Spawning Sites of the Japanese Eel in Relation to Oceanographic Structure and the West Mariana Ridge
et al. (2014) Spawning Sites of the Japanese Eel in Relation to Oceanographic Structure and the
West Mariana Ridge. PLoS ONE 9(2): e88759. doi:10.1371/journal.pone.0088759
Spawning Sites of the Japanese Eel in Relation to Oceanographic Structure and the West Mariana Ridge
Jun Aoyama 0
Shun Watanabe 0
Michael J. Miller 0
Noritaka Mochioka 0
Tsuguo Otake 0
Tatsuki Yoshinaga 0
Katsumi Tsukamoto 0
Martin Castonguay, Institut Maurice-Lamontagne, Canada
0 1 Atmosphere and Ocean Research Institute, The University of Tokyo, Chiba, Japan, 2 College of Bioresource Sciences, Nihon University , Kanagawa , Japan , 3 Faculty of Agriculture, Kyushu University , Fukuoka , Japan , 4 School of Marine Biosciences, Kitasato University , Kanagawa , Japan
The Japanese eel, Anguilla japonica, spawns within the North Equatorial Current that bifurcates into both northward and southward flows in its westward region, so its spawning location and larval transport dynamics seem important for understanding fluctuations in its recruitment to East Asia. Intensive research efforts determined that Japanese eels spawn along the western side of the West Mariana Ridge during new moon periods, where all oceanic life history stages have been collected, including eggs and spawning adults. However, how the eels decide where to form spawning aggregations is unknown because spawning appears to have occurred at various latitudes. A salinity front formed from tropical rainfall was hypothesized to determine the latitude of its spawning locations, but an exact spawning site was only found once by collecting eggs in May 2009. This study reports on the collections of Japanese eel eggs and preleptocephali during three new moon periods in June 2011 and May and June 2012 at locations indicating that the distribution of lower salinity surface water or salinity fronts influence the latitude of spawning sites along the ridge. A distinct salinity front may concentrate spawning south of the front on the western side of the seamount ridge. It was also suggested that eels may spawn at various latitudes within low-salinity water when the salinity fronts appeared unclear. Eel eggs were distributed within the 150-180 m layer near the top of the thermocline, indicating shallow spawning depths. Using these landmarks for latitude (salinity front), longitude (seamount ridge), and depth (top of the thermocline) to guide the formation of spawning aggregations could facilitate finding mates and help synchronize their spawning.
Freshwater eels of the genus Anguilla are catadromous fishes that
spawn over deep water at tropical latitudes and use the ocean for
their larval development before entering estuarine and freshwater
growth habitats [1,2]. All three northern temperate species of
anguillid eels consist of single panmictic populations , with all
of their reproductively maturing individuals migrating long
distances offshore to spawn in a single spawning area for each
Anguillid eel populations including those of the Japanese eel,
Anguilla japonica, have declined worldwide in recent decades [9,10],
but the exact causes of the declines are difficult to determine partly
because their reproductive ecology is hidden by the vast open
ocean. The spawning areas of the Atlantic eels, the European eel,
Anguilla anguilla, and the American eel, Anguilla rostrata, were
discovered early in the last century  and were later found to be
associated with temperature fronts in the Sargasso Sea based on
the distribution of small larvae [12,13], with spawning occurring
across a wide latitudinal area [14,15]. The spawning area of the
Japanese eel in the western North Pacific (Fig. 1) was discovered in
1991  and has been intensively studied in the last few decades
. The surveys that succeeded to collect newly hatched
larvae , eggs and spawning-condition adults [19,21,22]
indicated that spawning area of the Japanese eel is located
latitudinally from about 1215uN  within the continuous
westward flow of the North Equatorial Current (NEC) that is
present from about 817uN [23,24], and longitudinally along the
western side of the West Mariana Ridge, which is the southwestern
extension of the Izu-Bonin-Mariana Arc system (see Gardner
). Further, otolith analyses for the larvae collected during the
surveys showed that spawning of Japanese eels occur during new
moon periods [17,26].
