Lagrangian fronts in the ocean

We introduce the concept of Lagrangian fronts (LFs) in the ocean and describe their importance for analyzing water mixing and transport and the specific features and differences from hydrological fronts. A method of calculating LFs in a given velocity field is proposed. Based on altimeter velocity fields from AVISO data in the northwestern Pacific, we calculate the Lagrangian synoptic maps and identify LFs of different spatial and temporal scales. Using statistical analysis of saury catches in different years according to the Goskomrybolovstvo (State Fisheries Committee of the Russian Federation), we show that LFs can serve as good indicators of places that are favorable for fishing.     

Identification and Lagrangian analysis of oceanographic structures favorable for fishery of neon flying squid (<Emphasis Type="Italic">Ommastrephes bartramii</Emphasis>) in the South Kuril area

Based on the AVISO velocity field, we compute daily synoptic Lagrangian maps in the South Kuril area for the fishery seasons of 1998, 1999, and 2001–2005 from available catching data on neon flying squid (NFS). With the help of drift maps for artificial particles, we found that the majority of NFS fishing grounds featuring maximum catches are situated near large-scale Lagrangian intrusions: tongues of water penetrating the surrounding water of other Lagrangian properties. It is shown that the NFS catch locations tend to accumulate at places where waters with different magnitudes of certain Lagrangian indicators converge, mix, and produce filaments, swirls, and tendrils typical of chaotic advection. Potential NFS fishing grounds are mainly located near (1) Lagrangian intrusions of the Subarctic front, (2) intrusions of Okhotsk Sea and Oyashio waters around mesoscale anticyclones east of Hokkaido with subsequent penetration of catch locations inside eddies and (3) intrusions of subtropical waters into the central part of the South Kuril area due to interaction with eddies of different size and polarity. Possible reasons for increased biological production and fishery in the vicinity of Lagrangian intrusions are discussed.     

Variations of oceanic fronts and their influence on the fishing grounds of Ommastrephes bartramii in the Northwest Pacific

Two predominant currents, the warm Kuroshio Current and the cold Oyashio Current, meet in the Northwest Pacific Ocean. The dynamics of physical oceanographic structures in this region, including frontal zones and meandering eddies, result in a highly productive habitat that serves as a favorable feeding ground for various commercially important species. Neon flying squid, Ommastrephes bartramii, is an important oceanic squid, which is widely distributed in the North Pacific Ocean. Based on the catch data collected by Chinese squid jigging fleets and relevant environmental data, including sea surface temperature(SST) and fronts(represented by gradients of SST and thermocline) during 1998–2009, the variations of oceanic fronts and their influence on the fishing grounds of O. bartramii were evaluated, and the differences in distribution of fishing grounds of O. bartramii in 2000 and 2002 were compared by describing the differences in vertical temperature between 0–300 m. It was found that the preferred horizontal temperature gradient of SST for O. bartramii tended to be centered at 0.01–0.02°C/nm, which attracted nearly 80% of the total fishing effort, and the preferred horizontal temperature gradients at the 50 m and 105 m layers were mainly located at 0.01–0.03°C/nm, which accounted for more than 70% of the total fishing effort during August–October. The preferred vertical temperature gradient within the 0–50 m layer for O. bartramii tended to be centered at 0.15–0.25°C/m during August and September and at 0.10–0.15°C/m in October, implying that the mixed surface layer was distributed at depths of 0–50 m. It was concluded that the vertical temperature gradient was more important than the horizontal temperature gradient in playing a role in forming the fishing ground. The results improved our understanding of the spatial dynamics of the O. bartramii fishery.     

Growth and Survival of Pacific Saury Cololabis saira in the Kuroshio-Oyashio Transitional Waters

Growth and mortality rates of larval and early juvenile Pacific saury Cololabis saira were estimated for spring and autumn spawning seasons in the Kuroshio-Oyashio transitional waters and for winter spawning season in the Kuroshio waters in 9 years from 1990–1998, based on quantitative fish sampling and otolith daily ring readings. Growth and mortality rates were more variable in the Kuroshio-Oyashio transitional waters than in the Kuroshio waters. The estimated production of 40-mm preschooling juveniles was a positive function of larval production in the hatching length class (5.9–9.9 mm) in the Kuroshio waters. In the Kuroshio-Oyashio transitional waters, rather than larval production in the hatching length class, cumulative survival through the larval and early juvenile stages determined the juvenile production. Variable growth and survival rates of saury observed in the transitional waters seem to be associated with large environmental variability in the waters, including shifts of the Kuroshio and Oyashio fronts and development of streamers and eddies between the fronts. This revised version was published online in August 2006 with corrections to the Cover Date.     

