaido, Japan;CREST, Japan Science and Technology, Tokyo, Japan;Bioinformatics Center, Institute for Chemical Research, Kyoto University, Kyoto, Japan

Xinyu GuoCenter for Marine Environmental Studies, Ehime University, Ehime, Japan;Japan Agency for Marine‐Earth Science and Technology, Kanagawa, Japan

Akimasa HabanoAquatic Sciences, Faculty of Fisheries, Kagoshima University, Kagoshima, Japan

Daisuke HasegawaTohoku National Fisheries Research Institute, Japan Fisheries Research and Education Agency, Miyagi, Japan

Toru HasegawaSeikai National Fisheries Research Institute, Japan FisheriesResearch and Education Agency, Nagasaki, Japan

Kiyotaka HidakaNational Research Institute of Fisheries Science, JapanFisheries Research and Education Agency, Kanagawa, Japan

Yutaka HiroeSeikai National Fisheries Research Institute, Japan;Fisheries Research and Education Agency, Nagasaki, Japan

Makio C. HondaJapan Agency for Marine‐Earth Science and Technology, Kanagawa, Japan

Yingying HuCenter for Marine Environmental Studies, Ehime University, Ehime, Japan

Tadafumi IchikawaNational Research Institute of Fisheries Science, Japan Fisheries Research and Education Agency, Kanagawa, Japan

Sami KatoGraduate School of Oceanography, Tokai University, Shizuoka, Japan

Satoshi KitajimaSeikai National Fisheries Research Institute, Japan Fisheries Research and Education Agency, Nagasaki, Japan

Yoko KiyomotoSeikai National Fisheries Research Institute, Japan Fisheries Research and Education Agency, Nagasaki, Japan

Toru KobariAquatic Sciences, Faculty of Fisheries, Kagoshima University, Kagoshima, Japan

Yurie KobariAquatic Sciences, Faculty of Fisheries, Kagoshima University, Kagoshima, Japan

Kosei KomatsuGraduate School of Frontier Sciences, The University of Tokyo, Tokyo, Japan;Atmosphere and Ocean Research Institute, The University of Tokyo, Tokyo, Japan

Reo KondoAquatic Sciences, Faculty of Fisheries, Kagoshima University, Kagoshima, Japan

Gen KumeAquatic Sciences, Faculty of Fisheries, Kagoshima University, Kagoshima, Japan

Hiroshi KurodaHokkaido National Fisheries Research Institute, Japan Fisheries Research and Education Agency, Hokkaido, Japan;National Research Institute of Fisheries Science, Japan Fisheries Research and Education Agency, Kanagawa, Japan

Hiroomi MiyamotoHachinohe Branch, Tohoku National Fisheries Research Institute, Japan Fisheries Research and Education Agency, Aomori, Japan

Takeyoshi NagaiDepartment of Ocean Sciences, Tokyo University of Marine Science and Technology, Tokyo, Japan;Prof. Uda Memorial Archives Association, Tokyo University of Marine Science and Technology Library, Tokyo, Japan

Hiroshi NakanoProf. Uda Memorial Archives Association, Tokyo University of Marine Science and Technology Library, Tokyo, Japan

Kou NishiuchiSeikai National Fisheries Research Institute, JapanFisheries Research and Education Agency, Nagasaki, Japan

Masami NonakaJapan Agency for Marine‐Earth Science and Technology, Kanagawa, Japan

Yuji OkazakiTohoku National Fisheries Research Institute, JapanFisheries Research and Education Agency, Miyagi, Japan

Dorleta Orúe‐EchevarríaDepartament d’Oceanografia Física i Tecnològica, Institut de Ciències del Mar, Consejo Superior de Investigaciones Científicas, Unidad Asociada ULPGC‐CSIC, Barcelona, Spain

Kazuyuki OtsukaProf. Uda Memorial Archives Association, TokyoUniversity of Marine Science and Technology Library, Tokyo, Japan

Josep L. PelegríDepartament d’Oceanografia Física i Tecnològica, Institut de Ciències del Mar, Consejo Superior de Investigaciones Científicas, Unidad Asociada ULPGC‐CSIC, Barcelona, Spain

Hiroaki SaitoAtmosphere and Ocean Research Institute, The University of Tokyo, Chiba, Japan

Yoshikazu SasaiJapan Agency for Marine‐Earth Science and Technology, Kanagawa, Japan

Hideharu SasakiJapan Agency for Marine‐Earth Science and Technology, Kanagawa, Japan