Various types of research have been conducted recently to
understand the oceanic life histories of anguillid eels. The
physiological ecology of migration [27,28], biology of maturation
and spawning behavior , or geomagnetic sense  has
been studied in the laboratory. Pop-up satellite transmitting tags
have been used to learn about the migratory behavior of both
Northern [33,34] and Southern  Hemisphere anguillids.
These studies suggest that anguillid silver eels have incredible
longterm swimming abilities that might be guided in part by a
geomagnetic sense while they migrate through the ocean using
distinct diel vertical migration behaviors. Exactly how they find
their spawning areas and decide where to form spawning
aggregations has remained a mystery however  and spawning
eels have not yet been observed directly .
A shallow salinity front that forms within the NEC where
Japanese eels spawn [14,34] (Fig. 1) has been hypothesized to
affect the latitude of spawning [39,40]. Indeed, collections of small
leptocephali  or eggs  were made near the southern edge
of the spawning area when the salinity front was located far to the
south in two different years. Larger leptocephali were also found
south of the salinity front further to the west during their larval
transport . However, in some years there are no distinct
salinity fronts in the spawning area [17,41], so what determines
spawning locations at those times has remained unclear.
Understanding how Japanese eels decide their latitude of
spawning is of special importance because the NEC bifurcates
into both northward and southward flows (Fig.1). The latitude of
bifurcation of the two current flows can change in different months
or years , which may strongly affect how many Japanese
eel larvae get entrained into the southward flowing Mindanao
Current and transported away from their recruitment areas
[43,45]. Therefore, at what latitude the eels decide to spawn could
have a significant affect on the recruitment success of their larvae
This study analyzes the results of three sampling surveys for eggs
and pre-feeding stage preleptocephali in 2011 and 2012 that were
conducted to learn more about what factors may determine where
Japanese eels form spawning aggregations. The three sampling
surveys were designed to determine the distribution of eggs and
preleptocephali in the NEC along the ridge during new moon
periods to evaluate where spawning may have occurred and where
it did not occur by the same set of protocols. Because the
temperature structure of the warm surface layer of the NEC does
not include any distinct gradients or fronts at the latitudes where
the Japanese eel spawns [23,41,46], the salinity structure was
evaluated in relation to where eggs and larvae were collected. This
information along with the findings of previous studies is used to
propose a hypothesis for where this species will spawn along the
Materials and Methods
Survey Strategy of Cruises
The three cruises of this study were conducted during 24 June
10 July in 2011 (KH-11-6), and 13 May1 June (leg 1 of KH-12-2)
and 6 June28 June 2012 (leg 2 of KH-12-2, Table 1).
Oceanographic observations were made at the beginning of each
cruise to particularly know the location of the salinity front as it
crossed the seamount chain before deciding where to sample for
eggs just before new moon. Based on the salinity structure, a
region along the ridge was chosen for sampling for eggs just before
new moon, and an arrangement of stations was set along the west
side of the ridge. These stations were sampled until eggs were
collected, and then a new grid of stations was arranged around the
location of the first egg collection, which was intensively sampled
until the eggs would likely be hatching into preleptocephali. Then
transect surveys were conducted to find newly hatched
preleptocephali at different latitudes along the ridge to estimate where else
spawning may have occurred during each new moon period.
