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Anguillid eels are found globally in fresh, transitional and saline waters and have played an important role in human life for centuries. The population status of several species is now of significant concern. The threats to populations include direct exploitation at different life stages, blockages to migratory routes by dams and other structures, changes in river basin management that impact habitat carrying capacity and suitability, pollution, climate change, diseases and parasites. While much has been done to understand eel biology and ecology, a major challenge is to identify the key research and management questions so that effective and targeted studies can be designed to inform conservation, management and policy. We gathered 30 experts in the field of eel biology and management to review the current state of knowledge for anguillid eel species and to identify the main topics for research. The identified research topics fell into three themes: (a) Lifecycle and Biology; (b) Impacts and (c) Management. Although tropical anguillid eels are by far the least well understood, significant knowledge gaps exist for all species. Considerable progress has been made in the last 20 years, but the status of many species remains of great concern, particularly for northern temperate species. Without improved engagement and coordination at the regional, national and international level, the situation is unlikely to improve. Further, adaptive management mechanisms to respond to developments in science, policy and our knowledge of potential threats are required to ensure the future of these important and enigmatic species.

The ecology of leptocephali remains poorly known but they appear to feed on marine snow that can vary spatially and temporally according to the food web dynamics. This study provided new information about the position of leptocephali within the functional structure of microbial plankton and other food web components of the western South Pacific (WSP) region at a large geographic scale including the New Caledonia, Fiji, and Samoa islands. The hydrographic structure varied with latitude, and nutrient levels were generally low but somewhat variable. Stable isotopic signatures were examined in relation to the 3 current zones of the eastward flowing South Equatorial Countercurrent (SECC; north), the westward South Equatorial Current (SEC; mid-latitudes), and the eastward South Tropical Countercurrent (STCC; south), and all zones were found to be primarily based on a heterotrophic planktonic functioning that were co-limited by inorganic nitrogen and phosphorus, with biomasses of planktonic groups varying with depth. Isotopic signatures of leptocephali were compared to the signatures of other mesozoplankton, micronekton, and Trichodesmium components of the food web, and in relation to the signatures of particulate organic matter (POM) that varied among the 3 collection depths. The isotopic signatures of six taxa of leptocephali, other taxonomic groups and POM showed interesting variability according to latitude and among some stations. The presence of Trichodesmium at the surface in the STCC zone influenced the isotopic signatures of POM and thus the signatures of leptocephali. The signatures of leptocephali were therefore linked with the overall food web and were consistent with the larvae feeding on marine snow components of POM. The two apparent groups of leptocephali with different isotopic signatures that have also been observed in other oceanic areas may be explained by feeding behavior at different depths or on different types of marine snow.

Leptocephali of the shallow-water congrid eel Ariosoma balearicum are abundant during February–April in the Sargasso Sea, and larval and adult meristic data indicates this species includes several regional subpopulations/cryptic species. Four multiple-transect larval surveys (2011, 2014, 2015, 2017) were used to examine the geographic size distribution of two myomere-count types of A. balearicum leptocephali. High-count (HC) larvae were consistently mostly between 80 and 100 mm in size (60–132 mm; 87.9 ± 6.8 mm) as observed previously, and frequently had narrow size ranges. The usually larger LC larvae (78–176 mm; 111.4 ± 26.7 mm) were more abundant in western or central areas. HC larvae tended to decrease in size from west to east and increase from south to north. Catch rates were geographically variable relative to hydrographic structure/frontal positions across the wide 2015 sampling area. Mitochondrial 16 s rRNA sequences of HC and LC larvae show species-level differences, providing evidence of the existence of two cryptic species with different larval dispersal strategies in the Sargasso Sea subtropical gyre region. The HC larvae disperse widely into the gyre, seemingly through Gulf Stream recirculation or eastward frontal-jet flows, and apparently must use directional swimming to cross the Florida Current to recruit into the South Atlantic Bight. LC leptocephali may mostly be retained near the Bahamas, with few larvae dispersing into the gyre. This seems to indicate natural selection occurred for spawning location and larval behavior due to the powerful Florida Current/Gulf Stream, resulting in two completely different spawning and larval dispersal strategies within a local geographic region.