Seasonal Variations in the Sea off Sanriku Coast, Japan

The temperature and salinity data obtained by the Iwate Fisheries Technology Center for the 25-year period from 1971 to 1995 were analyzed to clarify the seasonal variations in the sea off Sanriku, Japan. The variations of three typical waters found in this region, the Tsugaru Current water, the Oyashio water, and the Kuroshio water are discussed in terms, of a T-S scatter diagram referring to the water mass classification proposed by Hanawa and Mitsudera (1986). The mean temperature and salinity fields averaged for each month show clear seasonal variation. Distributions of the Tsugaru Current water and the Oyashio water can barely be distinguished in the fields deeper than 200 m since the Tsugaru Current has a shallow structure; however, the fields at 100 m depth indicate remarkable seasonal variation in the area of the Tsugaru Current. At 100 m depth, the temperature and salinity fronts between the Tsugaru Current water and the Oyashio water gradually disappear in January through April, appear again in May, then become clearest in September. This revised version was published online in July 2006 with corrections to the Cover Date.     

Velocity Field of the Oyashio Region Observed with Satellite-Tracked Surface Drifters during 1999–2000

Based on the surface drifters that moved out from the Sea of Okhotsk to the Pacific, the surface velocity fields of mean, eddy, and tidal components in the Oyashio region are examined for the period September 1999 to August 2000. Along the southern Kuril Island Chain, the Oyashio Current, having a width of ∼100 km, exists with velocities of 0.2–0.4 m s⁻¹. From 40°N to 43°N, the Subarctic Current flows east- or northeastward with velocities of 0.1–0.3 m s⁻¹, accompanied by a meandering Oyashio or Subarctic front. Between the Oyashio and Subarctic current regions, an eddy-dominant region exists with both cyclonic and anticyclonic eddies. The existence of an eastward flow just south of Bussol' Strait is suggested. The 2000 anticyclonic warmcore ring located south of Hokkaido was found to have a nearly symmetric velocity structure with a maximum velocity of ∼0.7 m s⁻¹ at 70 km from the eddy center. Diurnal tidal currents with a clockwise tidal ellipse are amplified over the shelf and slope off Urup and Iturup Islands, suggesting the presence of diurnal shelf waves. From Lagrangian statistics, the single-particle diffusivity is estimated to be ∼10 × 10⁷ cm²s⁻¹.     

The biological oceanography of the East Australian Current and surrounding waters in relation to tuna and billfish catches off eastern Australia

The surface and sub-surface biological oceanography of tuna fishing grounds within the East Australian Current (EAC) was compared in 2004 with two other fishing areas further offshore. Our aim was to determine whether the biological oceanography of the region could explain the distribution and intensity of pelagic fishery catches inside and outside the EAC at that time. The EAC fishing area was noticeably warmer, less saline and lower in nutrients than waters in the other fishing areas. The EAC waters were dominated by large diatoms, the biomass of which was significantly higher than in the seamount and offshore areas, apparently the result of a cold core eddy beneath the EAC surface filament. Over the seamount and offshore more typical Tasman Sea waters prevailed, although the presence of a relatively deeper oxygen minimum layer over the seamount suggested topographically induced mixing in the area. Notably, sub-surface zooplankton and micronekton biomass was significantly higher around the seamount than in the two other areas. The offshore region was characterised by frontal activity associated with the Tasman front. Micronekton net biomass was generally highest in surface waters in this region. Examination of tuna catch records at that time showed yellowfin tuna (Thunnus albacares) dominated the catches of the EAC, whereas swordfish (Xiphias gladius) and bigeye tuna (Thunnus obesus) were the main species caught offshore. We suggest the yellowfin tuna concentrate in waters that are not only warmer but where prey species are concentrated near the surface. Offshore, deeper living species such as swordfish and bigeye tuna (T. obesus) can take advantage of prey species that are distributed deeper in the water column and along the flanks of the many seamounts in the region, or that are concentrated at fronts associated with the Tasman Front. Although only a snapshot of the region, relatively consistent catch data over time suggests the underlying biological oceanography may persist over longer time periods, particularly during the Austral spring.     

