The homing behavior and site fidelity to habitats in various fishes, including anguillid eels (genus Anguilla), are fascinating. However, little is known about how yellow-phase eels exhibit homing behavior and the sensory mechanisms involved. Using acoustic telemetry, we investigated the homing behavior of 18 Japanese eels, A. japonica, with total lengths ranging from 204 to 570 mm, in a narrow freshwater river in inland central Japan, where salinity gradient, tidal current, and magnetic sense cannot be used for their homing, but where olfaction could play a role. The tagged eels captured upstream and downstream were released downstream and upstream, respectively. The results showed that large eels, over approximately 400 mm in total length, exhibited homing behavior to their original sampling locations (likely to shelters and foraging sites, where they probably spent a longer time than in other locations and grew successfully) from outside their home ranges, predominantly during the dark period. Homing success was not affected by the two capture locations, indicating that eels did not use olfactory cues for short-range homing in freshwater rivers.

Although river-crossing structures can have a detrimental effect on the movement and distribution of anguillid eels (genus Anguilla), they have inhabited locations upstream of river-crossing structures in many rivers. However, the growth stage in which the eels start to climb river-crossing structures remains unclear. In this study, we directly observed, using infrared video camera systems, that the Japanese eel (Anguilla japonica) climbs a low-height vertical weir as a migratory barrier, confirming the ability of eels to climb over a low-height weir within a river. All eels climbed the weir at night, between sunset and sunrise. We observed that the total length of Japanese eels that climbed the weir ranged from 60 to 140 mm, suggesting that eels inhabiting the upstream area of a weir climbed it when they were small and then lived and grew upstream. Moreover, the general additive model showed considerable effects of water temperature and precipitation on eel climbing, suggesting that water temperature and precipitation are important for its activation. The results of this study also show the effectiveness of infrared video cameras in monitoring eel climbing, which could be applied to monitor fish migratory behavior through fish passages. The findings of this study are useful for the comprehensive management and conservation of wild eel stocks.

Abstract Scientific naming rules of animals are strictly defined by the International Code of Zoological Nomenclature, while those for common names are vague and not well defined. Specifically, the common names of freshwater eels of the genus Anguilla have become confused among scientific papers, pictorial books, and online resources in recent years. This disordered use of common names among freshwater eels demonstrates the urgent need for the standardization of common names for these species. In addition, freshwater eel populations have drastically decreased worldwide over the past few decades, resulting in their listing as endangered species. In the present study, we defined the following five rules for the common names of freshwater eels: to (1) use a representative locality name of the geographic distribution of the species or subspecies, (2) distinguish two sympatric species or subspecies as longfin or shortfin, (3) select a name that would enable the scientific name to be easily recalled, (4) value longstanding former common names if they adhere to the above three rules, and (5) use the shortest name possible. Based on current scientific knowledge and on these rules, we proposed 22 common names for all known 19 species and subspecies of the genus. As a result, 21, 6, 13, and 3 species and subspecies were named based on Rules (1), (2), (3), and (4), respectively.

The recently discovered freshwater eel species of the Luzon mottled eel, Anguilla luzonensis, has only been found in very limited area at the tip of the Northern Philippines. Here we investigated the early life history of A. luzonensis for the first time to help understand its larval dispersal pattern in the western North Pacific, and compared it to A. celebesensis and data from previous studies. Genetically-identified glass eels of A. luzonensis collected from the northern Luzon Island in January and February 2009 (n=3 and 8, respectively) were used to study the age of recruitment of this species using otolith microstructure. The leptocephalus stage was 129.6±10.7 days (mean±s. d.; range, 113 to 146), followed by 18.4±3.7 days (13 to 25) at the metamorphosing and 17.6±5.1 days (7 to 23) at the glass eel stages. These specimens were found to be hatched between August and September 2008. A previous study on a small number of the leptocephali of A. luzonensis collected offshore reported that the hatching dates were between February and March, and thus the spawning season of A. luzonensis is suggested to span at least a half of a year.

Comparisons of genetic sequences of the mitochondrial DNA 16S rRNA gene were used to match two species of congrid eels of the genus Ariosoma to two established Ariosoma-types of leptocephali. 18 Ariosoma sp. 7 leptocephali (40-151mmTL) were matched with an A. major adult (333mmTL) collected from Tosa Bay of eastern Japan with 0.2-0.7% sequence differences, and 1 Ariosoma sp. 8 leptocephalus (330mmTL) was matched with two A. shiroanago adults (317 and 320mmTL) also from Tosa Bay with no sequence difference. Ariosoma sp. 7 leptocephali have been collected in many regions of the western North Pacific (WNP), and the leptocephali from both the East China Sea and the WNP were matched with A. major, suggesting that the leptocephali of this species are widely distributed in the region. The leptocephali of Ariosoma sp. 8 appear to be much less common in most areas that have been sampled recently.

