Tahara, Japan
Tahara, Japan

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Kuroki M.,University of Tokyo | Okamura A.,IRAGO Institute Co. | Takeuchi A.,Nihon University | Tsukamoto K.,IRAGO Institute Co. | Tsukamoto K.,Nihon University
Fisheries Science | Year: 2016

Research efforts to achieve the production of artificial seedlings of the Japanese eel Anguilla japonica have progressed in recent decades. However, morphological deformities have been frequently observed in reared leptocephali and glass eels. We examined the effect of water current velocities (5.7–8.3 cm/s) on the body size and morphology of reared leptocephali and metamorphosed glass eels. As the current velocity increased, the size of leptocephali became smaller and the occurrence rate of notochord curvature increased. However, even in low velocities, water current had a long-term negative impact on their morphology. Sixty-five percent of metamorphosed glass eels had one of the eight types of vertebral deformities: compression, luxation, fusion, brachyspina, modification, lordosis, kyphosis, or scoliosis. Although their occurrence rate was unrelated to current velocity, there was a tendency for some deformities to be localised in a certain area of the vertebral column. In particular, compression frequently occurred in caudal vertebrae in faster currents. Most vertebral deformities began before the completion of metamorphosis. Therefore, appropriate management of the water current during the leptocephalus stage is important for establishing mass production of morphologically normal glass eels. © 2016 Japanese Society of Fisheries Science


Iida M.,University of Tokyo | Watanabe S.,University of Tokyo | Yamada Y.,IRAGO Institute Co. | Lord C.,French Natural History Museum | And 2 more authors.
Journal of Experimental Marine Biology and Ecology | Year: 2010

To understand the ecology and environmental tolerances of newly hatched larvae of the amphidromous fish Sicyopterus japonicus during their downstream migration, the salinity tolerance of eggs, 0-15 day old larvae, and adults, and the temperature tolerance, specific gravity and phototaxis of hatched larvae were examined. Tolerances of adults were measured as survival after a 24 h challenge in freshwater (FW), brackish water (1/3 SW) and seawater (SW). The survival rate of adult S. japonicus was 100% in FW and 1/3 SW, while none survived in SW. Hatching success of eggs (30 eggs each) was significantly higher in FW (mean: 73%) and 1/3 SW (73%) than in SW (19%). Tolerance of newly hatched larvae to salinity and temperature was investigated in different combinations of salinities (FW, 1/3 SW and SW) and temperatures (18, 23 and 28 °C). Larval survival was significantly different in each salinity and temperature. Survival rate was significantly higher in 1/3 SW than in FW and higher in SW than in FW at 23 °C and 28 °C. At the latter part of the experiment, there was no survival in FW and at 28 °C. Survival was higher in lower temperatures, but larval development did not occur in FW. Specific gravity of newly hatched larvae was 1.036 at 28 °C and 1.034 at 23 °C. When exposed to a light source on one side of an aquarium, larval distribution was not affected. Our results indicated larval S. japonicus are more adapted to brackish water and seawater than freshwater, while the adults and eggs are more adapted to freshwater and brackish water than seawater. This is consistent with their amphidromous life history with growth and spawning occurring in freshwater and the larval stage utilizing marine habitats. © 2009 Elsevier B.V. All rights reserved.


Okamura A.,IRAGO Institute Co. | Yamada Y.,IRAGO Institute Co. | Mikawa N.,IRAGO Institute Co. | Horie N.,IRAGO Institute Co. | And 2 more authors.
Aquaculture International | Year: 2015

