ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on June 30, 2007
ICES Journal of Marine Science: Journal du Conseil 2007 64(7):1387-1395; doi:10.1093/icesjms/fsm091
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Effect of El Niño on migration and larval transport of the Japanese eel (Anguilla japonica)
1 Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo 164-8639, Japan
2 Earth Simulator Centre, Japan Agency for Marine–Earth Science and Technology, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, Kanagawa 236-0001, Japan
Correspondence to H. Kim: tel: +81 3 5351 6504; fax: +81 3 5351 6508; e-mail: kimhy{at}ori.u-tokyo.ac.jp
Kim, H., Kimura, S., Shinoda, A., Kitagawa, T., Sasai, Y., and Sasaki, H. 2007. Effect of El Niño on migration and larval transport of the Japanese eel (Anguilla japonica). – ICES Journal of Marine Science, 64: –.To clarify the effect of an El Niño on the migration of Japanese eels (Anguilla japonica) in the western North Pacific, differences in migration patterns of eel larvae (leptocephali) in El Niño and non-El Niño years were compared qualitatively through a numerical particle-tracking model. Depending on interannual meridional displacements of the salinity front and bifurcation of the North Equatorial Current, transport of Japanese eel larvae to the Kuroshio was much less than to the Mindanao Current in an El Niño year, and recruitment to coastal habitats in Japan decreased in those years. In non-El Niño years, transport to the Kuroshio was twice as high, and recruitment to coastal habitats increased. If the spawning area of eels was independent of El Niño, transport differences between the two currents were not clear. In the western North Pacific, mesoscale eddies also played a significant role in dispersing eel larvae and prolonging their migration. Consequently, the changing oceanic conditions associated with climate change have resulted in decreased recruitment of Japanese eels, and the eddy effect on migration of the Japanese eel larvae needs to be added into the North Equatorial Current–Kuroshio system.
Keywords: El Niño, Japanese eel, larval migration, mesoscale eddy, NEC bifurcation, salinity front
Received 3 January 2007; accepted 24 May 2007; advance access publication 30 June 2007.
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Introduction |
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 Top Introduction Material and methodsResultsDiscussionReferences |
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The leptocephali of the Japanese eel (Anguilla japonica), a species that spawns only in the western North Pacific (Tsukamoto, 1992, 2006), have to migrate long distances to reach their preferred habitat in eastern Asia. Therefore, larval migration is probably governed by oceanic conditions, although it must have evolved strategies to maximize recruitment to continental habitats in eastern Asia. The Japanese eel stock has declined since the early 1970s. The Japanese average annual catch of glass eels, the typically eel-shaped transparent juvenile stage that follows the leaf-like leptocephalus larva stage, was
130 t in the 1960s, but just 17 t by the late 1990s (Tatsukawa, 2003). The catch of A. japonica glass eels, likely a single panmictic population (Ishikawa et al., 2001), showed a similar decreasing trend in Taiwan (Tzeng, 1997). Catches of Atlantic glass eels, Anguilla rostrata and Anguilla anguilla, have also declined seriously since the late 1970s, and the decrease has been hypothesized to be related to changes in ocean–atmosphere conditions (Knights, 2003; Friedland et al., 2007). Kimura et al. (1994, 2001) suggested that the decrease in eastern Asian eel stocks may be influenced strongly by recruitment failure attributable to fluctuating oceanographic conditions associated with climate change. Therefore, better knowledge of larval migration, which is clearly dominated by oceanographic conditions, is crucial to understanding the fluctuating recruitment of Japanese eels. Larval migration of Japanese eels is probably controlled by three environmental factors in the region of the North Equatorial Current (henceforth NEC) where their spawning area is located, the salinity front at the bifurcation of the NEC, allowing Ekman transport of leptocephali operating in an Ekman layer about 70 m deep (Kimura et al., 2001). The salinity front is suggested to be critical to spawning migration of Japanese eels (Kimura and Tsukamoto, 2006), and its interannual variability associated with El Niño/Southern