Noritaka Mochioka

Oceanic spawning ecology of freshwater eels in the western North Pacific

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.

Discovery of mature freshwater eels in the open ocean

Keywords Anguilla First catch Freshwater eels Matured males Spawning areaAll freshwater eels of the genus Anguilla are catadromous,in that they perform long-distance migrations to offshorespawning area after years of growth in freshwater and near-shore habitats [1]. Finding younger and smaller lepto-cephali indicated the spawning area to be located in theSargasso Sea for European and American eels (Anguillaanguilla and A. rostrata, respectively) [2, 3] and in thewestern North Equatorial Current (NEC) area of the wes-tern North Pacific to the west of Mariana Islands forJapanese and giant mottled eels (A. japonica and A. mar-morata, respectively) [4–6]. Recent investigation couldfurther narrow down the area and timing for spawning ofthe Japanese eel (A. japonica), where a large number ofsmall pre-leptocephali (n = 130, 4.2–6.5 mm in totallength) were collected during the new moon in June 2005around 14 N, 142 E to the west of the Suruga Seamount ofthe West Mariana Ridge [7]. No matured adult eels havebeen captured or observed to date in the spawning area, notonly for the four species mentioned above, but also for anyspecies of freshwater eel. The Fisheries Agency of Japanconcerned recent declines in catches of glass eel (smalljuveniles migrating to freshwater area), and dispatched R/VKaiyo Maru to capture adult Japanese eel for collectingbiological information. Here, we report the first discoveryof fully matured anguillid eels in the open ocean.We used a large mid-water trawl net (maximum mouthopening 65 m wide and 55 m height with 7 mm mesh codend) (Nichimo Co., Ltd., Tokyo), which could be towed at4–5 knots. A total of 21 nighttime tows were performed for0.5–4 h at a depth mainly of 170–300 m in the area adja-cent to the southern part of the West Mariana Ridge(Fig. 1a) during 25 May to 9 June 2008 (Fig. 1, closedcircles). Eventually, three anguillid eels were caught in twotows on 3 and 4 June operated at 13 000 Nto13 05

Seamounts, new moon and eel spawning: The search for the spawning site of the Japanese eel

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, 2000–3000 km 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.

Seasonal changes of the RNA/DNA ratio, size and lipid contents and immigration adaptability of Japanese glass-eels, Anguilla japonica, collected in northern Kyushu, Japan

RNA/DNA ratios and the condition factor, as determined by weight and length of glass-eels, Anguilla japonica, were evaluated for assessment of nutritional status. The negative correlation of RNA/DNA ratios and condition factor between capture months corresponded to an increase in DNA content and to a decrease in dry weight, respectively. These findings indicated that the body material decreased in the later caught glass-eels, but, in contrast, DNA content and ossification increased. These phenomena were explained by such factors as food density and metabolic energy requirements. We concluded that the development of the glass-eel with poor feeding is influenced by differences in ambient water temperature, and that expended endogenous nutrition originates in the leptocephali.

Factors influencing otolith strontium/calcium ratios in Anguilla japonica elvers

The effects of temperature and salinity on the concentration ratios of strontium (Sr) to calcium (Ca) within the sagittal otoliths of elvers of the Japanese eel, Anguilla japonica, were studied by spot analysis using a wavelength dispersive X-ray electron microprobe. A total of 340 elvers were used: 100 elvers were reared for 15 days under various salinity conditions (freshwater, one-third seawater, two-thirds seawater and pure seawater at 22 °C; 240 elvers were reared for 58 days under various water temperature conditions (12, 17, 22, 27 °C) in either freshwater or pure seawater. Otolith Sr/Ca ratios were found to be positively correlated with water salinity. On the other hand, the Sr/Ca ratios were not found to be significantly different among the various temperature groups. The above results strongly suggest that the physiological mechanism of incorporation of Sr and Ca within the otolith of an eurythermal fish, Japanese eel, does not change within this range of temperatures (12–27 °C).