Chiyuki SassaSeikai National Fisheries Research Institute, Japan Fisheries Research and Education Agency, Nagasaki, Japan

Eko SiswantoJapan Agency for Marine‐Earth Science and Technology, Kanagawa, Japan

Koji SuzukiFaculty of Environmental Earth Science, Hokkaido University, Hokkaido, Japan;CREST, Japan Science and Technology, Tokyo, Japan

Motomitsu TakahashiSeikai National Fisheries Research Institute, Japan Fisheries Research and Education Agency, Nagasaki, Japan

Youichi TsukamotoJapan Sea National Fisheries Research Institute, Japan Fisheries Research and Education Agency, Kyoto, Japan

Yusuke UchiyamaDepartment of Civil Engineering, Kobe University, Hyougo, Japan

Kazuyuki UeharaGraduate School of Oceanography, Tokai University, Shizuoka, Japan

Norihisa UsuiDepartment of Atmosphere, Ocean, and Earth System Modeling Research, Meteorological Research Institute, Ibaraki, Japan

Ignasi Vallès‐CasanovaDepartament d’Oceanografia Física i Tecnològica, Institut de Ciències del Mar, Consejo Superior de Investigaciones Científicas, Unidad Asociada ULPGC‐CSIC, Barcelona, Spain

Dharmamony VijaiHachinohe Branch, Tohoku National Fisheries Research Institute, Japan Fisheries Research and Education Agency, Aomori, Japan;State Key Laboratory of Marine Environmental Science, Xiamen University, Fujian Sheng, China

Daniel B. WhittNational Center for Atmospheric Research, Boulder, CO, USA

PREFACE

The warm streak of the Kuroshio current along the western boundary of the ocean basin looks exactly like one of the main arteries for sustaining marine ecosystems. Indeed, many pelagic fish and nekton spawn around the Kuroshio, and many commercially valuable fish, such as tuna and sardine, utilize this current to swim north to their feeding sites. On the other hand, it has been long recognized that the surface Kuroshio water is nutrient‐depleted and has low biological productivity. Therefore, it is one of the major enigmas in ocean sciences why and how these abundant fish populations are maintained in the Kuroshio region – the so‐called Kuroshio Paradox. This scientific problem has been tackled since 2011 when a research project, “Study of Kuroshio Ecosystem Dynamics for sustainable fisheries (SKED)”, led by Hiroaki Saito, one of the lead editors of this book, was funded by the Ministry of Education, Culture, Sports, Science and Technology–Japan (MEXT) for 10 years. To elucidate this scientific problem, a transdisciplinary scientific team was formed and many active observational and modeling studies have been carried out. One of the key targets of this project is to unravel the nutrient transport in the Kuroshio. Nutrient supply to the sunlit surface layer is crucial to support primary production by phytoplankton, higher trophic levels, and the contribution of the biological carbon pump to climate regulation through the uptake and sequestration of carbon dioxide. The North Atlantic counterpart of the Kuroshio, the Gulf Stream has been known since the 1990s to transport nutrients originating from the tropical ocean and even from the Antarctic Ocean to subpolar regions of the North Atlantic. The same functionality of the Kuroshio in the North Pacific has recently been reported in a few studies. However, the mechanisms for the generation, maintenance, and modulations of the nutrient streams have been elusive. As a result, the ecosystem structures and dynamics regarding the Kuroshio nutrient stream remain nearly unknown. This book synthesizes the recent research results of the physical, chemical, and biological aspects of the Kuroshio nutrient stream, aiming to provide the basis for comprehensive understanding and accurate prediction of the Kuroshio ecosystem responses to future climate variability.

The volume opens with an introductory chapter by Hiroaki Saito that summarizes the history of the research projects on the Kuroshio, especially for the biological and chemical aspects, introducing the influence of the Kuroshio on the Japanese culture. This is followed by a chapter by Takeyoshi Nagai that touches on the historical research advancement, especially on the physical aspect of the Kuroshio nutrient stream with episodes of earlier researchers. The rest of the first section introduces the nutrient stream of the North Atlantic, the Gulf Stream.

The second section addresses the physical and biogeochemical aspects of the Kuroshio, regarding the Kuroshio and the nutrient stream. The chapters in this section include recent research results on how the basin scale Kuroshio transports nutrients laterally, how the smaller scale processes could work together with the larger scale processes to supply nutrients along its path, and the mechanisms of the mixing and upwelling near the topographic features along the Kuroshio and the Kuroshio Large Meander.