Oscillation (ENSO) events probably leads to reduced larval transport into the Kuroshio Current, causing poor recruitment in eastern Asia (Kimura et al., 2001). The NEC bifurcates into the north-flowing Kuroshio and the south-flowing Mindanao Current (MC) at its westernmost boundary off the coast of the Philippines (Nitani, 1972; Toole et al., 1990). In the bifurcation zone, Japanese eels need to enter the northward flow leading to the Kuroshio and avoid being entrained into the south-flowing MC (Kimura et al., 1994). Its bifurcating position varies seasonally and interannually (Qiu and Lukas, 1996; Kim et al., 2004), and it shifts north with depth (Qu et al., 1998). Qui and Lukas (1996) failed to find significant seasonal fluctuations in the transport of the NEC near the coast of the Philippines, but noted that volume transports between the MC and the Kuroshio are 180° out of phase, because of the different speeds of the baroclinic Rossby waves at their respective latitudes. Using a high-resolution ocean general circulation model (OGCM), Kim et al. (2004) showed that meridional migration of the NEC bifurcation is strongly influenced by the ENSO; its northernmost and southernmost positions appear during El Niño years and La Niña years, respectively. The latitude of bifurcation acts as an indicator of partition of NEC mass, heat, and salt transport between the Kuroshio and MCs. Kimura et al. (1994) suggested that meridional variation in the NEC bifurcation zone can influence recruitment of Japanese eels into the East China Sea via the Kuroshio.
Many cruises to sample Japanese eel larvae, carried out over several decades (Kajihara, 1988; Kajihara et al., 1988; Tsukamoto, 1992, 2006; Ishikawa et al., 2002), have provided information on the spawning ecology and life history of the species. Tsukamoto et al. (2002) reported that Japanese eel larvae are usually distributed at the northern edge of the NEC, and that their size increases as they migrate west along the NEC. At the northern edge of the NEC, where there is notable variability in sea surface height, many mesoscale eddies propagating westwards are formed (Qiu, 1999). Moreover, the eddies cause great variability in the rate of migration of eel larvae and a broad geographical spread of larvae (Tsukamoto, 2003; Shinoda, 2004). Therefore, mesoscale eddies are considered to be a further mechanism that influences the migration route of eel larvae.
The objectives of this study were therefore to investigate the hydrographic structures linked to the transport of Japanese eel larvae, and also to understand the effect of climate change, such as an El Niño event, on the transport of larvae from a spawning site.
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Material and methods |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Our Lagrangian study of migrating eel larvae was based on a high-resolution circulation field that has been developed by the Frontier Research System for Global Change, Japan. The high-resolution ocean model for the Earth Simulator (hereafter referred to as OFES) is based on the Modular Ocean Model (MOM3), and the model domain covers a near-global region extending from 75°S to 75°N except for the Arctic Ocean, with horizontal grid spacing of 1/10°. There are 54 vertical levels, with varying distance between levels, from 5 m at the surface to 330 m at the maximum depth of 6065 m. The model topography is constructed from the 1/30 bathymetry dataset created by the OCCAM project at the Southampton Oceanography Centre (obtained through GFDL/NOAA). Monthly mean wind stresses averaged from 1950 to 1999 from the NCEP/NCAR re-analysis data are used for climatological seasonal integration. The surface heat flux is calculated by the same bulk formula as used by Rosati and Miyakoda (1988), using monthly mean values output from the NCEP/NCAR re-analysis for data. The precipitation rate from the same date of re-analysis is utilized to obtain the fresh-water flux. In addition to this fresh-water flux, the monthly mean sea surface salinity of the World Ocean Atlas 1998 (henceforth WOA98) was adopted as additional restoring. The restoring time-scale of the model is set at 1 d, and the model is integrated for 50 years from the annual mean temperature and salinity fields (WOA98), without motion. Detailed description of the basic setting of the model is given by Masumoto et al. (2004) and Sasaki et al. (in press). Flowfields for calculating the Lagrangian larval trajectories were based on 3D circulation fields.
