Horizontal distribution and annual fluctuations in abundance of settled juveniles of the black sea bream Acanthopagrus schlegelii in Hiroshima Bay, Japan

広島湾においてクロダイ稚魚の着底・成育に適した環境条件を把握する目的で，同湾沿岸の稚魚の生息密度を明らかにした。稚魚は2006–2009年6–7月に広島湾内18定点で採集した（n=2,488）。さらに，調査定点のうち最も高い稚魚密度（35.83 inds./100m2）を観測した能美島鹿田において，2013–2016年の7–9月にかけて稚魚を採集し（n=1,588），密度の年変動を観察した。その結果，広島湾の全平均密度は3.78 inds./100m2となり，密度の高い4定点はいずれも厳島，能美島西岸であった。密度が高い定点の特徴として，潮流が物理的に遮られた静穏域の砂浜帯で，淡水流入があることが挙げられた。密度には年変動があり，近年は減少傾向にあることが判明した。広島湾クロダイの資源加入量が低下している可能性が示唆された。To obtain basic information on the optimal nursery grounds for black sea bream Acanthopagrus schlegelii recruits, juvenile density in Hiroshima Bay was calculated. Juveniles (n=2,488) were collected from 18 sampling stations around Hiroshima Bay using surf zone nets during the early summer from 2006 to 2009. to assess the annual fluctuations of juvenile density, samples were continuously caught from one station (St. 1) on the western side of Noumishima Island from 2013 to 2016. Relatively high densities were recorded from four stations (St. 1, 2, 6 and 10) located along the Ohno Seto Strait and the western side of Noumishima Island. These sampling stations shared three common characteristics that likely explain the observed higher juvenile density, namely (1) moderate currents with calm areas protected by physical barrier, (2) sandy muddy beaches (tidal flats) and (3) freshwater inflows or run-off. Juvenile density showed an annual decreasing trend, suggesting that fish stock levels of black sea bream are declining in Hiroshima Bay

Sock discrimination for juvenile ayu, Plecoglossus altivelis altivelis from stomach of field-caught Japanese seabass, Lateolabrax japonicas in the Nuta River, Hiroshima Prefecture

広島県沼田川下流域でスズキに捕食された稚アユの由来判別を，耳石結晶化および耳石Sr/Ca比を指標に行った。稚アユ7個体のうち，3個体は耳石の結晶化が認められたことから人工放流種苗とした。正常な耳石を有していた4個体の耳石Sr/Ca比分析を行ったところ，3個体が人工放流種苗で，天然遡上は1個体となった。結局，稚アユ7個体の由来は，天然遡上が1個体で，6個体は放流された人工放流種苗と考えられた。耳石形状やSr/Ca比分析は，外敵に捕食された稚アユの由来判別に有効であると考えられた。Stock discrimination study for juvenile ayu, Plecoglossus altivelis altivelis from stomach of Japanese seabass, Lateolabrax japonicus in the Nuta River estuary was conducted using prey otolith morphology and Sr/Ca profiling. Out of 7 individuals, 3 individuals identified as hatchery-stock ayu that had crystallized otolith. Based on the Sr/Ca profiling for 4 individuals that have normal otolith, 1 individual was identified as an amohidromous form and 3 individuals were identified as hatchery-stock ayu. The results showed that otolith-based approches could provide insight into stock discrimination for even highly digested ayu that have lost all phenotypic characteristics

A list of fishes found on the oyster farming rafts by the underwater visual census in northern Hiroshima Bay, Seto Inland Sea, Japan