These transects were designed to detect spawning near the ridge
based on the presence of preleptocephali, but were not extensive
enough to exclude spawning in other areas with certainty if
preleptocephali were not collected if there were longitudinal
variations in where spawning may have occurred in relation to the
Locating the Salinity Front
Oceanographic observations were made to construct salinity
and temperature sections using profiles from either conductivity,
temperature, depth (CTD) sensor system (Seabird, USA) deployed
from a cable on the side of the ship (to a depth of 500 m depth) or
expendable X-CTD probes (Tsurumi Seiki Co. Ltd., Japan)
deployed from the back of the slowly moving ship (to a depth of
1000 m depth). Stations were planned east and west of the ridge,
but some stations were cancelled on one or the other side of the
ridge after the location of the sanity front had been determined
Collecting Eggs and Preleptocephali
Sampling to determine the horizontal distributions of fertilized
eggs (embryos) and preleptocephali was conducted using
standardized oblique tows of a 3-m diameter ORI-BigFish ring net
with 0.5 mm mesh that fished mostly in the upper 200 m. Salinity
structure was primarily used to decide the starting point of
sampling for eggs, and once eggs were found then their
distribution was examined in a grid of stations (Fig. 4). After the
new moon when the eggs hatched into preleptocephali during all
three cruises, transect surveys were carried out over the entire
length of the ridge within the spawning area. The stations were at
similar distances from the outer ridge, which shifts further to the
west and becomes wider with deeper seamounts in the southern
region, to determine the possible range of spawning latitudes based
on the distribution of preleptocephali (Fig. 4). Most stations had
only one ORI-BF tow, but a few had more than one tow. During
June 2011, 9 stations were sampled in the egg grid area (including
7 tows in the depth experiment at Stn. 4), followed by 46 stations
for preleptocephali after new moon. In May 2012, 8 stations in an
overlapping area as the later stations (14.616.0uN, not shown)
were sampled before new moon prior to sampling in the egg grid,
which was followed by 25 stations for preleptocephali. In June
2012, 13 stations (13.515.2uN, not shown) were sampled before
the egg grid, followed by 37 stations for preleptocephali. For
clarity and simplicity, catches of eggs and preleptocephali are
presented as number of specimens collected, which is directly
understandable, rather than as catch rate values, because their
presence or absence is the main factor to consider in this study, not
their quantitative relative abundances calculated using the amount
of water filtered by the net. The lengths of the preleptocephali
(,45 mm) were not evaluated in this study, because these are a
short duration pre-feeding stage of larvae, which do not grow until
they develop into leptocephali and begin feeding.
For these research surveys, all necessary permits to conduct the
biological sampling and hydrographic observations in the
Exclusive Economic Zone of the coastal states for both 2011 and 2012
were issued by the United States Department of State, Bureau of
Oceans and International Environmental and Scientific Affairs,
Department of Foreign Affairs, Federated States of Micronesia
and the Ministry of State of the Republic of Palau. No specific
animal welfare permit is required in Japan for collecting
Eggs (Fig. S1) and preleptocephali (Fig. S2) were sorted out of
the plankton samples and morphologically identified before
subsamples were genetically confirmed onboard. After being
sorted out of the plankton samples, the morphology of the eggs
and preleptocephali were examined and photographed using a
Nikon SMZ1500 dissecting microscope and a Nikon DMX1200F
digital imaging system (Nikon, Tokyo Japan) (Fig. S1, S2).
Specimens were then mostly preserved in 99% ethanol to enable
later DNA analyses.
Onboard Genetic Identification
Before deciding to conduct a grid survey around an egg
collection location, eggs with the appropriate size of about 1.6 mm
and morphology  were genetically identified using an onboard
real-time polymerase chain reaction (PCR) based ABI PRISM
7300 Sequence Detection System (Applied Biosystems, USA)
[48,49] as described previously [19,47]. Some preleptocephali
were also confirmed onboard to be of the Japanese eel using this
Vertical Distribution of Eggs
During the KH-11-6 survey, the vertical distribution of eggs was
studied by making multiple tows at the same station after eggs
were collected there. This was conducted on 29 June 2011 at Stn.
4 (13u00N, 141u559E) (Fig. 4C). The ORI-BF net was deployed in
7 tows that had 20 min of horizontal towing at one of 6 depths (60,
120, 150, 180, 250, 420 m) from 07:38 to 16:20 during the day.
The catch rate of eggs in each tow was used as a measure of
Table 1. Overview of the number of stations sampled during the different parts of each of the three surveys in relation to the
timing of new moon and the number of eggs or preleptocephali that were collected in 2011 (KH-11-6, new moon 1 July) and 2012
(KH-12-2: leg 1, new moon 20 May 2; leg 2, new moon 19 June).