Passive larvae released at different spawning points and different climate conditions were tracked over periods of 1 year. Trajectories of 1000 passive larvae were stepped forward using a 3D advection–diffusion scheme. The position [X p(t + 
t)] of a particle at time-step t + t is given by X p(t + t) = [X p(t) + u(t)t] + ldiffX p(t) = (xpt, ypt, zpt) and represents the position of the particle at the previous time-step t. t = 1 h for the scheme, so it requires interpolation between the 3D mean velocity fields of OFES. The velocity u (xp, yp, zp, t: here zp = 50- and 150-m depth), which was weighted by the distances from each grid point for four velocities in a grid field, was used to calculate the advection of particles. Leptocephali of Atlantic eels (A. rostrata) and A. anguilla >5 mm total length (TL) perform diel vertical migration (Castonguay and McCleave, 1987). Those two species, ranging from 5.0 to 19.9 mm TL, are mostly at 100–150 m by day and between 50 and 100 m by night. A similar distribution pattern has been suggested for A. japonica leptocephali by Otake et al. (1998). Further, the night-time distribution of A. japonica leptocephali tends towards shallower water with increasing body length (Kajihara et al., 1988; Otake et al., 1998). In our model of migration, vertical distributions by day and night were adopted by the use of a flowfield at fixed depth, 150 m by day and 50 m depth by night before metamorphosis, and at 50 m by both night and day after metamorphosis. ldiff is a diffusion scheme added to the position of a particle by advection. For diffusion of particles, 1 x 102 m2 s–1 was adopted as the horizontal eddy diffusivity.
In the western equatorial Pacific, greater precipitation than the annual average net evaporation dilutes seawater (Weare et al., 1981; Donguy, 1987; Lukas and Lindstrom, 1991), and a salinity front is generated between the diluted water and high-salinity North Pacific Tropical Water (NPTW, >34.8 psu), caused by excessive evaporation, as shown in Figure 1. However, during an El Niño event, this circumstance changes, because the anomaly of evaporation minus precipitation is larger in the central and eastern equatorial Pacific (Kessler and Taft, 1987; Ando and McPhaden, 1997). Hence, the salinity front moves south, and variations in its position appear to be related to the spawning migration of adult Japanese eels (Kimura et al., 1994, Kimura and Tsukamoto, 2006). Interannual variability of the salinity front, as demonstrated by Kimura et al. (2001), is applied to determine the spawning sites of the Japanese eel (Tsukamoto, 2006).
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Estimates of the spawning season of Japanese eels has usually been based on age determination using otoliths, because increments in otoliths deposited daily have been noted at both early larva and juvenile stages (Tabeta et al., 1987; Tzeng and Tsai, 1992; Arai et al., 2000; Marui et al., 2001). Using Japanese eels collected from various localities in Japan, China, and Taiwan, Tsukamoto (1990) estimated hatching dates between April and November. Lee (1999) suggested that estimating the spawning season of Japanese eels by back-calculation using otoliths of glass eels can have two types of error; one attributable to difficulty in recognizing the daily deposition of increments late in the leptocephalus stage, and the other the possibility of otolith increments not being deposited daily. However, through accumulation of glass eel samples and the development of measuring tools (Shinoda, 2004), it has now been suggested that hatching dates of Japanese eels range from May to December, with a peak in August. Therefore, the spawning season of Japanese eels used for this langrangian study followed the best currently available information, i.e. that of Shinoda (2004).
Estimating the daily age of leptocephali collected in the NEC region in July 1991 indicated that A. japonica leptocephali of 10–30 mm TL could be separated clearly into two groups, May- and June-born, and they roughly coincided with the new moon each month (Tsukamoto et al., 2003). This means that Japanese eels do not spawn continuously throughout the long spawning season, but rather spawn around the time of the new moon each month. Therefore, the hatching dates of the eel larvae in this study are based on the time of the new moon.
Additionally, to investigate differences in the transport of A. japonica leptocephali attributable to climate change deduced from variations in the Southern Oscillation Index (SOI) (Figure 2a), we selected specific years in which there was an El Niño (1997/1998), along with years in which there was not (1995/1996). The conditions for the Lagrangian model are listed in Table 1.
