Current Dependent Dispersal Characteristics of Japanese Glass Eel around Taiwan

Japanese eel larvae are passively transported to the East Asian Continental Shelf by the North Equatorial Current, Kuroshio and Kuroshio intrusion currents, and coastal currents. Previous studies have investigated the dispersal characteristics and pathways of Japanese glass eels. However, there are still limitations in these studies. According to long-term (2010–2020) catch data from the Fisheries Agency in Taiwan, the distribution and time series of glass eels recruitment to Taiwan are closely related to the surrounding ocean currents. Recruitment begins in eastern Taiwan via the mainstream Kuroshio and in southern Taiwan via the Taiwan Strait Warm Current. In central Taiwan, recruitment occurs from southern Taiwan, as well as from mainland China via the southern branch of the China Coast Current (CCC). The latest recruitment occurred in northern Taiwan and mainly comprised glass eels from mainland China via the northern branch of the CCC. A stronger monsoon during the La Niña phase could affect the recruitment time series in northern and eastern Taiwan. This study suggests that the recruitment directionality of glass eels is an indicator of the flow field of ocean/coastal currents and elucidates the dispersal characteristics of glass eels in the waters around Taiwan.

North Equatorial Current and Kuroshio Velocity Variations Affect Body Length and Distribution of the Japanese eel Anguilla japonica in Taiwan and Japan

The larval stage of Japanese eel travels a substantial distance over a long duration through the North Equatorial Current (NEC) and the Kuroshio, and the spawning behavior of mature eels leads to monthly arrival waves in eastern Taiwan between November and February. The total length (TL) of the glass eel relates to its larval duration and age; therefore, the TL can indicate the larval duration. The monthly mean TLs of eels along eastern Taiwan from 2010–2021 were used to estimate the batch age, and the recruitment patterns and relative abundances were compared. The TLs of glass eels followed a normal distribution, and the estimated ages were highly correlated with their mean TLs. Early recruit TLs were significantly greater than those of late recruits. The mean tracer drift time was longer in early recruitment months (November–December) than in later dates (February–March). The recruitment lag was approximately 1–1.5 months, with relative recruitment higher in the early recruitment months than in later months. Cohorts followed the main streams of the NEC and Kuroshio, and the monthly velocity changes of these currents could affect the TLs as well as the distribution patterns of Japanese glass eels in Taiwan and Japan.

What Speleothems Tell Us about Long-Term Rainfall Oscillation throughout the Holocene on a Planetary Scale

Within the context of anthropogenic warming, rainfall oscillations may induce especially important societal impacts worldwide. In this article, we propose to study potential underlying mechanisms related to precipitation changes on a planetary scale by taking advantage of the recent theory of Rossby waves of long periods winding around subtropical gyres, the Gyral Rossby Waves (GRWs). The stable oxygen isotopic compositions of speleothems are used to regionalize and reconstruct the evolution of long-term rainfall oscillation during the Holocene. The method applied here consists in estimating the wavelet power of dated series of stable oxygen isotopic composition (δ18O) in speleothems within period bands representative of subharmonic modes. Our findings highlight: (1) hydrological processes resulting from friction between the North Equatorial Current (NEC) and the North Equatorial Counter Current (NECC) to explain the weakening of ENSO activity in mid-Holocene, and (2) the quasi-resonance of the equatorward migration of the summer Inter Tropical Convergence Zone (ITCZ) during the Holocene, because of the progressive decrease of the thermal gradient between the low and high latitudes of the gyres. The results of this study suggest that the spatial and temporal variations in the amplitude of the rainfall oscillations are related both on the acceleration/deceleration phases of the western boundary currents and on the shrinkage of the Hadley cell. The latitudinal shift of the summer ITCZ in response to changes in the thermal gradient is of the utmost importance in predicting the expansion of deserts resulting from anthropogenic warming.