*4 eggs were collected on 21 May (one day after New moon) after the egg grid survey.
Figure 2. Hydrographic stations along the West Mariana Ridge. The locations of CTD or X-CTD profile stations during the three surveys in
2011 and 2012 are shown with black dots. The estimated location and angle of the salinity front crossing the ridge (red line) and the northern extent
of the layer of low salinity water usually associated with the salinity fronts (#34.5) (blue lines) if present are also shown. Three shallow seamounts
(SM) previously investigated are shown (red triangles).
abundance at each depth layer based on the amount of water
filtered by the net calculated from the flow meter revolutions. Two
tows were made at 150 m to obtain more eggs for a different study,
so the catch rates of the two tows were averaged at that depth in
Figure 5. The depth of the net was monitored acoustically with a
net depth monitoring system (Scanmar, Norway). The net was
open during each tow, but there was no evidence of contamination
from other layers, because eggs were only collected at two layers.
The hydrographic sections of the upper 500 m of the ocean
from the three different surveys showed similar general patterns of
temperature and salinity structure, except for the patterns of the
lower salinity water in the upper 100 m. Water temperatures in
the upper 100 m ranged from 2630uC in the 3 sections, and no
temperature fronts were present (Fig. 3A,C,E). The depth of the
warmest water in the surface layer became shallower in the north
in all three sections, but 30uC water was only detected in June
2012 (Fig. 3E). The patterns of fluctuations of the depth of the
isotherms appeared to vary in conjunction with salinity structure
in all three pairs of sections in Figure 3. Water with a salinity of
34.5 has been found to be a reliable marker associated with the
salinity fronts [16,39,40,46], so this salinity value was used in the
present study to show the distribution of salinity levels. Surface
water with a salinity of #34.5 is shown by red lines in
Figure 3B,D,F that correspond to the blue-green shaded water
in Figure 4 (left panels). In all three sections the lower salinity
water (#34.5) that is usually associated with the salinity fronts was
present in the upper 100 m, but its depth and latitudinal extent
varied among surveys (Fig. 3B,D,F). Each hydrographic section
showed that the salinity increased with depth until about 75
150 m where there was a layer of high salinity water of various
thickness and northerly extension (Fig. 3,4). This subsurface layer
of high salinity water is referred to as the North Pacific Tropical
In the June 2011 survey there was a shallow layer of lower
salinity water #34.5 extending almost to 16uN (solid red line in
Figure 2B), but there was a distinct salinity front extending deeper
between about 12.5 and 13.5uN. Eggs were collected at two
stations within the frontal zone (n = 147, 2 days before new moon),
and preleptocephali were only collected in the same area (10
stations, n = 83) (Fig. 4A,B,C). No preleptocephali were found
either to the north or south of that area after new moon (Fig. 4B).
In leg 1 of the survey in May 2012 a deeper layer of #34.5
water extended slighter further to the north compared to in June
2011 and almost reached 16.5uN (Fig. 2D). Eggs were collected in
9 tows at 6 stations near 15uN (n = 128, 03 days before new
moon, and 4 eggs collected one day after new moon) about 1u
south of a possible weak subsurface salinity front associated with
the northern limit of the low salinity water (Fig. 4D,E,F). There
were 153 preleptocephali collected at 7 stations from 13u3015uN
within the low salinity water after new moon (Fig. 4D,E).
During the next month in the June 2012 leg 2 survey, the layer
of lower salinity water #34.5 extended all across the study area
and to the north of 17uN (Fig. 2F). There was a pool of even lower
salinity water #34.3 (dotted red line near surface) extending
almost to 14.4uN and a separated patch of that water was also
detected in the north. Eggs were collected at 8 stations near 14uN
at the northern edge of that southern layer of #34.3 water
(n = 284, 13 days before new moon) (Fig. 4G,H,I).