利用基于表温因子的栖息地模型预测西北太平洋柔鱼(Ommastrephes bartramii)渔场

根据1999—2004年8—10月主渔汛期间我国鱿钓船在150—165°E海域的鱿钓生产数据,结合其表温及表温梯度,分别将作业次数百分比和单位渔船日产量作为适应性指数,利用算术平均法(AM)和联乘法(GM)分别建立基于表温因子(表温和表温水平梯度)的综合栖息地指数模型。结果表明,AM栖息地指数模型和GM栖息地指数模型均拟合较好,在HSI大于0.6的海域,1999—2004年间其作业次数平均比重分别在70%以上,平均日产量均在2t/d以上。但AM模型稍优于GM模型。利用2005年8—10月生产数据及表温资料对AM模型进行验证,分析认为作业渔场主要分布在HSI大于0.6海域,其作业次数比重达到80%以上,各月平均CPUE均在3.0t/d以上。研究表明,基于表温和表温水平梯度的AM栖息地模型能获得较好预测西北太平洋柔鱼中心渔场。     

东太平洋赤道海域茎柔鱼小型群体资源分布及其渔场环境特征

茎柔鱼(Dosidicus gigas)是我国远洋渔业的重要捕捞对象。当前针对茎柔鱼渔场分布及其与环境关系的研究多集中于秘鲁海域,针对赤道海域茎柔鱼特定种群小型群体资源分布及其渔场环境特征研究较少。根据2019年12月至2020年2月茎柔鱼生物学数据,2019年12月至2020年4月生产和环境数据,运用胴长-体重关系拟合、地统计插值、广义可加模型(GAM)探究其资源分布及渔场环境状况。结果表明：东太平洋赤道海域茎柔鱼胴长范围为136~407 mm,体重范围为117~1557 g；2019年12月至2020年4月各月渔获量呈先增加后减小趋势,2月渔获量最高；CPUE曲线除2月增加外,总体呈下降趋势；渔场集中分布于0°~3°S、105°W~114°W海域,不同月份渔场重心经向变化明显；渔场最适SST范围是24.5~25.5 °C,最适Chl-a范围是0.16~0.20 mg/m3,月份是影响茎柔鱼CPUE的主要因子。研究表明：该海域茎柔鱼渔获主要为小型群体；小型群体生长发育期(2–3月)对渔场分布有重要影响,生长发育期前茎柔鱼集群度高,生长发育期后逐渐分散活动；单一影响因子与茎柔鱼CPUE相关性不显著,综合考虑其他环境因素及其交互影响是今后的研究方向。     

Intermediate Circulation in the Northwestern North Pacific Derived from Subsurface Floats

In order to examine the formation, distribution and synoptic scale circulation structure of North Pacific Intermediate Water (NPIW), 21 subsurface floats were deployed in the sea east of Japan. A Eulerian image of the intermediate layer (density range: 26.6–27.0σ_θ) circulation in the northwestern North Pacific was obtained by the combined analysis of the movements of the subsurface floats in the period from May 1998 to November 2002 and historical hydrographic observations. The intermediate flow field derived from the floats showed stronger flow speeds in general than that of geostrophic flow field calculated from historical hydrographic observations. In the intermediate layer, 8 Sv (1 Sv ≡ 10⁶ m³s⁻¹) Oyashio and Kuroshio waters are found flowing into the sea east of Japan. Three strong eastward flows are seen in the region from 150°E to 170°E, the first two flows are considered as the Subarctic Current and the Kuroshio Extension or the North Pacific Current. Both volume transports are estimated as 5.5 Sv. The third one flows along the Subarctic Boundary with a volume transport of 5 Sv. Water mass analysis indicates that the intermediate flow of the Subarctic Current consists of 4 Sv Oyashio water and 1.5 Sv Kuroshio water. The intermediate North Pacific Current consists of 2 Sv Oyashio water and 3.5 Sv Kuroshio water. The intermediate flow along the Subarctic Boundary contains 2 Sv Oyashio water and 3 Sv Kuroshio water. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of Ocean Climate Changes on the Korean Stock of Pacific Saury, <Emphasis Type="Italic">Cololabis saira</Emphasis> (BREVOORT)