One of the difficulties to rear Anguilla japonica larvae is the frequent occurrence of notochord deformities. We tested the effect of salinity on the occurrence of the deformities, because we have been using 50 % diluted seawater (50 % SW) for glass eel production, on the basis of the fact that intermediate salinity saves energy due to lower cost for osmoregulation and contributes higher survival and growth rates. We reared 6-day-old larvae in 50 and 100 % SW for 85 days and observed their morphology. The occurrence rate of deformed larvae, including kyphosis and scoliosis, was significantly higher in 50 % SW (35.8 %) than in 100 % SW (25.4 %), while survival rate was significantly higher in 50 % SW (69.8 %) than in 100 % SW (32.3 %) and growth in 50 % SW (mean body depth: 7.9 ± 5.3 mm) was better than in 100 % SW (6.8 ± 4.6 mm). We speculate that the most of severely deformed larvae could not survive in the tougher condition of 100 % SW, showing the lower occurrence of deformed larvae. Eventually, the yield of normal larvae after 85 days rearing was 1.9-fold higher in 50 % SW than in 100 % SW, implying that the advantage of 50 % SW for rearing eel larvae outweighs the risk of deformities. © 2015 Springer International Publishing Switzerland


Utoh T.,IRAGO Institute Co. | Horie N.,IRAGO Institute Co. | Okamura A.,IRAGO Institute Co. | Mikawa N.,IRAGO Institute Co. | And 5 more authors.
Aquaculture | Year: 2013

To develop seed production techniques for aquaculture of the common Japanese conger, Conger myriaster, we attempted to induce ovarian maturation and ovulation without exogenous hormone treatment and examined the changes in steroid hormones to understand the endocrine condition of the fish during their maturation process. Juveniles caught in November 2001 (20 to 30. cm in total length) were reared for 1.5. years in seawater at temperatures ranging from 10 to 20. °C before the experiments. Then, 141 randomly selected female eels (58 to 82. cm in total length) were maintained at a water temperature of 6. °C from April 2003 to March 2004. The initial GSI values (5.0 to 12.0) and oocyte diameters (300 to 400. μm) recorded in April 2003 had increased to maximum values of 30.0 and 650. μm, respectively, at the end of experiment, in March 2004. In late December 2003, 12 fish with oocytes measuring over 600. μm in diameter were selected and divided into an elevated temperature group from 6 to 10. °C and a control group of 6. °C for 43. days. The mean oocyte diameter in the elevated temperature group increased significantly, and one fish (estimated GSI, 47.0) ovulated eggs with an average diameter of 995 ± 46. μm on the 41st day at a water temperature of 10. °C, in contrast, the control group showed no significant changes in oocyte diameter, and no fish ovulated. Plasma T levels, examined via ELISA, increased with oocyte maturation, with the highest level detected at the migratory nucleus stage, and declined after ovulation. In contrast, plasma E2 was maintained at high levels during the vitellogenic stage, exhibiting the highest value after ovulation. The present study demonstrates that ovarian maturation and ovulation in the common Japanese conger could be achieved through water temperature manipulation. © 2013 Elsevier B.V.


Okamura A.,IRAGO Institute Co. | Yamada Y.,IRAGO Institute Co. | Mikawa N.,IRAGO Institute Co. | Horie N.,IRAGO Institute Co. | And 3 more authors.
Aquaculture | Year: 2011

Technologies for rearing Japanese eel Anguilla japonica larvae (leptocephali) have improved markedly over the past decades. Notochord deformities, however, have been observed frequently in reared leptocephali. To learn more about these deformities, we studied notochord alterations in the leptocephalus stage of Japanese eels in captivity. Of 277 A. japonica leptocephali examined 170 to 307. days post hatching, 140 (50.5%) had kyphosis, a dorsal curvature of the notochord column; 4 (1.4%) had kyphosis-lordosis, a complication of the dorsal and ventral curvatures; and 21 (7.6%) had scoliosis, a lateral zig-zag curvature. Kyphotic leptocephali had various degrees of body curvature and often showed abnormal swimming behavior, continually whirling in the water column. The frequency of this whirling motion increased with increasing degree of body curvature. Even in the absence of whirling, the bodies of all reared larvae showed significantly greater curvature when compared with wild caught leptocephali, indicating that all reared larvae were potentially abnormal. Unlike leptocephali with scoliosis, those with kyphosis and kyphosis-lordosis often metamorphosed into glass eels, but retained their deformities, which would affect subsequent feeding behavior. Possible causes of these deformities, including nutritional imbalance, salinity, water temperature, and water current, are discussed. © 2011 Elsevier B.V.