The third section includes the ecosystem research results on the lower trophic levels, including phytoplankton and zooplankton, and mesopelagic fishes in the Kuroshio region, and the higher tropic levels in the Kuroshio ecosystem. These chapters delineate using the cutting edge monitoring that biological production and diversity would depend on the nutrient dynamics in the Kuroshio and its adjacent waters.

Our intention is to provide the new integrated insights of the role of the Kuroshio as a nutrient stream. We acknowledge that this book will not answer all questions in the Kuroshio nutrient stream, but we expect that many young ocean researchers worldwide in the future generations that will be stimulated by this book will pursue active researche in the related topics.

The editors would like to thank a number of people who supported the publication of this book, including Dr. Rituparna Bose, Kshitija Iyer and Karthiga Mani at John Wiley & Sons, Inc. All the papers in this book have been peer reviewed and we thank all the reviewers for their time and valuable comments to improve the contents of the book. The cover photo obtained from Himawari‐8 (Japan Meteorological Agency) was supplied by the P‐Tree System, Japan Aerospace Exploration Agency (JAXA). Takeyoshi Nagai thanks Dr. Kazunori Kuroda for useful comments, Dr. Hideo Tameishi for the aerial photo of the surface front, support from Capt. Tsugio Nagai, Capt. Ukekura (R/V Natsushima), Capt. Inoue (R/V Kaiyo), Capt. Ryouno (R/V Shinsei), Capt. Uchiyama (T/V Kagoshima‐maru), Capt. Noda (R/T/V Umitaka), TUMSAT, MIT‐Hayashi fund and JSPS. Koji Suzuki shows gratitude to JST‐CREST. Motomitsu Takahashi appreciates all the support from FRA, and we all acknowledge MEXT and SKED scientists.

Takeyoshi NagaiHiroaki SaitoKoji SuzukiMotomitsu Takahashi

Section IIntroduction

1The Kuroshio: Its Recognition, Scientific Activities and Emerging Issues

Hiroaki Saito

Atmosphere and Ocean Research Institute, The University of Tokyo, Chiba, Japan

ABSTRACT

The Kuroshio, the western boundary current of the North Pacific Ocean, transports large amounts of heat, salt, chemical materials, and organisms. This transport influences the climate and ecosystems of the region along the Kuroshio axis (Kuroshio region), and also the economy and culture of human society through the continuous supply of marine ecosystem services. Owing to the scientific and social importance of the Kuroshio, a large amount of effort has been put into physical, chemical, and biological oceanography and fisheries sciences in the Kuroshio region. In spite of the oligotrophy, various fish use the Kuroshio region as spawning and nursery grounds, and good fishing grounds are formed in the Kuroshio region. I named this inconsistency of high fisheries production in oligotrophic environment as the Kuroshio Paradox. To solve the paradox, an interdisciplinary approach encompassing physical oceanography to fisheries sciences is essential. In this book, recent scientific developments in the physical, chemical, and biological aspects of the Kuroshio and those of the Gulf Stream, the western boundary current of the North Atlantic Ocean, are described and compared to understand the similarity and differences between them. In this introductory chapter, the history of the recognition of the Kuroshio and scientific research including interdisciplinary projects are reviewed, and recent developments in research focused on solving the Kuroshio Paradox are summarized. Better understanding of the Kuroshio is essential not only to solve the paradox but also to develop sustainable use of marine ecosystem services that our society is dependent on.

1.1. INTRODUCTION

The Kuroshio, the western boundary current of the North Pacific Ocean, transports a large amount of heat, salt, chemical materials, and organisms to the north; these characterize the physical, chemical and biological properties of the region along the Kuroshio (Kuroshio region) (Figure 1.1). Owing to the warm, saline water transported from the tropical Pacific Ocean, the sea surface temperature (SST) of the Kuroshio region is higher than surrounding waters, and clear temperature and salinity boundaries are formed at the northern and western edges of the Kuroshio axis. The Kuroshio also transports organisms. Various tropical and subtropical species are dispersed to the north, or use the Kuroshio as a migration route. For example, Japanese sardine Sardinops melanostictus lay eggs in the south of Kyushu and Honshu, Japan, and the egg and larvae are transported to the north by the Kuroshio, which is advantageous for feeding migration to the Oyashio region (Watanabe et al., 1995). Tropical fish Epinephelus fasciatus dispersed to north along the Kuroshio and the phylogenetic variation increased at settlement cites (Kuriiwa et al., 2014). Coral distribution reaches 35°N (Veron & Minchin, 1992) and the coral reef that exists at the highest latitude in the world (33°48′ N) is observed in the Kuroshio region (Yamano et al., 2001). The dispersion of organisms along the Kuroshio contributes to it having the highest biodiversity of the world oceans (Tittensor et al., 2010).