Atmospheric, hydrographic and fishery biological factors indicated that the abnormal northward shift in the distribution of Pacific saury might be the result of a strong flux of warm water into the Japan/East Sea in the late 1970s. Recruitment failure of saury in the late 1970s was attributed to the limitation of the productive area for primary production caused by oligotrophic warm water, and to mismatch of the time of the spring outburst with the earlier arrival of saury to the feeding ground. Comparison of monthly upper mixed layer depth (MLD) and critical depth supported the possibility of the mismatch phenomenon. However, an appropriate management scheme should be implemented to prevent recruitment overfishing for the stock when any sign of ocean climate changes is detected.     

距今三万年来西北太平洋洋流系统变化的初步研究

本文探讨了西北太平洋流系统变化的研究方法,并利用这些方法恢复了距今三万年以来这一地区洋流系統变化的过程。     

Recent variability in the Pacific western subarctic boundary currents and Sea of Okhotsk

The objective of this research is to describe physical processes which are the cause of the recent variability of the Pacific western subarctic waters. Rapid thermohaline changes have occurred within the Oyashio and Kamchatka Current during the last decade. This variability has included a warming of the Kamchatka Current warm intermediate layer, but a cooling and freshening of the upper layer in the Oyashio and Sea of Okhotsk. The example presented here uses data obtained during the Canada/Russia INPOC and WOCE projects, as well as the new Russian studies with high resolution station grid.The possible physical mechanism that generated the upper layer freshening during the thermohaline transition is examined. Major components of the fresh water budget of the Okhotsk Sea are considered in order to describe the dramatic changes in salinity which have recently occurred in the Pacific subarctic. Significant changes in precipitation and other fresh water inputs are demonstrated. It is suggested that upper layer of the Oyashio and Kamchatka Current became cooler and fresher because of the export of cold, fresher waters from the Bering and Okhotsk seas. These waters from the marginal seas have cooled the bottom of the halocline, reducing evaporation and acting as a feedback that has kept the upper layer of the western subarctic boundary currents fresh. It is also shown that the outflow of the cold Sea of Okhotsk water has changed its path during this recent thermohaline transition.     

An overview of the Oyashio ecosystem

The Oyashio shelf region and the seasonally ice-covered areas north of Hokkaido are highly productive, supporting a wide range of species including marine mammals, seabirds and commercially important species in the western subarctic Pacific. The fishes include gadids, such as walleye pollock and Pacific cod, and subarctic migratory pelagic fishes such as chum salmon and pink salmon. It is also an important summer feeding ground for subtropical migrants such as the Japanese sardine, Japanese anchovy, Pacific saury, mackerels, Japanese common squid, whales and seabirds. In recent decades, some components of the Oyashio ecosystem (i.e., phytoplankton, mesozooplankton, gadid fish, and subtropical migrants) have shown changes in species abundance or distribution that are correlated with environmental changes such as the 1976/1977 and 1988/1989 regime shifts. The First Oyashio Intrusion moved northward from the mid-1960s until the late 1970s, when it moved southward until the 1980s, after which it returned to the north again after the mid-1990s. The sea-surface temperature in spring decreased after the late 1970s, increased after the late 1980s, and remained high during the 1990s. The extent of ice cover in the Sea of Okhostk also decreased during the latest warming in the 1980–1990s but has increased again since the late 1990s. This and other variabilities affect the Oyashio ecosystem and the surrounding region.     

基于空间自相关模型的西北太平洋日本鲭渔场时空变动研究

日本鲭(Scomber japonicus)是西北太平洋渔业捕捞的主要种类,了解其渔场变动对探究日本鲭种群分布、资源评估、开发利用和管理等意义重大.为获知其渔场的时空变动特征,本研究根据中国2014-2019年西北太平洋公海灯光围网渔业统计资料,运用全局莫兰指数、局部热点分析、重心迁移轨迹模型和标准差椭圆模型对西北太平...     