Okamura A.,IRAGO Institute Co | Yamada Y.,IRAGO Institute Co | Mikawa N.,IRAGO Institute Co | Horie N.,IRAGO Institute Co | Tsukamoto K.,University of Tokyo
Canadian Journal of Zoology | Year: 2012

We assessed the effects of starvation, body size, and water temperature on the onset of metamorphosis in leptocephali of Japanese eel (Anguilla japonica Temminck and Schlegel, 1846) as determined by the morphological criteria of proportion, preanal length, and body depth to total length. Leptocephali of mean total length 55.6 mm that had been reared in captivity for 241 days from hatching were divided into unfed (n = 28) and fed (n = 30) groups in triplicate and reared for an additional 2 weeks. The mean percentage of larvae starting metamorphosis within 2 weeks was significantly higher in the unfed than in the fed groups (70% vs. 28.6%), suggesting that food deprivation acted as a cue for metamorphosis. The critical size for metamorphosis was a total length of 50-55 mm; smaller larvae did not start metamorphosis even in the absence of food, whereas larvae reaching that critical size were induced to undergo metamorphosis by starvation. The start of metamorphosis under unfed conditions was independent of diel-varying water temperature (day 23 °C; night 21-29 °C), suggesting a high plasticity in response to a wide range of environmental temperatures. These findings suggest methods for the efficient production of glass eels, as well as new insights into the mechanism of eel metamorphosis.


Ahn H.,University of Tokyo | Ahn H.,Kinki University | Lee K.M.,University of Tokyo | Inokuchi M.,University of Tokyo | And 6 more authors.
Fisheries Science | Year: 2015

The onset of osmoregulation for seawater adaptation was examined during the early life stages of the Japanese eel. Ingested seawater was detected in the digestive tract by using fluorescent dextran as an inert marker. Ingested seawater remained in the forepart of the digestive tract at 0 and 1 days post hatching (dph), but reached the anus at 2 dph. Scanning electron microscope observations showed that the mouth appeared as a slit at 0 dph and developed into a hole-like shape at 2 dph. Expressions of Na+, K+, and 2Cl− cotransporter 2β (NKCC2β), and Na+ and Cl− cotransporter β (NCCβ) mRNAs were detected mainly in the intestine and rectum, respectively. These results are consistent with those of the adult eel, suggesting that the intestine and rectum are the sites of active ion absorption in larvae also. Expression levels of NKCC2β steeply increased from 4 to 6 dph, while NCCβ levels were highest on the day of hatching, presumably due to a maternal factor. The expression levels of NCCβ decreased by 3–4 dph and then increased to a constant status at 7 dph. Our findings confirmed that osmoregulation started as early as the day of hatching and stabilized within a week. © 2014, Japanese Society of Fisheries Science.


Ahn H.,University of Tokyo | Yamada Y.,IRAGO Institute Co. | Okamura A.,IRAGO Institute Co. | Horie N.,IRAGO Institute Co. | And 4 more authors.
Aquaculture | Year: 2012

To obtain information about how water temperature affects the embryonic development of Japanese eel (Anguilla japonica) eggs, the developmental rates and hatching times of three lots of eggs were quantified at 16, 19, 22, 25, 28 and 31°C. Hatching time was shortened at higher temperatures with eggs at 28°C hatching within 23-28. h after fertilization, while eggs at 19°C took 48-54. h for their hatching. At 22 and 25°C, eggs hatched out within 35-43. h and 27-32. h, respectively. There was no successful hatching at 16 or 31°C. The developmental stages until hatching were divided into morula, blastula, gastrula, eye vesicle and ear vesicle formation and heart formation. Development of the embryos were also faster at higher temperature especially after the gastrula stage, but the morphological features of each stage were the same at all water temperatures. The final developmental stage before the eggs died at 16 and 31°C differed among the lots. Eggs maintained at 25°C showed a high hatching rate (mean: 69%), compared to other water temperatures (mean: 12% at 19°C, 32% at 22°C, 4% at 28°C). The results from present study indicate that the optimal temperature for egg incubation is around 25°C, similar to the likely temperatures experienced by eggs and early larvae in the spawning area. © 2011 Elsevier B.V.


