To establish a simple and reliable index for determining silvering stages of the Japanese eel, Anguilla japonica, we observed the colorations of various body parts and biological characteristics of the eels collected in a coastal area of Japan (Mikawa Bay). The four silvering stages are characterized by the colorations of pectoral fins and ventral skin as follows: (1) Y1, yellow eel without a metallic hue at the base of pectoral fins, (2) Y2, late yellow eel with a metallic hue at the base of the pectoral fins but without melanization at the tip of pectoral fins, (3) S1, silver eel with complete melanization at the tip of pectoral fins but without full pigmented belly in black or dark brown, and (4) S2, late silver eel with black or dark brown belly. The body size, eye diameter and sexual maturity of each stage increased in the order of Y1, Y2, S1 and S2 stages, whereas the digestive tract degenerated in the same order, suggesting a sequential development of these ontogenetic stages identified in the study. The Y1, Y2 and S1 stages could be also distinguished by canonical discriminant function analysis using three internal (gonad-somatic index, GSI; hepato-somatic index, HIS; and gut index) and two morphometric (condition factor and eye index) parameters, supporting the significance of these stages. This method of staging for the silvering process of the Japanese eel appeared to be applicable to all specimens of this species, since this index used only simple external characteristics that would be easy to observe during field surveys.
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 . 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 (4.2-8.7 mm) was limited to a narrow area around 14°N, 142°E just west of the southern part of the seamount chain, while leptocephali (7.7-62.0 mm) were widely distributed at increasing size westward in the North Equatorial Current (NEC) to the region east of Taiwan. Metamorphosing larvae (52.7-61.2 mm) were collected only in the area 21-26°N, 121-129°E to the east of Taiwan, while glass eels (51.3-61.2 mm) occurred only within or west of the Kuroshio. These distributions suggest that leptocephali begin to
To map quantitative trait loci (QTL) for growth and carcass traits in a purebred Japanese Black cattle population, we conducted multiple QTL analyses using 15 paternal half-sib families comprising 7860 offspring. We identified 40 QTL with significant linkages at false discovery rates of less than 0.1, which included 12 for intramuscular fat deposition called marbling and 12 for cold carcass weight or body weight. The QTL each explained 2%-13% of the phenotypic variance. These QTL included many replications and shared hypothetical identical-by-descent (IBD) alleles. The QTL for CW on BTA14 was replicated in five families with significant linkages and in two families with a 1% chromosome-wise significance level. The seven sires shared a 1.1-Mb superior Q haplotype as a hypothetical IBD allele that corresponds to the critical region previously refined by linkage disequilibrium mapping. The QTL for marbling on BTA4 was replicated in two families with significant linkages. The QTL for marbling on BTA6, 7, 9, 10, 20, and 21 and the QTL for body weight on BTA6 were replicated with 1% and/or 5% chromosome-wise significance levels. There were shared IBD Q or q haplotypes in the marbling QTL on BTA4, 6, and 10. The allele substitution effect of these haplotypes ranged from 0.7 to 1.2, and an additive effect between the marbling QTL on BTA6 and 10 was observed in the family examined. The abundant and replicated QTL information will enhance the opportunities for positional cloning of causative genes for the quantitative traits and efficient breeding using marker-assisted selection.
Temperate eels Anguilla anguilla (European eel), A. rostrata (American eel) and A. japonica (Japanese eel) are three catadromous species which have been declining since the 1970s/1980s despite their remarkable adaptive capacity. Because of their specific life cycles, which share distant oceanic spawning grounds and continental growth stage, eels are affected by five components of the global change: (a) climate change affecting larval survival and drift, (b) an increase in pollution leading to high levels of contamination exacerbated by their high lipid levels, (c) increasing fragmentation and habitat loss that reduce dramatically the amount of available habitats and induce increased spawner mortality, (d) the appearance of Anguillicola crassus a parasitic alien nematode that impairs spawning success, and (e) the impact of commercial and recreational fisheries for all life stages of eel. In this context, the rapid increases of pressures during the “Great Acceleration” have surpassed the adaptive capacity of eels. This illustrates that cumulative effects of global change can lead to the collapse of species, even in species that have amazingly high adaptive capacities.
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.
– Yellow‐phase Japanese eels (Anguilla japonica) were investigated in the Hamana Lake system, Japan, from 2003 to 2004 to understand how their demographic attributes vary within the lake system. A total of 779 yellow eels were collected during sampling in two inlet rivers and two brackish/saltwater lakes within the lake system. Female eels predominated, constituting 84% of the 75 sex‐determined eels in the river, and 50% of the 151 sex‐determined eels in the lakes. Total lengths (TL) of all eels examined ranged from 54.2 to 715.0 mm (mean = 320.4 ± 145.4 SD). In the inlet river, the TL of eels showed a significant positive relation with the distance from the river mouth. The estimated relative abundances of eels ranged from 0 to 1.8 eels·m−2 effort (mean: 0.3 ± 0.41) in the river and was negatively correlated with the distance from the river mouth. This suggested that larger eels might tend to be distributed at lower abundances in upstream reaches of the river. Mean age of yellow eels determined by their otolith annuli was younger in the lake (N = 117, 3.3 ± 1.4 years) than in the river (N = 214, 4.3 ± 1.7 years). Growth rate was higher in the lake than in the river at age 1–2 years (131.9 and 104.4 mm·year−1, respectively). The results of this study suggest that, although Japanese eels can adapt to various types of environments, significant differences can occur in population structures and growth patterns among habitats.
We successfully discriminated wild and cultured anguillid eels without artificial tagging. For the purpose of population restoration of anguillid eels, stocking has been widely conducted in the European Union (EU) and Japan, although the net benefits of stocking remain unclear. To evaluate the effectiveness of eel stocking for population restoration, the survival, growth, escapement, and reproduction of stocked eels should be tracked. In this study, we explored the potential of using otolith oxygen and carbon stable isotope ratios as a natural tag to discriminate between wild and cultured eels, because cultured eels are often stocked into natural rivers and lakes. A discrimination model was developed based on wild (n = 95) and cultured (n = 314) Japanese eels as a training dataset. The results of accuracy estimation based on leave-one-out cross-validation were 96.8%. We then tested the applicability of the model to stocked–recaptured eels (n = 20); 100.0% were successfully identified as cultured eels, indicating that these eels spend their early continental phase in aquaculture ponds before stocking. This method could be widely applied to assess the effectiveness of eel stocking by determining the proportion of stocked eels among those captured from rivers, coastal areas, or spawning grounds.
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