Figure 1.1 SST distribution of the North Pacific in April 1986 (color contour, FORA‐WNP30, http://synthesis.jamstec.go.jp/FORA/e/index.html) and schematic illustration of the path of the Kuroshio and Oyashio (top). The path of the Kuroshio near Japan (bottom).

The Kuroshio has also functioned as a maritime route for human transportation and trade since the prehistoric age. In the Paleolithic, human migrated to the southern and central Ryukyu Islands from the south, probably from Taiwan (30–36 ka BP) (Kaifu & Fujita, 2012; Kaifu et al., 2015). Because the Ryukyu Islands were never connected to Taiwan or Kyusyu in the late Pleistocene, it is strongly suggested that people used sailing skills to travel up to 220 km across channels around the Ryukyu Islands (Miyako Island to Okinawa Island). In the mid‐Yayoi period (4 BCE to 1 CE), active trade between Okinawa and Kyushu is evident from bracelets made of the large marine snail Strombus latissimus dug from various ruins in Kyushu and western Honshu (Oda, 1992). This snail occurs only in the water around the Ryukyu Islands and the mass processing archeological sites of the snail in this period were excavated in Okinawa Island.

The Kuroshio also influences human society and culture. Services and goods from the Kuroshio ecosystem contribute significantly to form Japanese culture. The Chinese historical text “Records of Three Kingdoms” (~290 CE) contains descriptions of how Japanese people would dive to collect fish and shell fish. Artisanal diving fishing is still observed in Japan mostly along the Kuroshio. Ise Jingu is the biggest Shinto shrine in Japan. Nihon Shoki (720, The Chronicles of Japan), the second oldest chronicle in Japan, details that the shrine was built in Ise because it was an appropriate environment facing the sea where waves come from eternally. Since its construction (ca 1500 years ago), a ceremony has been offered every morning and evening for the sun goddess Amaterasu. The offering includes dried bonito, fresh or dried fish, sea weeds, and salt. At three main festivals a year, lobster, turban shell, dried shark, and dried sea cucumber are also dedicated to the god, which illustrates how the people of Japan have considered various sea foods as important bounties from the wild ocean. Skipjack tuna, lobster, and turban shell are typical products from the Kuroshio. Accordingly, the marine ecosystem services from the Kuroshio have been essential culture of Japan. In addition, the abundant fishery resources brought by the Kuroshio affect the dietary habit of the region along the Kuroshio to be more sea‐food dependent. Sea food consumption per capita in Japan is more than twice the world average value, which is one of the reasons why it has one of the world's highest life expectancy (Shimazu et al., 2007). The above indicate that in Japan various sea foods are regarded as important supply from environment. The marine ecosystem services from the Kuroshio are essential to the culture of Japan.

1.2. RECOGNITION OF THE KUROSHIO AND THE HISTORY OF KUROSHIO STUDY

In Geographia Generalis by Varenius (1650), there is a description of strong northerly current off the Philippines and Japan (after Jones & Jones, 2002). This is regarded as the first description of the current system of the western North Pacific. The Kuroshio name first appeared on the map of Japan as a strong current off Hachijo‐jima (Nagakubo, 1775, after Kawai, 1994). A detailed description of the Kuroshio appears in Izu Kaitoh‐Fudoki (Description of culture, geography and climate of Izu Islands) (Sato & Yoshikawa, 1781) as:

(The fast currents off Hachijo‐jima) flow from the southeast or west‐southwest to east. The two currents are quite fast, and sound like a waterfall. People in Hachijo‐jima called the currents Kuroshio, or Yamashio, and are afraid of them, as a ship can be driven tens of miles off course if it enters the currents

(translated from Japanese by the author)

Hachijo‐jima Island (Figure 1.1) is located near the axis of the Kuroshio. The term Kuroshio (or Kurosegawa, meaning black undulated current) appeared in various literature of the late 18th century, suggesting that the presence of this strong current was widely recognized by this period. However, most literature described the Kuroshio as a regional system 2 km in width and a few hundred kilometers in length at most. The presence of strong currents west of the Ryukyu Islands and south of Kyushu was also recognized from the 17th century, but only as local systems. The view of a continuous flow of the Kuroshio from Okinawa to Honshu or Okinawa to Kyushu in this period had not yet been established.