Leptocephali of the shallow-water congrid eel Ariosoma balearicum are abundant during February–April in the Sargasso Sea, and larval and adult meristic data indicates this species includes several regional subpopulations/cryptic species. Four multiple-transect larval surveys (2011, 2014, 2015, 2017) were used to examine the geographic size distribution of two myomere-count types of A. balearicum leptocephali. High-count (HC) larvae were consistently mostly between 80 and 100 mm in size (60–132 mm; 87.9 ± 6.8 mm) as observed previously, and frequently had narrow size ranges. The usually larger LC larvae (78–176 mm; 111.4 ± 26.7 mm) were more abundant in western or central areas. HC larvae tended to decrease in size from west to east and increase from south to north. Catch rates were geographically variable relative to hydrographic structure/frontal positions across the wide 2015 sampling area. Mitochondrial 16 s rRNA sequences of HC and LC larvae show species-level differences, providing evidence of the existence of two cryptic species with different larval dispersal strategies in the Sargasso Sea subtropical gyre region. The HC larvae disperse widely into the gyre, seemingly through Gulf Stream recirculation or eastward frontal-jet flows, and apparently must use directional swimming to cross the Florida Current to recruit into the South Atlantic Bight. LC leptocephali may mostly be retained near the Bahamas, with few larvae dispersing into the gyre. This seems to indicate natural selection occurred for spawning location and larval behavior due to the powerful Florida Current/Gulf Stream, resulting in two completely different spawning and larval dispersal strategies within a local geographic region.

Leptocephali of the shallow-water congrid eel Ariosoma balearicum are abundant during February–April in the Sargasso Sea, and larval and adult meristic data indicates this species includes several regional subpopulations/cryptic species. Four multiple-transect larval surveys (2011, 2014, 2015, 2017) were used to examine the geographic size distribution of two myomere-count types of A. balearicum leptocephali. High-count (HC) larvae were consistently mostly between 80 and 100 mm in size (60–132 mm; 87.9 ± 6.8 mm) as observed previously, and frequently had narrow size ranges. The usually larger LC larvae (78–176 mm; 111.4 ± 26.7 mm) were more abundant in western or central areas. HC larvae tended to decrease in size from west to east and increase from south to north. Catch rates were geographically variable relative to hydrographic structure/frontal positions across the wide 2015 sampling area. Mitochondrial 16 s rRNA sequences of HC and LC larvae show species-level differences, providing evidence of the existence of two cryptic species with different larval dispersal strategies in the Sargasso Sea subtropical gyre region. The HC larvae disperse widely into the gyre, seemingly through Gulf Stream recirculation or eastward frontal-jet flows, and apparently must use directional swimming to cross the Florida Current to recruit into the South Atlantic Bight. LC leptocephali may mostly be retained near the Bahamas, with few larvae dispersing into the gyre. This seems to indicate natural selection occurred for spawning location and larval behavior due to the powerful Florida Current/Gulf Stream, resulting in two completely different spawning and larval dispersal strategies within a local geographic region.

It has been known for about a century that European eels have a unique life history that includes offshore spawning in the Sargasso Sea about 5000–7000 km away from their juvenile and adult habitats in Europe and northern Africa. Recently hatched eel larvae were historically collected during Danish, German and American surveys in specific areas in the southern Sargasso Sea. During a 31 day period of March and April 2014, Danish and German research ships sampled for European eel larvae along 15 alternating transects of stations across the Sargasso Sea. The collection of recently hatched eel larvae (≤12 mm) from 70° W and eastward to 50° W showed that the European eel had been spawning across a 2000 km wide region of the North Atlantic Ocean. Historical collections made from 1921 to 2007 showed that small larvae had also previously been collected in this wide longitudinal zone, showing that the spatial extent of spawning has not diminished in recent decades, irrespective of the dramatic decline in recruitment. The use of such a wide spawning area may be related to variations in the onset of the silver eel spawning migration, individual differences in their long-term swimming ability, or aspects of larval drift.

Catadromous anguillid eels (genus Anguilla) migrate from their freshwater or estuarine habitats to marine spawning areas. Evidence from satellite tagging studies indicates that tropical and temperate eel species exhibit pronounced diel vertical migrations, from between 150-300 m nighttime depths to 600-800 m during the day. Collections of eggs and larvae of Japanese eels A. japonica suggest they may spawn at these upper nighttime migration depths. How anguillid eels navigate through the ocean and find their spawning areas remains unknown; thus, this study describes the salinity, temperature and geostrophic currents between 0 and 800 m depths within 2 confirmed and 3 hypothetical anguillid spawning areas during likely spawning seasons. Within the 4 ocean gyres in which these spawning areas are located, many eels would encounter subducted 'Subtropical Underwater' water masses during their nighttime ascents that could provide odor plumes as signposts. Four of the spawning areas are located near the western margins of where subducted water masses form cores of elevated salinities (∼35.0 to 36.8) around 150 m depths, and one is located near the center of subduction. Low salinity surface waters and fronts are present in some of the areas above the high-salinity cores. Spawning may occur at temperatures between 16 and 24°C where the thermocline locally deepens. At spawning depths, weak westward currents (∼0 to 0.1 m s-1) prevail, and eastward surface countercurrents are present. Anguillid eels possess acute sensory capabilities to detect these hydrographic features as potential signposts, guiding them to their spawning areas.