Preleptocephali were collected both north and south of the egg collection
area, with 2 being caught in the patch of #34.3 water in the north,
and 1 being caught in the southwest area after new moon
The tows made at Stn. 4 to study egg depth distribution in the
June 2011 survey collected 21 eggs in two depth layers (150,
180 m), but no eggs were caught in the tows at layers above and
below (60, 120, 250, 420 m) as shown in Figure 5. The layer of egg
collection was near the top of the thermocline within the center of
the high salinity water and just below the chlorophyll maximum,
with the most eggs being caught in the 150 m layer.
This study collected both eggs and preleptocephali along the
West Mariana Ridge during each of three sampling surveys that
were timed in relation to the new moon periods of May or June of
201l and 2012. The eggs were found just before new moon at
three different latitudes along the ridge, which corresponded to the
locations of the salinity front or being within the latitudinal extent
of the low salinity surface water. Egg catches to the south of the
strong salinity front in June 2011 were consistent with the
hypothesis that the salinity front influences the latitude of
spawning [16,39,40]. The catch locations of preleptocephali were
also limited to the frontal area during that survey. This suggests
that spawning eels may have focused on that particular area to
form spawning aggregations. The first collections of Japanese eel
eggs in May 2009 were also in a similar area just south of a salinity
front, with no preleptocephali being collected at stations to the
north of the front . Surveys in previous years have found no
evidence of spawning on the east side of the ridge , which is
why there were no stations there during this study.
However, there were no clear salinity fronts in the two 2012
surveys and in some previous studies [17,41], so the eels must
sometimes have to decide where to form spawning aggregations
without a distinct salinity front as a landmark. In May and June of
2012 of this study, eggs were collected within the latitudinal range
of the low salinity surface water and the catch locations of
preleptocephali suggested that spawning had also occurred at a
wide range of latitudes. Eddies could redistribute some larvae to
slightly different latitudes than those at which they were actually
spawned, but in the two 2012 surveys, it is unlikely to account for
the broad latitudinal distribution of preleptocephali just a few days
after they were spawned. A similar situation was found in 2008
when preleptocephali were collected over almost 2 degrees of
latitude within the low salinity water [19,41]. Although no
preleptocephali were collected near the northern edge of the low
salinity water in May of 2012, the two surveys in that year suggest
that if there is no distinct salinity front, Japanese eels may spawn at
multiple latitudes south of the northern limit of the lower salinity
surface water. The limited distribution of stations that sampled for
preleptocephali was not wide enough in each of the three surveys
of this study to determine all places where spawning did not occur,
but the collections of preleptocephali were good indications of the
general latitudes where spawning likely did occur.
The importance of the salinity front for the Japanese eel was
first suggested in 1991 when many leptocephali mostly about
1020 mm were collected further west in the NEC, just south of a
distinct salinity front around 16uN . Similarly, leptocephali
were found in waters south of a salinity front in 1994  and in
2002 . The distributions of eggs and preleptocephali revealed
the blue-green color in the left panels shows low salinity water #34.5, light-purple shows 35.0 salinity, and dotted-red lines show contours spaced at
0.2 salinity intervals above or below the bottom of the #34.5 water in the surface layer. In the middle panels, light-green circles show
preleptocephalus catches and black dots show no-catch stations, and in the right side insets, light-blue circles show egg catches and white circles
show no-catch stations, with numbers of specimens shown with numbers. Four eggs were caught at a station sampled again in the preleptocephali
transect (light-blue circle in E). Orange shows shallower depths associated mostly with the West Mariana Ridge and the Mariana Ridge including
Guam (middle panels), and blue shows deeper depths down to 10,000 m in the Mariana Trench, which is the deepest place in the worlds oceans.
Scale bar shows 50 km (H). More than one tow was made at stations 4 and 8 in (C) and (F), respectively.
in the present study showed that the salinity structure across the
spawning area can influence the range of latitudes at which
spawning may occur during each new moon period within the
spawning season. When a distinct salinity front is present, eels may
attempt to aggregate and spawn at or south of the front as
occurred in June 2011 and May 2009  even if it is located in
the southern part of the NEC (,12uN). Using a landmark such as
a front might facilitate finding mates and increase the successful
fertilization of the eggs by having many eels of both sexes involved
in the spawning aggregations, rather than just a few as a result of
spawning being spread over a wide range of latitudes.