Numerical simulation of the upper ocean currents in South China Sea

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Contrast in life histories of exploited fishes and ecosystem structures in coastal waters off west Canada and east Korea

By reviewing the history of fishery exploitation in the coastal waters of west Canada and east Korea, related with contrasting life history strategies of the dominant species, the fishery management challenges that each country would face in the upcoming decades were outlined. In the ecosystem of the Canadian western coastal waters, the dominant oceanographic feature is the coastal upwelling domain off the west coast of Vancouver Island, the northernmost extent of the California Current System in the eastern North Pacific. In the marine ecosystem of the eastern coasts of Korea (the Japan/East Sea), a major oceanographic feature is the Tsushima Warm Current, a branch of the Kuroshio Current in the western North Pacific. Fishes in the Canadian ecosystem are dominated by demersal, long-lived species such as flatfish, rockfish, sablefish, and halibut. During summer, migratory pelagic species such as Pacific hake, Pacific salmon, and recently Pacific sardine, move into this area to feed. In the late 1970s, Canada declared jurisdiction for 200 miles from their coastline, and major fisheries species in Canadian waters have been managed with a quota system. The overall fishing intensity off the west coast of Vancouver Island has been relatively moderate compared to Korean waters. Fishes in the ecosystem of the eastern Korean waters are dominated by short-lived pelagic and demersal fish. Historically, Korea has shared marine resources in this area with neighbouring countries, but stock assessments and quotas have only recently (since the late-1990s) been implemented for some major species. In the Korean ecosystem, fisheries can be described as intensive, and many stocks have been rated as overfished. The two ecosystems responded differently to climate impacts such as regime shifts under different exploitation histories. In the future, both countries will face the challenge of global warming and subsequent impacts on ecosystems, necessitating developing adaptive fisheries management plans. The challenges will be contrasting for the two countries: Canada will need to conserve fish populations, while Korea will need to focus on rebuilding depleted fish populations.     

海渔况信息产品的制作原理及其应用

海渔况信息产品对于渔业资源的研究、调查、预测、评估具有重要意义。利用计算机制作的海渔况图对于信息的表达一目了然,具有速度快、时效新、反应及时等特点,因此计算机制图已经成为目前海渔况信息产品的主要制作手段。论文将以《太平洋大眼金枪鱼延绳钓渔场与SST叠加分布图集》和《东海中心渔场预报图》为例,简要介绍海渔况信息产品的制作原理及其应用。     

The shedding of mesoscale anticyclonic eddies from the Alaskan Stream and westward transport of warm water

The south-flowing waters of the Kamchatka and Oyashio Currents and west-flowing waters of the Alaskan Stream are key components of the western sub-Arctic Pacific circulation. We use CTD data, Argo buoys, WOCE surface drifters, and satellite-derived sea-level observations to investigate the structure and interannual changes in this system that arise from interactions among anticyclonic eddies and the mean flow. Variability in the temperature of the upstream Oyashio and Kamchatka Currents is evident by warming in mesothermal layer in 1994–2005 compared to 1990–1991. A major fraction of the water in these currents is derived directly from the Alaskan Stream. The stream also sheds large anticyclonic (Aleutian) eddies, averaging approximately 300 km in diameter with a volume transport significant in comparison with that of the Kamchatka Current itself. These eddies enclose pools of relatively warm and saline water whose temperature is typically 4 °C warmer and salinity is 0.4 greater than that of cold-core Kamchatka eddies in the same density range. Aleutian eddies drift at approximately 1.2 km d⁻¹ and retain their distinctive warm and salty characteristics for at least 2 years. Selected westward pathways during 1990–2004 are identified. If the shorter northern route is followed, Aleutian eddies remain close to the stream and persist sufficiently long to carry warm and saline water directly to the Kamchatka Current. This was observed during 1994–1997 with substantial warming of the waters in the Kamchatka Current and upstream Oyashio. If the eddies take a more southern route they detach from the stream but can still contribute significant quantities of warm and saline water to the upstream Oyashio, as in 2004–2005. However, the eddies following this southern route may dissipate before reaching the western boundary current region.