To describe the histological characteristics of the oocyte chorion in wild adult and artificially matured Japanese eels, we investigated changes in chorion thickness during artificially induced oogenesis and compared the chorion thickness and ultrastructure between wild and artificially matured eels. In artificially maturing eels, the chorion thickness and volume increased significantly with increasing follicle diameter, peaking at approximately 450 A mu m; beyond this point, the chorion thinned significantly, whereas there were no significant changes in volume. A significant positive correlation was observed between the number of salmon pituitary extract (SPE) injections and chorion thickness. In wild post-spawning adult eels, chorion thickness varied among individuals, and two had chorions that were significantly thinner than those of artificially matured eels. Ultrastructural examination revealed electron-dense layers in the chorions of wild post-spawning adult eels, as was seen in artificially matured eels. This result is inconsistent with our hypothesis that the formation of an electron-dense layer is unique to artificially maturing eels due to repeated SPE injections. These results suggest that the formation cycle of the chorion might be affected by SPE injections in artificially maturing eels, whereas that of wild eels might be synchronized with behavioral and/or environmental fluctuations that occur during the oceanic spawning migration.

To describe the histological characteristics of the oocyte chorion in wild adult and artificially matured Japanese eels, we investigated changes in chorion thickness during artificially induced oogenesis and compared the chorion thickness and ultrastructure between wild and artificially matured eels. In artificially maturing eels, the chorion thickness and volume increased significantly with increasing follicle diameter, peaking at approximately 450 A mu m; beyond this point, the chorion thinned significantly, whereas there were no significant changes in volume. A significant positive correlation was observed between the number of salmon pituitary extract (SPE) injections and chorion thickness. In wild post-spawning adult eels, chorion thickness varied among individuals, and two had chorions that were significantly thinner than those of artificially matured eels. Ultrastructural examination revealed electron-dense layers in the chorions of wild post-spawning adult eels, as was seen in artificially matured eels. This result is inconsistent with our hypothesis that the formation of an electron-dense layer is unique to artificially maturing eels due to repeated SPE injections. These results suggest that the formation cycle of the chorion might be affected by SPE injections in artificially maturing eels, whereas that of wild eels might be synchronized with behavioral and/or environmental fluctuations that occur during the oceanic spawning migration.

To describe the histological characteristics of the oocyte chorion in wild adult and artificially matured Japanese eels, we investigated changes in chorion thickness during artificially induced oogenesis and compared the chorion thickness and ultrastructure between wild and artificially matured eels. In artificially maturing eels, the chorion thickness and volume increased significantly with increasing follicle diameter, peaking at approximately 450 A mu m; beyond this point, the chorion thinned significantly, whereas there were no significant changes in volume. A significant positive correlation was observed between the number of salmon pituitary extract (SPE) injections and chorion thickness. In wild post-spawning adult eels, chorion thickness varied among individuals, and two had chorions that were significantly thinner than those of artificially matured eels. Ultrastructural examination revealed electron-dense layers in the chorions of wild post-spawning adult eels, as was seen in artificially matured eels. This result is inconsistent with our hypothesis that the formation of an electron-dense layer is unique to artificially maturing eels due to repeated SPE injections. These results suggest that the formation cycle of the chorion might be affected by SPE injections in artificially maturing eels, whereas that of wild eels might be synchronized with behavioral and/or environmental fluctuations that occur during the oceanic spawning migration.

The Japanese eel, Anguilla japonica, is one of the most important species of the aquaculture industry of East Asia. Supply of glass eels for aquaculture is completely dependent on wild catch. However, glass-eel catches in Japan declined linearly from over 200 tons in the early 1960s to 20 tons at present. In recent years an unstable situation has occurred with glass-eel catches not meeting the demand of aquaculture farms. In order to address this rapid decline, a challenging research project for artificial production of glass eels was commenced in the late 1960s. Since then, through a continuing process of trial and error, the production of second-generation larvae was finally achieved in 2010. Throughout the research, a new application of several substances has caused breakthroughs in artificial induction of sexual maturation and larvae rearing. This article reports on a study of controlled eel reproduction focusing on the finding of substances that accelerated progress. In addition, guideline research on eel reproduction in the wild was conducted aiming at overcoming current bottlenecks that impeded the establishment of largescale glass-eel production in captivity, by investigating maturing eels and eggs collected in their spawning area in the vicinity of the West Mariana Ridge.