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Three sampling surveys for A. japonica leptocephali were conducted by the RV "Hakuho Maru" of the Ocean Research Institute, University of Tokyo, in the western North Pacific during the periods 10 June–6 July 2004, 29 May–13 June 2005, and 30 June–15 July 2005. Leptocephali were collected with a 3 m Isaacs–Kidd midwater trawl (IKMT) net (8.7 m2 mouth opening; 1.0 or 0.5 mm mesh) in 2004, and with an ORI net (Big Fish Net) of 3 m diameter (7.1 m2 mouth opening; 0.5 mm mesh) as well as the IKMT net in 2005. Collections were made at 109 stations in 2004 and at 181 stations in 2005. CTD observations were made at the same stations as leptocephali were caught.
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Results |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Criteria from the literature were used to define the latitude of the salinity front and the NEC bifurcation. The meridional location of the salinity front was said by Kimura et al. (2001) to lie at 34.5 psu, based on salinity averaged between the sea surface and 30 m deep over the latitude range 137–142°E (Figure 2b). Based on climatology of the OFES model, we defined the latitude of the NEC bifurcation as the position where 2°-longitude-averaged meridional velocity east of the coast of the Philippines is zero (Kim et al., 2004). Annual mean meridional velocity averaged within a longitudinal band off the coast of the Philippines showed that the bifurcation shifted north with increasing depth, extending north of 16.6°N at 500 m (Figure 3 of Kim et al., 2004). The NEC bifurcation latitude varies in almost the same phase as ENSO, and is correlated with the position of the main thermocline, ranging from 75 to 250 m deep (Kim et al., 2004). Therefore, the interannual variation of the NEC bifurcation latitude was represented by a position averaged within the main thermocline (Figure 2c). The time-series was smoothed with a 12-month Butterworth low-pass filter (Roberts and Roberts, 1978), to remove the variance associated with the seasonal cycle.
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The positions of the salinity front and the NEC bifurcation latitude varied according to ENSO conditions. Variation in the SOI indicates that variations in atmospheric pressure lead the migrations of the salinity front and the NEC bifurcation zone by 5 months and 1 month, respectively (Kimura et al., 2001; Kim et al., 2004). The long-term average location of the salinity front was 15°N, and its meridional movement corresponded to the SOI with a time-lag of 5 months. Its northernmost position was during El Niño years (1982/1983, 1997/1998), and it was near its average location during non-El Niño years (1995/1996, 2000/2001). Similarly, the NEC bifurcation was at its northernmost position during El Niño events and to the south during non-El Niño events. The meridional displacements of the salinity front and the NEC bifurcation showed distinct interannual variation associated with ENSO, likely influencing the basic conditions at the spawning site of Japanese eels as well as the migration route of the larvae.
The left panels and tabulation in Figure 3 show the results of simulated larval transport from spawning points depending on meridional displacements of the salinity front. Transport to the Kuroshio and the MC were defined by counting particles passing through a line at 24.5°N (line A in Figure 3) between Taiwan and Iriomote Island, the westernmost island of the Ryukyu Islands, and a line at 10°N (line B in Figure 3) west of Mindanao Island, respectively. Comparing the differences in transport to the Kuroshio and the MCs, the influence of the changed ocean systems associated with climate change was clear. In an El Niño year, passive larvae transported to the Kuroshio were fewer than to the MC, whereas transport to the Kuroshio was twice as much in a non-El Niño year. When the larvae were released at a fixed spawning site of 15°N without considering climate change (right panels in Figure 3), the transport differences between the two currents during El Niño and non-El Niño periods were not clear.
Migration routes became more complicated as eel larvae moved near the western boundary east of the Luzon Strait between Taiwan and the northern Philippines (Figure 3). These transport routes could not be explained by just the NEC–Kuroshio Current System. Mesoscale eddies are oceanic conditions that influence the intricate paths of transport. When present, they probably result in Japanese eel larvae being transported along more northerly pathways and dispersing more widely. Moreover, the eddies play a role in lengthening the period of larval migration and retaining A. japonica leptocephali around the eastern area of the Luzon Strait, where a number of larvae >200 days old have been found in some years (Shinoda, 2004).
