[Show abstract][Hide abstract] ABSTRACT: The natural reproductive ecology of freshwater eels remained a mystery even after some of their offshore spawning areas were discovered approximately 100 years ago. In this study, we investigate the spawning ecology of freshwater eels for the first time using collections of eggs, larvae and spawning-condition adults of two species in their shared spawning area in the Pacific. Ovaries of female Japanese eel and giant mottled eel adults were polycyclic, suggesting that freshwater eels can spawn more than once during a spawning season. The first collection of Japanese eel eggs near the West Mariana Ridge where adults and newly hatched larvae were also caught shows that spawning occurs during new moon periods throughout the spawning season. The depths where adults and newly hatched larvae were captured indicate that spawning occurs in shallower layers of 150-200 m and not at great depths. This type of spawning may reduce predation and facilitate reproductive success.
[Show abstract][Hide abstract] ABSTRACT: The distribution of all larval stages of the Japanese eel, Anguilla japonica, were examined using historical catch records and original data in the western North Pacific (WNP) to evaluate existing information
about the larval distribution and migration of this species. A total of 148 preleptocephali, 2547 leptocephali, 6 metamorphosing
larvae, and 21 glass eels were collected during 37 cruises over a 52-year period (1956–2007). Sampling effort was spatio-temporally
biased in latitude/longitude among seasons with sampling effort being concentrated near the western margin of the subtropical
gyre near Taiwan in the winter season and extensive effort occurring near the spawning area to the east near the seamount
chain of the West Mariana Ridge in summer during the spawning season. The distribution of preleptocephali (4.2–8.7mm) was
limited to a narrow area around 14°N, 142°E just west of the southern part of the seamount chain, while leptocephali (7.7–62.0mm)
were widely distributed at increasing size westward in the North Equatorial Current (NEC) to the region east of Taiwan. Metamorphosing
larvae (52.7–61.2mm) were collected only in the area 21–26°N, 121–129°E to the east of Taiwan, while glass eels (51.3–61.2mm)
occurred only within or west of the Kuroshio. These distributions suggest that leptocephali begin to metamorphose within or
just east of the Kuroshio, then after completion of metamorphosis the glass eels detrain from the current and migrate inshore.
The relationship between catch date and body size of leptocephali suggested that the spawning season is from April to August,
but further sampling is needed to eliminate possible effects of sampling bias. This analysis is consistent with the existing
hypothesis that Japanese eel larvae born near the West Mariana Ridge are transported westward in the NEC and then transfer
to the Kuroshio to recruit to East Asia, although more sampling effort is needed for later stage larvae in the NEC bifurcation
region to help understand the larval migration in relation to the possible impacts of ocean–atmosphere changes.
–North Equatorial Current–Kuroshio–Larval dispersal
Reviews in Fish Biology and Fisheries 01/2011; 21(3):591-611. · 2.27 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: After analyzing all the collection data for larvae of the Japanese eel, Anguilla japonica, in the western North Pacific, we found that the spawning site of this species appears to be near three seamounts in the West Mariana Ridge, 2000–3000km away from their freshwater habitats. These seamounts are located in the westward flow of the North Equatorial Current and are hypothesized to provide cues for migrating silver eels and to serve as possible aggregation sites for spawning. Back-calculated birth dates based on otolith microstructure of leptocephali indicate that the Japanese eel does not spawn continuously during the long spawning season from April to November, but is synchronized to spawn periodically once a month during new moon. This lunar periodicity of spawning and the seamount spawning hypothesis are new developments in the millennium-old mystery of eel spawning that has fascinated naturalists since the time of Aristotle.
Environmental Biology of Fishes 01/2003; 66(3):221-229. · 1.31 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The location and timing of spawning of the Japanese eel Anguilla japonica was studied during two research cruises of the R/V Hakuho Maru and R/V Suruga Maru in the North Equatorial Current of the western North Pacific Ocean during June–September 1998. There were 38 A. japonica leptocephali (10.0–43.2 mm in total length (TL)) collected in three areas: 24 specimens around the Arakane and Pathfinder Seamounts in June (approx. 16°N and 143°E) and five specimens at the southernmost station (13°N) and nine specimens at the northernmost station (17°N) of a transect along 137°E in September. The average total lengths of the leptocephali were significantly different among the three areas, with those around the seamount being smallest, those at the northern station being largest, and none being collected along the easternmost 144°E transect. This and the currents in the region suggested that spawning of A. japonica occurred near some of the seamounts in the West Mariana Ridge. Back calculated spawning dates indicated that most leptocephali were born during the new moon, supporting the hypothesis that A. japonica spawns around the new moon. Analysis of otolith daily rings found a strong correlation between total length and age (r = 0.97), and the average daily growth rate was about 0.5 mm/day.
[Show abstract][Hide abstract] ABSTRACT: Two sampling surveys of the R/V Tansei Maru were conducted in the Kuroshio region south of Kyushu Island of Japan and to the south in the western North Pacific, to study the distribution patterns of the larval stages of the Japanese eel Anguilla japonica as they approach their recruitment areas in East Asia. Nine fully-grown premetamorphic leptocephali (49.5-58.3mm TL) were collected during November 1996 to the east of Taiwan. During November and December 2000, nine early stage glass eels (51.3-57.0mm TL, pigmentation stage II-IV) that were still in the late metamorphosis stages were collected in the Kuroshio. These findings suggest that metamorphosing Japanese eel leptocephali that recruit to the northern part of their species range migrate in the Kuroshio. They may detrain from the Kuroshio at the pigmentation stage IV-VA and begin their coastal migration.
[Show abstract][Hide abstract] ABSTRACT: Eggs of the Japanese eel Anguilla japonica collected in the western North Pacific were identified by onboard species-specific polymerase chain reaction (PCR) and DNA nucleotide sequencing after the cruise. Fish eggs of various species were collected by large plankton net tows at 12 stations along the southern part of the West Mariana Ridge on 19–25 May 2009. A total of 43 fish eggs were distinguished morphologically as possibly being of A. japonica. Thirty-one of those were analyzed by PCR, which included 15 eggs collected at 12°50–55′N, 141°15–20′E (in 5 tows) that showed positive results. The 16S ribosomal RNA (rRNA) gene sequences of eggs determined after the cruise indicated that 31 A. japonica eggs had been collected. The remaining eggs were of mesopelagic eel species (Serrivomeridae and Derichthyidae), or unidentified species. The morphological characteristics of the A. japonica eggs were consistent with those of artificially spawned eggs, except they had a slightly larger diameter. The egg diameter range did not overlap with those of mesopelagic eels of the Serrivomeridae, which often spawn in the same area as A. japonica. These results suggest that egg diameter and embryo shape can be used to morphologically identify naturally spawned A. japonica eggs.