The Japanese eel, Anguilla japonica, is one of the most important species of the aquaculture industry of East Asia. Supply of glass eels for aquaculture is completely dependent on wild catch. However, glass-eel catches in Japan declined linearly from over 200 tons in the early 1960s to 20 tons at present. In recent years an unstable situation has occurred with glass-eel catches not meeting the demand of aquaculture farms. In order to address this rapid decline, a challenging research project for artificial production of glass eels was commenced in the late 1960s. Since then, through a continuing process of trial and error, the production of second-generation larvae was finally achieved in 2010. Throughout the research, a new application of several substances has caused breakthroughs in artificial induction of sexual maturation and larvae rearing. This article reports on a study of controlled eel reproduction focusing on the finding of substances that accelerated progress. In addition, guideline research on eel reproduction in the wild was conducted aiming at overcoming current bottlenecks that impeded the establishment of largescale glass-eel production in captivity, by investigating maturing eels and eggs collected in their spawning area in the vicinity of the West Mariana Ridge.

Chapter I. Aquatic Environment. Ken FURUYA and Ichiro YASUDA : chapter_1.pdf, Chapter II. Biology and Ecology of Aqua-Shere. Toyoji KANEKO, Katsumi TSUKAMOTO, Atsushi TSUDA, Yuzuru SUZUKI and Katsufumi SATOH : chapter_2.pdf, Chapter III. Aquatic Resource and Production. Ichiro AOKI, Kazuo OGAWA, Taku YAMAKAWA and Tomoyoshi YOSHINAGA : chapter_3.pdf, Chapter IV. Chemistry of Aquatic Organism and Their Utilization. Hiroki ABE, Shugo WATABE, Yoshihiro OCHIAI, Shigeru OKADA, Naoko YOSHIKAWA, Yoshiharu KINOSHITA, Gen KANEKO and Shigeki MATSUNAGA : chapter_4.pdf, Chapter V. Relation between Aqua-Shere and Human Life. Hisashi KUROKURA, Hirohide MATSUSHIMA, Shingo KUROHAGI, Haruko YAMASHITA, Akinori HINO, Kazumasa IKUTA, Satoquo SEINO, Masahiko ARIJI, Ken FURUYA, Junichiro OKAMOTO and Nobuyuki YAGI : chapter_5.pdf, Part of "Introduction for Fisheries and Aquatic Biology"

The Japanese eel, Anguilla japonica, is one of the most important species of the aquaculture industry of East Asia. Supply of glass eels for aquaculture is completely dependent on wild catch. However, glass-eel catches in Japan declined linearly from over 200 tons in the early 1960s to 20 tons at present. In recent years an unstable situation has occurred with glass-eel catches not meeting the demand of aquaculture farms. In order to address this rapid decline, a challenging research project for artificial production of glass eels was commenced in the late 1960s. Since then, through a continuing process of trial and error, the production of second-generation larvae was finally achieved in 2010. Throughout the research, a new application of several substances has caused breakthroughs in artificial induction of sexual maturation and larvae rearing. This article reports on a study of controlled eel reproduction focusing on the finding of substances that accelerated progress. In addition, guideline research on eel reproduction in the wild was conducted aiming at overcoming current bottlenecks that impeded the establishment of largescale glass-eel production in captivity, by investigating maturing eels and eggs collected in their spawning area in the vicinity of the West Mariana Ridge.

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The effects of density, starvation and size difference on aggressive behaviour in juvenile yellowtails, Seriola quinqueradiata (Temminck and Schlegel), wee investigated. When fish were acclimated to densities of 1, 2, 4, 8, and 16 fish L-1, the frequency of aggressive behaviour per fish decreased significantly. Following starvation for 0, 4, 8, 12 and 24 h, the highest aggression was observed at 24 h, while levels of aggression were equal between 0 and 8 h. According to a pair match test from one batch among three groups of different size (total length mm: 16.6 ± 2.0, 21.1 ± 4.9), the highest aggression was observed in the group with the largest size difference. therefore, in view of the three parameters investigated, aggressive behaviour in the yellowtail was concluded to be positively starvation- and size-dependent, but negatively density-dependent. Synthesizing all relevant results, including abiotic factors such as the light condition, a practical method is presented to reduce mortality caused by aggressive behaviour in yellowtail cultures., Journal of Applied Ichthyology, 14(1-2), pp.9-13; 1998