Miller M.J.,University of Tokyo | Chikaraishi Y.,Japan Agency for Marine - Earth Science and Technology | Ogawa N.O.,Japan Agency for Marine - Earth Science and Technology | Yamada Y.,IRAGO Institute Co. | And 2 more authors.
Biology Letters | Year: 2013

What eel larvae feed on in the surface layer of the ocean has remained mysterious. Gut contents and bulk nitrogen stable isotope studies suggested that these unusual larvae, called leptocephali, feed at a low level in the oceanic food web, whereas other types of evidence have suggested that small zooplankton are eaten. In this study, we determined the nitrogen isotopic composition of amino acids of both natural larvae and laboratory-reared larvae of the Japanese eel to estimate the trophic position (TP) of leptocephali. We observed a mean TP of 2.4 for natural leptocephali, which is consistent with feeding on particulate organic matter (POM) such as marine snow and discarded appendicularian houses containing bacteria, protozoans and other biological materials. The nitrogen isotope enrichment values of the reared larvae confirm that the primary food source of natural larvae is consistent only with POM. This shows that leptocephali feed on readily available particulate material originating from various sources closely linked to ocean primary production and that leptocephali are a previously unrecognized part of oceanic POM cycling. © 2012 The Author(s) Published by the Royal Society. All rights reserved.


Okamura A.,IRAGO Institute Co. | Horie N.,IRAGO Institute Co. | Mikawa N.,IRAGO Institute Co. | Yamada Y.,IRAGO Institute Co. | And 2 more authors.
Ecology of Freshwater Fish | Year: 2014

Remarkable progress in techniques for maturation of adults and rearing of larvae has been achieved for the Japanese eel Anguilla japonica over the past 50 years, but recent efforts have not yet succeeded in the mass production of glass eels. This article reviews recent advances in techniques for artificial production of A. japonica glass eels. Successful new protocols for obtaining viable eggs and larvae of A. japonica are largely resulting from using artificially feminised eels instead of wild silver eels. The feminisation technique by administration of 17β-estradiol to glass eels not only provides an opportunity for using females as experimental broodstock throughout the year, but also accelerates their oocyte development or even increases the competence of females to respond to salmon pituitary extract. Induced spontaneous spawning with paired eels can improve the success rate for obtaining good-quality eggs compared with the traditional stripping-insemination method. As a result of the accumulation of field data on natural environmental conditions, such as an optimum temperature, where eel eggs and larvae were collected, the survival and growth rate of captive eel leptocephali have been much improved under these conditions. However, 50% diluted sea water (17.5 psu) that is far from the natural condition can result in better growth and survival performance in A. japonica leptocephali, possibly because it reduces the energy necessary for osmoregulation. Starvation can be a cue for triggering the onset of metamorphosis in A. japonica leptocephali, reducing the prolonged duration of the larval stage in captivity. To reach the mass production of glass eels at a commercial level, further improvement in quality of an artificial larval diet not containing eggs of the endangered shark Squalus acanthias is needed. The control of deformities in captive leptocephali and glass eels is also an important task. New techniques reducing the excessive use of exogenous hormones with parent eels should also be developed, to avoid public perception of an unhealthy product. If these techniques are completely established, we can exchange artificially produced glass eels with some part of the wild glass eels that are heavily exploited and provided to the eel aquaculture industry, and this can help to reduce the impact of eel consumption, which will contribute to the conservation of eel species worldwide. © 2013 John Wiley & Sons A/S.

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