To examine the genetic diversity of Sicyopterus japonicus, 448 sites of the control region of the mitochondrial DNA were analyzed in 77 specimens from the four localities of Okinawa, Kochi, Wakayama, and Shizuoka in Japan. A total of 74 haplotypes were found in the individuals examined. The same haplotypes occurred in Okinawa and Kochi, Kochi and Wakayama, and two specimens in Wakayama. The average sequence in genetic characteristics within localities varied from 1.7% in Kochi, 1.6% in Wakayama to 1.4% in Okinawa. There was no significant difference in genetic characteristics among the three locations of Wakayama, Kochi, and Okinawa (10,000 times permutation test, P>0.05). The number of specimens from Shizuoka was too small to compare to the other three locations. The neighbor-joining tree of the mitochondrial DNA haplotypes for all specimens constructed from the Kimura's two-parameter distances suggested no evidence of genetic subdivision of S. japonicus. These results suggested that this species has a single panmictic population and their larvae probably have a high dispersal ability during their oceanic stage.

The population structure of Anguilla bicolor bicolor was evaluated using their total number of vertebrae and genetic analyses of the mtDNA control region. Based on the likely geographic population structure of this subspecies in the Indian Ocean, the data were combined into East (Madras, Sumatra Island and Australia) and West groups (South Africa, Madagascar, Reunion and Seychelles) according to their sampling localities. Significant differences were not found in the range and mean total number of vertebrae between the East (N=74) and West groups (N=47) in the Indian Ocean, which were 107 to 113 (108.29±1.26), and 106 to 113 (109.60±1.47), respectively. Furthermore, the polymerase chain reaction-restriction fragment length polymorphism analysis for the mitochondrial control region using a total of 18 specimens from three localities showed no genetic differences between the East (N=14, Myanmar and Sumatra Island) and West groups (N=4, Madagascar) in the Indian Ocean. Morphological and genetic characters examined in the present study suggested no population structure for A. bicolor bicolor in the east and west side of the Indian Ocean, whereas they were ecologically assumed to be different populations. This contradiction suggested that these populations were in the beginning of speciation.

沿岸漁業漁獲量が減少し続ける中で、スズキの資源水準が長期的に安定している原因が、本種による河川利用にあるという仮説を検証することを研究目的としている。仙台湾、丹後海、別府湾の3水域をフィールドとして、どの発育段階や年齢で、どのくらいの割合が、どのようなタイミングで河川を利用するのかという生態特性を調査し、繁殖と生き残りにおける河川利用の生態学的意義を明らかにする。 3水域で採集したスズキ成魚84個体(標準体長388-736mm、年齢3-23歳、仙台湾21個体, 丹後海38個体, 別府湾25個体)の耳石を年齢査定し、耳石Sr/Ca比をEPMAにより点分析した。その結果、幼稚魚期（0歳魚期）に河川（汽水域＋淡水域）を利用した個体の割合は、仙台湾52％、丹後海61％、別府湾36％であった。河川利用個体の割合は年齢とともに減少し、0歳魚の平均51％から5歳魚では17％となった。成熟する3歳前後まで、河川利用個体に雌雄差は見られなかったが、4歳以降は雌の割合が増加した。2歳以上で河川を利用した個体のうち、約8割は幼稚魚期にも河川を利用しており、幼稚魚期に河川を成育場とした個体が成体でも河川に遡上する傾向が認められた。 スズキ成魚の行動と環境要因との関係を調べるため、由良川の河口から48km上流までの12地点に超音波受信機を設置した。2020年10月－2021年3月の間に由良川及び近隣の河川で釣りにより採集したスズキ成魚24個体に超音波発信機を装着して、由良川中流域に放流した。10－11月に放流した17個体中追跡できた13個体では、7個体が12月初旬までに降海し、6個体が冬季にも由良川に残留した。降海した7個体のうち3個体は翌年春－夏季に由良川に再遡上した。耳石Sr/Ca比分析とバイオテレメトリーデータを総合することにより、スズキ成魚の河川利用行動の詳細が解明できると期待される。

本州中部から琉球列島までの南日本、台湾及び東南アジアの魚類について、分類学、集団遺伝学及び動物地理学的観点から研究した。その結果、日本の温帯部に分布する魚類の中には、黒潮が障壁となって種レベルあるいは個体群レベルで、南方集団と北方集団に分かれているケースがあることが分かった(例:ハタ科やアジ科)。一方、ハゼ科のボウズハゼの稚魚・成魚のDNA解析を行い、黒潮によって長距離分散していることを明らかにした。