Ontogenetic changes in whole-body immunoreactive cortisol concentrations (IRC) and aggressive behavior were examined in yellowtailSeriola quinqueradiata(Temminck & Schlegel; Carangidae). Baseline IRC significantly increased during the transition from larval to juvenile stage and was correlated with the onset of aggressive behavior. Handled fish (13.1 ± 2.6 ng/g tissue) showed an IRC level about three times higher than unhandled fish (4.7 ± 1.4 ng/g tissue), indicating that whole-body immunoreactive cortisol level may be an indicator of stress in juvenile yellowtails. Behaviorally subordinate fish (8.6 ± 1.6 ng/g tissue,n = 4) showed IRC levels significantly higher than dominant fish (0.6 ± 0.3 ng/g tissue, n = 4). Whole-body immunoreactive cortisol levels may thus reflect stress and social status in juvenile yellowtails, and the inverse relationship between social rank and IRC may result from agonistic interactions., General and Comparative Endocrinology, 109(2), pp.286-292; 1998

種子島沿岸での、1991年11月(広島大学豊潮丸)および12月(東京大学淡青丸)の採集により、計242尾のウナギ目レプトケファルス幼生と計5520尾の他目の仔稚魚が採集された。ニホンウナギの幼生は採集されなかった。採集されたレプトケファレス幼生の54.5%をギンアナゴ Gnathophis nystromi nystremi が占めた。ギンアナゴ幼生は種子島の西岸および南岸の水深100-10mの測点で、水深や昼夜を間わず採集された。ギンアナゴ幼生、および他目の仔稚魚の中で優先したネズミギス Gonorynchus abbreviatus・ソコイワシ Bathyltagus ochotensis・ハダカイワシ科 Myctophidae spp. は、いずれも通常は沖合い水域に出現する魚種である。当該海域の水温・塩分濃度は表層から底層まで極めて均一であり、しかも同じ時期の黒潮の水温・塩分濃度とよく一致した。以上より、種子島の西岸と南岸には黒潮水が直接流れ込むことが明らかとなった。沖合い性の仔稚魚は黒潮によって運ばれ、種子島沿岸に達するものと推察される。同時に行なわれた、種子島西海岸長浜なぎさにおける採集では多数のニホンウナギのシラスが採集された。, A total of 242 non-anguillid leptocephali was collected in the coastal water (20-60 m deep) around the Tanegashima Island, especially in the southern and western coasts. The dominant was Gnathophis nystromi nystromi (54.5%). The species was caught in any depth and at any time in the coastal water. Many oceanic larvae and juveniles of non-Auguilliformes fishes were also captured. Hydrographic characteristics including temperature and salinity of the coastal water showed that the water was extremely homogeneous horizontally and vertically, and was very similar with the Kuroshio water in quality. All the data obtained in the present study imply that the Kuroshio Current flows directly into the coastal water of the island and transports many oceanic larvae and juveniles of fish species there. Routes of the coastal migration of Anguilla japonica could not be clarified.

土佐湾及び四隅沖黒潮流域で、1990年1月にウナギ目魚類レプトケファルス幼生の採集を行った。採集は丸稚ネット(口径1.3m、網口面積1.33㎡、目合O.33mm)による傾斜曳あるいはステップ曳と高速稚魚ネット(網口面積2.03㎡、目合2.0mm)による表層曳で行われた。計41尾のレプトケファルス幼生が黒潮流域で採れた。その内訳はギンアナゴGnathophis nystromi nystromi(JORDAN et SNYDER)(33尾)、ニラミアナゴ属sp. 5 Ariesoma sp. 5(1尾)、ニラミアナゴ属sp. 7 Ariosoma sp. 7(2尾)、クロアナゴConger japonicus BLEEKER(1尾)、クロアナゴ亜科sp. 1 Congrinae sp. 1(2尾)、メクラアナゴDysomma anguillare BARNARD(2尾)であった。全個体の82.9%(34尾)が黒潮南外側域に分布していた。またギンアナゴは全個体が表層曳で、それ以外は全でステップ曳(ワイヤー長;500, 400, 300, 200, 100m)で採集された。, A total of 41 leptocephali was collected in the Kuroshio waters off Shikoku Island. They included 33 Gnathophis nystromi nystromi (JORDAN et SNYDER), one Ariosoma sp. 5, two Ariosoma sp. 7, one Conger japonicus BLEEKER, two Congrinae sp. 1, and two Dysomma anguillare BARNARD. Most of leptocephali collected (82.9% of a total catch) were occurred in the southern boundary of the Kuroshio Current. All the Gnathophis leptocephali were collected with the surface horizontal tow of High Speed Larva Net, and the other leptocephali were with step tow of Larva Net.