カキ筏には多様な付着生物が生育することが知られており，魚類にとっても寄り付きやすい環境であることが示唆されている。しかし，カキ筏に具体的にどのような魚類が出現するのかについての知見は乏しい。そこで，カキ筏が数多く係留されている広島湾北部の江田島沖合において，カキ筏に出現する魚類の潜水目視調査を2011年8月と10月に実施した。マガキの垂下された水深5-9mを筏に沿って約50m 遊泳しながら，水中構造内に存在する魚種と個体数を記録した。各月とも，マガキの成長した筏とマガキが垂下されて間もない筏について調査し，のべ400分の潜水観察により総計16科24種の魚類を確認した。そのほぼすべてが，沿岸岩礁域およびガラモ場にみられる魚種であった。クロダイ，アカオビシマハゼ，アミメハギ，カワハギの４種は，調査を実施した筏すべてにおいて出現が認められ，カキ筏環境への順応性の高さが伺われた。出現魚種数および出現個体数は，マガキの成長した筏が有意に高く，マガキの成長に伴い，より多様な魚が数多く寄り付きやすくなることが示唆された。The oyster farming rafts occupy 200ha of the water surface in Hiroshima Bay. Each floating raft (ca.11m wide x 26m long) hangs a total of 600 wires (ca. 10m depth) each attaching 40 oysters’ collectors, providing a complex underwater structure. Though the raft structure has been suggested to provide nursery habitats for fishes, information of fishes found around the oyster raft has been very limited hitherto. In order to obtain data of the species composition and the abundance of fishes occurring on the raft structure, we conducted visual census at off Eta-shima Island, northern Hiroshima Bay, during the daytime on August 5 and October 3, 2011. The collecters were dropped in 0-15m depth from floating rafts. Five observers swam for 52m along the outer edge of the raft in 5-9m depth, with recording fish species and individual numbers occurring within the structure (ca. 100min observation for each raft). The underwater survey was conducted for two types of the rafts in each survey month; a raft with well-grown oysters after overwintering (called the late raft) and one with oyster spats (called the early raft). A total of 24 fish species (16 families) were confirmed and almost of them (23 species) have ever been known as occurring on rocky reefs in Hiroshima Bay. Four fishes, a sparid Acanthopagrus schlegelii, a gobiid Tridentiger trigonocephalus, monacanthids Rudarius ercodes and Stephanolepis cirrhifer, commonly occurred in all four surveyed rafts, suggesting their high abilities in fitting for the environmental conditions of the oyster rafts. The late rafts maintained significantly higher values both in mean fish species numbers and in mean fish abundance than the early rafts, implying that the environmental conditions would become attractive for fishes as a nursery habitat in accordance with the growth of oysters mooring for longer periods.本研究は平成23年度広島大学地域連携推進研究事業の支援を受け，プロジェクト「魚類によるカキならびにアサリの食害防除に関する生物学的研究」の一環として実施したものである

Genetic variation of Japanese loach inferred from restriction fragment length polymorphism analysis of mitochondrial DNA

Genetic analysis using restriction fragment length polymorphisms (RFLPs) of D-loop region in the mitochondrial DNA including the part of 12s RNA and cytochrome b genes was made to clarify genetic variations and relations and relationships among eleven populations of Japanese loach Misgurnus anguillicaudatus. Ten haplotypes were detected using seven restriction enzymes (Alu I, Hinc II, Msp I, EcoR I, Hinf I, Hae III and Taq I). The differences between each pair of the populations were significant (

Insight into the migratory history using otolith Sr : Ca ratio of the anadromous masu salmon Oncorhynchus masou masou from the Tonegawa River

利根川上流域で採集した大型ヤマメ8個体の回遊性を耳石Sr:Ca比分析を用い検証した。その結果，3個体が降海型，5個体が非回遊型と推定された。最も上流で釣獲された検体（降海型）は，4つの堰を通過し，利根川河口から243km遡上していた。利根川には降海型サクラマスと非回遊型の大型ヤマメが生息しており，環境に応じて柔軟に回遊パターンを変化させている可能性が示唆された。The migratory histories of Oncorhynchus masou masou in the Tonegawa River were evaluated from the ontogenetic changes in otolith Sr:Ca ratios. We sampled 8 fish at 180-243 km upstream from the river mouth. Three fish were categorized as anadromous whereas five individual were categorized as nonanadromous life mode with the constantly low Sr:Ca ratios. Intraspecific variation in the migration modes recorded for O. masou masou suggest a plasticity strategy for anadromous behaviors

The migratory histories of the Cottus sp. (middle-egg type) in the Ota River

広島県太田川下流の高瀬堰周辺で採集されたカジカ中卵型の回遊性を，耳石Sr:Ca比分析によって確認した。高瀬堰周辺で採集した24個体の耳石Sr:Ca比は発育初期に高く，成長に従って減少した。よって，これらの個体は回遊型であると断定できた。一方，高瀬堰の上流に生息していた1個体は陸封型であった。カジカ中卵型は，回遊パターンに対して柔軟性を有することで環境に適応していると考えられた。The migratory histories of Cottus sp. (middle-egg type) were insighted from ontogenetic changes in otolith Sr:Ca ratios. We sampled 25 fish in the upstream and downstream of the Takase Weir in the Ota River. Twenty-four fish were categorized as amphidromous form whereas one individual caught in the upstream, showed landlocked life mode. Intraspecific variations in the migration pattern recorded for Cottus sp. (middle-egg type) suggest a plasticity strategy for diadromous behaviors