Temperature fronts have been hypothesized to have the same function for
spawning of Atlantic eels to stop their migration in the Sargasso
Sea and look for mates [51,52].
Although the surveys of this study and previous ones have
usually had the problem of not having enough time after new
moon to adequately determine the precise spatial extent of
spawning by sampling for preleptocephali in multiple gridlines at
different longitudes, an interesting pattern has emerged
nevertheless. Eggs and preleptocephali have been collected at various
latitudes mostly between 12.215.3uN on the west side of the West
Marina Ridge and not on the east side during multiple years
[18,19], indicating that the seamount chain acts as a landmark of
the eastern edge of the spawning area. The location of the salinity
front has been linked to the estimated spawning locations in
several different years [16,19,39] including 2011, but in some
years low salinity water extends across the spawning area and no
distinct fronts are present  as was the case in 2012.
When salinity fronts form within the latitudes of the spawning
area, which all types of data suggest is from about 1216uN [17
19], spawning will occur at or south of the front and the eggs will
be found along the west side of the seamount ridge. When no front
is present within the latitudes of the spawning area, spawning can
occur at various latitudes within the lower salinity surface water
and eggs may appear in many places along the west side of the
ridge. It is unclear if other factors such as differences in current
velocities of parts of the NEC may have some influence on the
latitude of spawning in the absence of a distinct salinity front, but
there has been no clear indication of this occurring.
In the vertical axis of ocean depth, the eggs were only collected
at the two layers of 150 and 180 m and not in the 2 shallower and
2 deeper layers. Previous catches of preleptocephali [19,21] within
the same depth layer as the eggs in this study suggest spawning
occurs near this depth stratum located at the top of the
thermocline. However, since eggs and preleptocephali may be
positively buoyant compared to seawater at these approximate
temperatures , the eggs may rise up until they reach the depth
stratum where they seem to accumulate. The development time of
Japanese eel eggs at these temperatures are about 1.52 days
before hatching , so they have time to float up to the
thermocline. Catches of adult eels in the spawning area in the
upper 250 m [19,21,22] also suggest that spawning probably
occurs at depths of about 160250 m. Combining all this
information, spawning by the Japanese eel seems to be relatively
shallow as previously hypothesized  and not much deeper as
was indirectly suggested by the observation from a submersible of
a possible American eel at a depth of about 2000 m on the bottom
in the Bahamas far from the American eel spawning area . A
possible male Japanese eel was observed 2 days before new moon
within the spawning area along the West Mariana Ridge by a
underwater camera system Deep-Tow(JAMSTEC) at a depth of
179 m in July 2012 , but like the possible American eel seen
previously, the eel could not be identified for certain.
In the temporal axis, spawning of the Japanese eel appears to be
timed to occur in the few days just before new moon. This has
been shown by otolith analyses of leptocephali in several different
years that showed back-calculated hatching dates were centered
only on new moon periods [17,26]. More direct evidence of new
moon spawning has been found by the collections of eggs just
before new moon in 2009 , and in 2011 and 2012 in this study.
Similarly, preleptocephali have only been collected near or just
after new moon in 2005 , 2007, 2008, 2009  and in this
study. Spawning condition adults were also only caught within the
spawning area during new moon periods [19,21,22].
Therefore, it is hypothesized that Japanese eels spawn just
before new moon near or below the top of the thermocline along
the seamount ridge, which is determined latitudinally by the
northern extent of shallow lower salinity waters. How they
determine where these locations are before each new moon period
and why these locations would have been established as preferred
spawning locations are still unclear though, because little is known
about silver eel migrations in the ocean . Pop-up tag studies for
the Japanese eel  and other anguillid eels [33,3537] showed
that they mostly migrate between depths of about 100400 m at
night (much deeper during the day). However, they can sometimes
come into even shallower layers at night [34,35] at depths close to
where they would be able to detect the low salinity surface layers
described in this study. There is no evidence that these eels have
reached their spawning areas though, so their behaviors might
drastically change once they reach the spawning area to include
more movements into depths closer to the surface where they
could find some aspect of the low salinity water mass, or its
absence. Japanese eels and other anguillids are known to have a
geomagnetic sense [32,56,57] that might be used to help find their
spawning areas, but there has also been speculation that they
might use various types of odors associated with fronts or different
water masses, or pheromones from each other, to locate their
spawning sites and to find mates [31,51,52]. However, it is
essentially still a mystery how the eels reach their spawning areas
and find mates for spawning.