Spawning sites of the Japanese eel in relation to oceanographic structure and the West Mariana Ridge

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.

Leptocephalus eel larvae will feed in aquaria

SynopsisPremetamorphosing larvae of Muraenesox cinereus (Muraenesocidae) and Conger myriaster (Congridae), caught and kept alive in aquaria, repeatedly bit pieces out of a lump of squid paste. By coloring the paste with Brilliant Red, ingestion and defecation were clearly seen through the transparent gut. The paste is not their natural food, but nevertheless a possible food item for rearing eels from eggs; most previous efforts have failed inasmuch as the larvae have starved.

A pre-leptocephalus larva of Conger myriaster (family Congridae) collected from Suruga bay, Central Japan

One specimen of leptocephalus larva, measuring 16.0 mm in total length, was collected from the northern part of Suruga Bay (34°56′N, 138°39′E) by the T/V Bosei Maru II of Tokai University on November 7, 1982. This larva is identified asConger myriaster because of having melanophores under the eye, simple gut with melanophores, 142 total number of myomeres and the position of the last vertical blood vessel at the 52nd myomere. Judging from the body length, appearances of teeth and fins, and disposition of the anus, the present larva belongs to the preleptocephalus stage. The presence of this early larva in this area suggests that one of the spawning grounds ofC. myriaster exists in or near Suruga Bay in autumn.

Determination of the freshwater mark in otoliths of Japanese eel elvers using microstructure and Sr/Ca ratios

The microstructure, in particular checks within the otolith edge, of Anguilla japonica glass-eels and elvers and changes in otolith Sr/Ca ratios were examined to ascertain the environmental history of the eels, especially with regard to the time when glass-eels entered the river, and as a benchmark for count daily increments. The percentage of glass eels and elvers with checks and the mean number of checks within the otoliths of glass-eels caught at four localities, Tosa Bay off Tosa City, the mouth of the Gokase River, the mouth of the Saigo River and the dam of the Tsuri River were 0% (0), 15.0% (0.2), 51.6% (1.0) and 100.0% (4.2), respectively. The Sr/Ca ratios and Sr content peaked in the region where checks were formed and the values decreased rapidly towards the edge of the checks; on the other hand, these decreased gradually in the otolith when checks were not formed. These checks were estimated to be formed by stress when the glass-eels were affected by ambient fresh water within the river. The innermost check was called the ‘freshwater mark’ in the present study and this mark may be useful as a benchmark in studying the growth history of the eel before and after entering freshwater.

Light-sensitive vertical migration of the Japanese eel Anguilla japonica revealed by real-time tracking and its utilization for geolocation

Short-time tracking (one to eight days) of the Japanese eel (Anguilla japonica) using ultrasonic transmitter was performed in the tropical-subtropical area adjacent to the spawning area and temperate area off the Japanese Archipelago. Of 16 eels (11 wild and five farmed) used, 10 wild eels displayed clear diel vertical migration (DVM) from the beginning, while the other five farmed eels tracked for 19 to 66 hours did not. During daytime, a significantly positive correlation between migration depth and light intensity recorded on the vessel was observed in the 10 wild eels, indicating that the eels were sensitive to sunlight even at the middle to lower mesopelagic zone (500 to 800 m). During nighttime, the eel migration depth was observed to be associated with the phase, rising and setting of the moon, indicating that the eels were sensitive to moonlight at the upper mesopelagic zone (<300 m). Two of 10 wild eels were in the yellow stage but shared similar DVM with the silver stage eels. Swimbladders of three silver stage eels were punctured before releasing, but very little effect on DVM was observed. The eels very punctually initiated descent upon nautical dawn and ascent upon sunset, enabling us to determine local times for sunrise and sunset, and hence this behavior may be used for geolocating eels. In fact, estimated positions of eels based on the depth trajectory data were comparable or even better than those obtained by light-based archival tag in other fish species.