Reconstruction of the migratory history of the catadromous fourspine sculpin Cottus kazika from the Monobe and Shimanto River, Kochi prefecture in southern Shikoku, using otolith Sr/Ca concentration ratios

カマキリ当歳魚の遡上履歴を推定する目的で，耳石輪紋の日周性ならびに耳石Sr/Ca比と塩分濃度との関係について調べた。その結果，本種の耳石輪紋は日周輪であること，および耳石Sr/Ca比は海水中で顕著に上昇することを確認した。これらの結果をもとに，高知県四万十川および物部川で採集したカマキリ当歳魚の遡上時期を推定したところ，海水から淡水への移行期は，いずれの河川においても3月上旬と推定された。The fourspine sculpin (Cottus kazika) is a catadromous member of the family Cottidae that is endemic to Japan. Our objective was to evaluate the utility of using sagittal otolith Sr/Ca concentration ratios to document the movement of juvenile sculpin between the estuarine areas and freshwater of the Shimanto and Monobe Rivers in southern Shikoku. We confirmed the daily periodicity of increment formation and the correlation between the otolith Sr/Ca ratio and ambient salinity at laboratory condition. Otolith profiles suggested that juveniles tended to move from seawater to freshwater at the beginning of March in both river systems

Discrimination of amphidromous and landlocked ayu, Plecoglossus altivelis altivelis (Teleostei: Plecoglossidae) in the Nuta River, Hiroshima Prefecture, using the otolith Sr/Ca ratio

6～8月にかけて主に沼田川中流域で採集したアユ30尾の耳石Sr/Ca比を調べ，由来判別を行った。その結果，30尾中12尾が天然遡上アユであることが判明した。また，支流の椋梨ダム湖（白竜湖）より上流の椋梨川で採集した13尾は，全ての個体で耳石Sr/Ca比の変動が認められなかったことから，これらはダム湖で再生産した陸封アユと考えられた。Ayu, Plecoglossus altivelis altivelis, is a commercially important species of amphidromous fish. Thus it is being targeted for stock replenishment to sustain harvest numbers. Stock identification between amphidromous and hatchery fish was conducted using the otolith Sr/Ca concentration ratio on fish taken from the Nuta River in Hiroshima Prefecture, where 170,000 hatchery ayu had been previously released. Of 30 fish sampled from the middle-reaches, 12 were identified as an amphidromous form. Of the 13 fish sampled from the Mukunashi River, a branch of the Nuta River, all were identified as landlocked fish that would have completed their life cycle between the Mukunashi River and the artificial lakes formed by the Mukunashi Dam.本研究は平成19年度広島大学地域貢献研究において，東広島市沼田川漁業協同組合より課題提案された「東広島市沼田川におけるアユの資源保全と有効利用」の一環として行った

Discrimination of stock origin of spawning population of ayu Plecoglossus altivelis altivelis at the Ota River, Hiroshima Prefecture

太田川下流域で採集したアユ産卵群の由来判別を外部形態および耳石Sr：Ca比分析で行った。太田川にて放流された人工種苗の耳石Sr：Ca比のチャートパターンは耳石中心から300μmで減少が認められ，耳石中心から400μm以遠で比が減少する天然型と異なっていた。耳石Sr：Ca 比のチャートパターンに基づくと，産卵群への人工種苗の混入率は28%と考えられた。採集した産卵群の側線上方横列鱗数は13～26枚（n=173）で，側線上方横列鱗数が17枚以下を人工種苗，18枚以上を天然アユと仮定すると，産卵群への人工種苗の混入率は31%と見積もられた。太田川産卵群では概ね3割が放流された人工種苗であると考えられた。Stock discrimination in the spawning population of ayu, Plecoglossus altivelis altivelis in the Ota River, western Japan was conducted using number of scales above the lateral line and otolith Sr:Ca ratio. Chart pattern in otolith Sr:Ca ratio of hatchery-stocked ayu was different in amphidromous form. Based on chart pattern of otolith Sr:Ca ratio, mixing ratio of hatchery-stocked ayu to the spawning population was estimated at 28%. Number of scales above the lateral line of spawning population marked from 13 to 26 (n=173). Based on number of scales above the lateral line (hatchery-stocked ayu ≦ 17; amphidromous ