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Okazaki, Japan

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.

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.

Fukuba T.,Japan Agency for Marine - Earth Science and Technology | Miwa T.,Japan Agency for Marine - Earth Science and Technology | Watanabe S.,Nihon University | Mochioka N.,Kyushu University | And 9 more authors.
Fisheries Science | Year: 2015

Spawning-condition Japanese eels Anguilla japonica, fertilized eggs, and newly-hatched preleptocephali have been captured, and studies for observing spawning eels with underwater camera systems have begun. This study describes a new, less invasive, free-drifting underwater camera observation system that was deployed from the research vessel (R/V) Natsushima in June 2013. Three drifting buoy camera systems (Una-Cam) with lights-on/lights-off programmed sequencing during daytime and nighttime hours were deployed over a period of seven days at 20 locations south of a salinity front along the southern West Mariana Ridge. Live artificially matured A. japonica eels held in transparent chambers were used as an attractant source through the release of reproductive pheromones and other odors. Each system was suspended from a buoy array at a depth of 174–200 m, with four cameras and three lights pointed downward at different angles towards the eel chamber. The Una-Cam systems were stable and were effective at recording images of fish, crustaceans, and gelatinous zooplankton. Olfactory cues may have attracted male and female Derichthys serpentinus eels, which showed what seemed to be reproductive behavior and attraction to the Japanese eels in the chamber. Una-Cam systems are capable of recording images of anguillid eels, if they approach, and may be useful for observing spawning eels in their offshore spawning areas. © 2014, Japanese Society of Fisheries Science.

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.

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