Captain James King, an officer from Captain James Cook‘s expedition (1776–1780) described a strong current off Japan that reached a speed of 1.5–3 knots. As is evident from the literature as well as a long navigation history (Jones & Jones, 2002), the presence of the Kuroshio as a strong current off Japan was well known in seamen in the 19th century. In the map of Berghaus (1837), the presence of a current system from Okinawa to east of Honshu was described as Japanische Strom. Since broader description was observed in various maps in mid‐19th century, the synoptic view of the Kuroshio was established by this period (Kawai, 1994). Bent (1856) reported that “Kuro‐Siwo” flowed along Taiwan, the Ryukyu Islands, and mainland Japan as a continuous current system of the western North Pacific. This is the first appearance of the term Kuroshio outside of Japan. It is worth noting that the full title included “Kuro‐Siwo, or Gulf Stream of the North Pacific Ocean”. Thus, in this period the Kuroshio was first recognized as the current system of the North Pacific comparative to one of the North Atlantic.

Scientific research on the Kuroshio, however, did not begin until the last decades of the 19th century. The first synoptic survey of the surface current of Kuroshio was carried out by means of drifting bottles in 1894–1895 by Dr. Yuji Wada. From 1965, an international project the Cooperative Study of the Kuroshio and Adjacent Regions (CSK), was organized by the Intergovernmental Oceanographic Commission, UNESCO. During this long‐term (1965–1979) international cooperation project, intensive marine science research was carried out and understanding of the Kuroshio, especially its physical aspects, greatly progressed. Physical oceanography of the Kuroshio obtained during the CSK was well documented in the book Kuroshio: Physical aspects of the Japan Current (Stommel & Yoshida, 1972). The CSK also provided a good opportunity to bring together chemical and biological data on the Kuroshio obtained by each member country; these were then compiled in the proceedings (Marr, 1970; Sugawara, 1972).

The number of publications with “Kuroshio” in the title cited in the Web of ScienceⓇ increased after the CSK and the number of annual publications after 2012 exceeded more than 200 (Figure 1.2). This does not mean that the study of the Kuroshio was inactive in the past. More than 600 publications were published before 1968 (Yoshida & Shimizu, 1972). Most of them are not cited in Web of ScienceⓇ because they are written in Japanese or Russian, or published in discontinued journals. The recent increase in publications on the Kuroshio reflects an increase in scientific efforts in the Kuroshio region. In particular, developments in sensors and remote sensing techniques, such as Argo floats, sea glider, satellite altimeter, and an improvement in mathematical modeling skills contribute to improving our understanding of the Kuroshio.

Figure 1.2 The number of scientific publications with “Kuroshio” in the title cited in Web of ScienceⓇ (https://apps.webofknowledge.com).

1.3. AN INTERDISCIPLINARY APPROACH TO FISHERIES OCEANOGRAPHY IN THE KUROSHIO

In Japan, due to the large demand for marine fisheries products, fisheries science has led the study of the Kuroshio as well as physical oceanography. The pioneering physical oceanographic study by Dr. Wada was based on a proposal submitted to the Board of Fisheries Inquiry. A survey of fish eggs and larvae in the Kuroshio region, led by Dr. Jinjiro Nakai, started in 1937. After the interruption of World War II, the synoptic survey of fish and fish larvae, mainly targeted at Japanese sardine, Japanese anchovy, and chub mackerel, restarted in 1947 and has been continued until now (Figure 1.3). Observation efforts were intensified from 1978 and the annual average number of stations during 1978– 2015 was 3820 covering 1.2 × 106 km2 (sum of monthly coverage) (Oozeki et al., 2007; Takasuka et al., 2008). The original purpose of the study was to monitor egg production and to estimate the recruitment of target species.

Figure 1.3 Egg and larval survey sampling stations off the Pacific coast of Japan from 1978–2015.

Modified and updated from Figure 2 of Takasuka et al. (2008).

The observational data had only been used in one discipline, i.e., fisheries science, even though physical and biological oceanographic data were also collected. The situation changed following the hypothesis that climate influences fisheries resources (Cushing, 1982; Kawasaki, 1983). This hypothesis stimulated collaboration between physical oceanographers and fisheries scientists, and several interdisciplinary science programs including chemical and biological oceanographers were launched collaborating with international GLOBEC and IMBER programs, such as BIOCOSMOS (1989–1999), VENFISH (1997–2003), POMAL (2007–2012), SKED (2011–) (Saito, 2008, 2013; Watanabe, 1991; Yatsu et al., 2013). Physical and biological data obtained in fish egg and larval survey are now used in other discipline, e.g., routinely obtained CTD data play a critical role in data assimilation of mathematical models that forecast the Kuroshio path (Kuroda et al., 2017; Miyazawa et al., 2009).