Regardless of how the eels accomplish the challenge of reaching
their spawning area, for the Japanese eel, the patterns of currents
in the western North Pacific have the potential to influence the
recruitment success of its larvae. Southward shifts of the spawning
locations following El Ni no events or shifts of the NEC bifurcation
latitude have been hypothesized to increase entrainment of
Japanese eel leptocephali into the Mindanao Current and reduce
their recruitment to East Asia [39,40,43,45]. Evidence that this
can occur was seen when a 42.8 mm A. japonica leptocephalus was
collected in the Celebes Sea  (Fig. 1). The modeling studies
showed that a slight difference (1u) in latitudes of spawning can
theoretically have a significant effect on the proportions of larvae
entering the Kuroshio (chance for successful recruitment) or the
southward branch entering the Mindanao Current (recruitment
failure) [43,45]. Other ocean-atmosphere or climatic factors have
been also suggested to be related to the recruitment success of the
Japanese eel or other species, including productivity changes
affecting larval survival . The detection of unusual
temporal patterns of glass eel recruitment has also raised the
question about the possibility of there being a shift in the spawning
season recently . Which of these factors may be more
important in influencing the decline or interannual recruitment
fluctuations of the Japanese eel is not known. It is also not known if
having a clear landmark to assist Japanese eels to decide where to
spawn might improve their spawning and recruitment success,
compared to when spawning aggregations are more spread out
Even though some aspects of where and when Japanese eels
spawn are now known, many mysteries remain to be determined
about how these remarkable fish are able to find their spawning
areas and then detect small differences in salinity or other
oceanographic characteristics before they form aggregations to
spawn. Future efforts to observe spawning aggregations using
camera systems  may help to understand the reproductive
ecology of the Japanese eel, and efforts to learn how their
leptocephali are transported westward from different spawning
latitudes also needs to be investigated in relation to seasonal
patterns of recruitment. These types of information may be
important components of the process of finding out how to best
manage and conserve this species to help prevent further declines
in its population.
Figure S1 Japanese eel, Anguilla japonica, eggs from the
spawning area. Photographs of various stages of freshly caught
Japanese eel eggs (embryos) caught along the West Mariana Ridge
during the three surveys at the locations shown in Figure 4. Some
late-stage embryos hatched out while being observed (A, bottom
right; B, right). Many early-stage eggs did not survive the agitation
and temperature shock of capture by the net (C, right).
Figure S2 Japanese eel, Anguilla japonica,
preleptocephali from the spawning area. Photographs of a 5.7 mm
early-stage Japanese eel, Anguilla japonica, preleptocephalus
(prefeeding stage larva) with a large oil globule and undeveloped head
(A), and a 5.0 mm late-stage preleptocephalus with a jaws, teeth,
and pigmented eyes, which were both collected at 13u00.1N,
141u24.9E on 21 June 2012 (B). Both larvae were confirmed to be
A. japonica using onboard Real-Time PCR. Scale bars show 1 mm.
We thank the captains and crew members of the R/V Hakuho Maru whose
excellent help made these research cruises successful. All the other scientists
and technicians who helped deploy the sampling and hydrographic gear
and carefully sort the many plankton samples looking for the tiny eggs and
preleptocephali during the cruises are also gratefully acknowledged.
Conceived and designed the experiments: KT TO JA SW. Performed the
experiments: KT NM JA TY SW MJM. Analyzed the data: JA SW MJM.
Contributed reagents/materials/analysis tools: TO NM JA MJM TY SW.
Wrote the paper: JA MJM.
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