In contrast to the abundance of A. japonica leptocephali just south of the salinity front seen previously (Tsukamoto, 1992), intensive grid surveys around the spawning grounds of the species in 2004 and 2005 showed a distinct feature. The station with most larvae was close to a cyclonic mesoscale eddy in 2004 (Figure 4a). During the first grid survey in 2005, when a number of 2-day-old pre-leptocephali were collected (Tsukamoto, 2006), the stations that yielded many pre-leptocephali were usually near a cyclonic eddy accompanied by upwelling, recognized by the vertical profiles of salinity (Figure 4b, c). Formation of the cyclonic eddy was verified through the absolute dynamic topography of Archiving, Validation and Interpretation of Satellite Oceanographic (AVISO) data (obtained at http://www.aviso.oceanobs.com/) (Figure 5). Many A. japonica leptocephali were collected near an upwelling area, but the number decreased away from that area (Figure 6). This result supported a belief that the eddy flow may be a mechanism that brings up, then diffuses pre-leptocephali spawned in deep water, because a cyclonic eddy with a diameter (140 km) larger than the Rossby deformation radius (90 km) can cause upwelling by means of the rotating effect of the earth (Gill, 1982). A larger cyclonic eddy with diameter 300 km appeared north of the first one, but no A. japonica leptocephali were found around it.
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Recruitment numbers of Japanese eel larvae spawned each month, deduced from the migration model, are shown in Figure 7. Age at recruitment suggested from daily increments in otoliths of glass eels sampled at nine East Asian coastal stations was in the range 98–227 d (Shinoda, 2004). Most Japanese eel larvae hatched during El Niño years cannot reach the Japanese coast during the catching season (November–May) for glass eels in Japan. However, many larvae arrive at the Japanese coast during catching seasons of non-El Niño years. Therefore, El Niño events clearly have a great influence on eel recruitment to Japan. The difference in recruitment levels between El Niño and non-El Niño years is caused by differences in the migration paths (Figure 3), the southward shift of the spawning site and the northward shift of the NEC bifurcation associated with an El Niño event preventing eel larvae from transferring from the NEC to the Kuroshio. Moreover, recruitment along the Japanese coast varies seasonally, because of seasonal-meridional displacements of the location of the NEC bifurcation, as suggested by Qu and Lukas (2003) and Kim et al. (2004). Qu and Lukas (2003) said that the NEC bifurcation is at its southernmost position (14.8°N) in July and at its northernmost position in December. Recruitment of December-spawned eel larvae in an El Niño year increases numerically more than that of larvae born in other months, because the latitude of the bifurcation is at its northernmost position in December and it shifts south in early spring when the larvae arrived at the western boundary.
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Discussion |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Our migration model has shown interannual variation in recruitment of Japanese eels associated with climate change. An El Niño event results in the spawning areas of Japanese eels shifting south and the bifurcation of the NEC moving north, which reduces the likelihood of recruitment of eels to coastal habitats in Japan. The ecological responses of eels to climate change are likely to provide the answer to why recruitment of Japanese eels has declined. Mesoscale eddies caused eel larvae to be transported along more northerly pathways and to disperse more widely, lengthening the larval migration period and retaining A. japonica leptocephali around the eastern side of Taiwan. The eddies also brought leptocephali soon after hatching closer to the surface.