One of the highlights from these projects is understanding the mechanism of a climate‐induced ecosystem regime shift in the Kuroshio region. Fish species alternation between sardine and anchovy is a drastic event of the Kuroshio ecosystem regime shift (Kawasaki, 1983). Most fish species alternations in the world’s oceans occur in upwelling zones and the regime shift is related to the variation in coastal wind (Lluch‐Belda et al., 1989). However, upwelling related to coastal wind does not explain the fish species alternation in the Kuroshio region. In 1988, the winter SST in the Kuroshio Extension region suddenly increased in conjunction with a rapid decline in sardine stock. Noto and Yasuda (1999) found that the winter SST in the Kuroshio Extension region was correlated with the recruitment failure of Japanese sardine. Since the egg production of sardine was at a historical high (>1 × 1015 eggs) and the survival rate of eggs improved in this period, the collapse of the Japanese sardine stock was suggested to be due to a high mortality of larvae and juveniles (Watanabe et al., 1995). Nonaka et al. (2012) revealed, by means of an eddy‐resolving global ocean circulation model, that the SST change with the intensified Kuroshio Extension was due to the climate shift in the eastern North Pacific. The change in the wind field induced a high sea surface height (SSH) anomaly in the eastern North Pacific in 1984. The high SSH anomaly was propagated west by Rossby wave and reached off Honshu in 1988. The arrival intensified the Kuroshio and increased the SST and shoaled wintertime mixed layer depth. As a result of the change in physical oceanographic property, spring primary production in the Kuroshio Extension region decreased and the production peak shifted two months earlier than the arrival of larval Japanese sardine (Nishikawa & Yasuda, 2008; Nishikawa et al., 2013). The rapid collapse of Japanese sardine stock at this time was due to the mismatch between larval sardine arrival and zooplankton production, which was induced by climate shift in a remote area (eastern North Pacific) far from the Kuroshio region. The collaborations between physical oceanographers, biological oceanographers, and fisheries scientists have contributed significantly to furthering our understanding of the mechanisms of ecosystem dynamics in the Kuroshio region.

1.4. THE KUROSHIO PARADOX

The Kuroshio transports oligotrophic subtropical water. Because of the low nutrient concentration, the water color of the Kuroshio is dark ultramarine and distinctive from blue or greenish (i.e., phytoplankton rich) coastal water. The name Kuroshio comes from its water color: “kuro” in Japanese means black or dark, and “shio” means current. The name clearly indicates its low phytoplankton concentration. In spite of the oligotrophic condition, various fishes use the region as a spawning and nursery ground throughout the year (Okazaki et al., Chapter 15 this book) and, as a result, good fishing grounds have been formed, which target spawning populations and their predators (Table 1.1). The Kuroshio species that use the Kuroshio region as a spawning and/or nursery ground comprise 59.9% of marine fisheries production in Japan. A question arises. What mechanisms realize high fisheries production in an oligotrophic condition? I named this inconsistency of high fisheries production in the oligotrophic Kuroshio region as the Kuroshio Paradox.

Table 1.1Catches of fishes and squid using the Kuroshio Region as a spawning ground in Japan (2016). [Only pelagic spawners are included, and fishes that lay eggs at the bottom or close to the shore are excluded. Total catch does not include aquaculture. Data from the Ministry of Agriculture, Forestry and Fisheries (https://www.e‐stat.go.jp/stat‐search/file‐download?statInfId=000031655414&fileKind=0).]

Species

Catch (100 ton)

Japanese sardine

3746

Japanese anchovy

1727

Round herring

970

Sardine and anchovy larvae

629

Pacific saury

1139

Japanese jack mackerel

1287

Amberstripe scad

265

Chub mackerel/blue mackerel

4891

Bluefin tuna

93

Amberjack

*

1048

Japanese flying squid

678

Total

16 473

% of total catch in Japan (2 750 200 ton)

59.9

*Including Japanese amberjack, yellowtail amberjack, greater amberjack.

One reason why fishes select the Kuroshio region as a spawning ground is the high temperature. Rapid growth at high temperature reduces the duration of the vulnerable early life stages of the fish and the associated mortality (Meekan et al., 2006