Distribution of plutonium in the Pacific Ocean and implications for tracing of ocean circulation

This study examined plutonium (Pu) sources and distribution in the Pacific Ocean based on extensive field datasets over the past 50 years. The basin wide 240Pu/239Pu atom ratios in surface and deep seawater ranged from 0.192 to 0.279 averaging 0.235 ± 0.019, a value consistently higher than that of global fallout at ~ 0.180. The distribution of 240Pu/239Pu atom ratios exhibited a decreasing trend along the North Equatorial Current-Kuroshio to their extension areas. The activity levels of 239+240Pu in Pacific Ocean surface seawater ranged widely from 0.2 to 43.5 mBq m− 3, and increased with latitude. We determined the Pu sourced from the Pacific Proving Grounds (PPG) and global nuclear fallout in the Pacific Ocean based on the Pu isotopic composition. Using a mixing model, we found that the PPG made the dominant Pu contribution (average = 69.6 ± 14.4%) to Pacific Ocean surface seawater. The depth range of maximal 239+240Pu activity in the Pacific Ocean was well defined, averaging 608 ± 137 m. The vertical distribution of the 239+240Pu inventory showed most of Pu retained in the upper 3000 m, namely, the contributions of 239+240Pu inventories at 0-1000 m and 0-3000 m depths accounted for 43.5 ± 9.0% and 75.1 ± 12.0% of the total, respectively. We identified the transport pathway of Pu-PPG in the Pacific Ocean and demonstrated that Pu isotopes hold great promise as tracers of ocean circulation. Finally, via this extensive compilation of Pu isotopic compositions in the Pacific Ocean water, we established a Pu baseline in the region.

Distribution and size variability of Japanese eel leptocephali in their Pacific Ocean spawning area using carbon and nitrogen stable isotope ratio analyses

The diets of larval (leptocephali) anguillid and marine eels are poorly understood, despite studies on their gut contents or stable isotope ratios suggesting marine snow particles represent a food source. Concerns for Japanese eel Anguilla japonica stock conservation necessitate an improved knowledge of their larval ecology to better understand the causes of their recent decline in numbers and fluctuating recruitment into East Asia. To understand the distribution of and variation in size of leptocephali in relation to their feeding, we examine carbon and nitrogen stable isotope ratios of larvae from seven research cruises (2002–2013) in the North Equatorial Current spawning area. Preleptocephali (2–3 days old, ~5 mm total length) isotope ratios reflect maternal ratios, but feeding-stage leptocephali (8–56 mm) tend to have higher ∂15N values with decrease of latitude typically in areas south of a salinity front. Neither ∂15N nor ∂13C ratios are clearly related to longitude or larval size < 30 mm, but ∂13C values of larvae > 40 mm are lower further downstream in the North Equatorial Current and Subtropical Countercurrent. Differences in ∂13C values might be a function of varying spatial baselines in the two currents apart from the spawning area. Although among-year larval isotope ratio differences may reflect temporal baseline variation related to the location of the salinity front, more research with much wider range observations in the spawning season is required because ingested marine snow particles might differ with larval growth and location.

Western Boundary of the North Atlantic Subtropical Gyre: Decadal Change

&lt;p&gt;The A20 is a meridional hydrographic section located at 52&amp;#186;W on the western North Atlantic Subtropical Gyre that encloses the path of the water masses of the Atlantic Meridional Overturning Circulation (AMOC). Using data from three A20 hydrographic cruises carried out in 1997, 2003 and 2012 together with LADCP-SADCP data and the velocities from an inverse box model, the circulation of the western North Atlantic Subtropical Gyre is estimated. The main poleward current of the AMOC is the Gulf Stream (GS) which carries 129.0&amp;#177;10.5 Sv in 2003 and 110.4&amp;#177;12.2 Sv in 2012. Due to the seasonality, the GS position is shifted southward in 2012 - relative to that of 2003 - as both cruises took place in different seasons. In opposite direction, the Deep Western Boundary Current (DWBC) crosses the section twice, first at 39.3-43.2&amp;#186;N (-34.9&amp;#177;7.5 Sv in 2003 and -25.3&amp;#177;9.4 Sv in 2012) and then at 7.0-11.7&amp;#186;N (42.0&amp;#177;8.0 Sv in 2003 and 48.0&amp;#177;8.1 Sv in 2012). Additionally, two zonal currents contribute with westward transport below 20&amp;#186;N: the North Equatorial Current and the North Brazil Current; with a net value of -28.0&amp;#177;4.1 Sv in 2003 and -36.7&amp;#177;3.6 Sv in 2012.&lt;/p&gt;