Several hypotheses have been advanced for the declining recruitment of American and European eels (Castonguay et al., 1994a; Moriarty and Dekker, 1997), including overfishing, habitat loss attributable to the construction of barrages and dams, and global climate change. Castonguay et al. (1994b) hypothesized that climate changes in the Atlantic Ocean influence American and European eels, and recruitment to some parts of the range of the European eel correlates with certain ocean/atmosphere parameters (Knights, 2003; Friedland et al., 2007). Atlantic eels spawn in a west-flowing current, as do Japanese eels, and migration of the former is affected by the Gulf Stream in a similar manner to the way the Kuroshio influences migration of Japanese eels. Moreover, recruitment of European glass eels declined drastically in the early 1970s, similar to the sudden decrease in Japanese glass eel recruitment in the late 1970s. These phenomena with essentially similar timing in different ocean basins imply that oceanic effects associated with global climate change rather than man-induced or natural short-term effects are probably critical parameters in causing the declining recruitment of European, American, and Japanese eels. Therefore, interannual variability in the NEC bifurcation and salinity front locations in the western North Pacific associated with climate change may be good indices of interannual fluctuations in Japanese eel resources.
Eddies largely influence the distribution and migration of Japanese eel larvae between their spawning areas and the western boundary of their distribution. In our migration model, most eel larvae not entrained into the Kuroshio or the MC piled up east of Taiwan and the Philippines. Distribution of the eel larvae collected from 1956 to 2002 revealed that metamorphosing larvae of A. japonica appeared with leptocephali >50 mm in the eastern area of Taiwan and the Luzon Strait (Shinoda, 2004). The simultaneous appearance of several development-stage larvae is likely caused by differences in transport speed attributable to eddy flows. Ichikawa (2001) suggested that a west-propagating mesoscale eddy in the western North Pacific was partly captured by the Kuroshio south of Okinawa, then moved downstream along the path of the Kuroshio. Therefore, eel larvae transported to the east of Taiwan by eddies probably eventually entrain into the Kuroshio and then reach the Japanese coast.
To recruit into coastal habitats in Taiwan, eastern China, Japan, and Korea, the eel larvae first enter the East China Sea with the Kuroshio. However, our migration model could not reproduce the migration across the continental shelf necessary for recruitment of eel larvae to coastal habitats. As the eel larvae are transported along the mainstream of the Kuroshio, they do not separate from the current, so can only cross over onto the continental shelf by swimming or in oceanographic disturbances such as a frontal eddy. Metamorphosis of the Japanese eel starts upstream of the Kuroshio east of the Philippines and terminates during passive migration with the dominant current (Sakakura et al., 1996), and metamorphosis into glass eels is almost completed within the Kuroshio (Otake et al., 2006). After metamorphosis into glass eels, recruitment to the estuaries of Taiwan takes place either by swimming or in coastal currents (Tzeng, 1985). In an estuary of the French Atlantic coast, glass eels (A. anguilla) migrate upstream using tidal currents (Gascuel, 1986). Inshore migration from mainstream currents offshore and recruitment to appropriate continental habitats consequently requires an ability to swim to be able to make the most of tidal currents and coastal current systems.
In this study, our larval transport model using advection and diffusion with Japanese eel larvae as passive tracers reproduced the migration routes of Japanese eels rather better than did a previous study (by Kimura et al., 1999), using the results of field sampling to designate the location of spawning areas. This means that eel larvae migrate to their recruitment areas in East Asia using mainstream currents that provide the most efficient pathway, to decrease energy consumption of eel larvae until the end of their metamorphosis into glass eels. Indeed, Otake et al. (2006) provided direct evidence for the migration of metamorphosing leptocephali and glass eels through the Kuroshio.
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Acknowledgements |
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We thank all those who helped with net sampling, plankton sorting, and identification of leptocephali, and especially the captain and crew of RV "Hakuho Maru" for their assistance throughout the cruises in 2004 and 2005. The OFES simulation on which our Lagrangian study calculating the transport of eel larvae was conducted based on the Earth Simulator under the support of Japan Agency for Marine–Earth Science and Technology (JAMSTEC). The research was supported financially by a research fellowship from the Japan Society for the Promotion of Science for Postdoctoral Fellowship for Foreign Researchers, Japan (JSPS). Valuable comments on an earlier version of the manuscript were provided by Brian Knights and Liam Fernand. Finally we thank Katsumi Tsukamoto and M. J. Miller for support and encouragement.
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References |
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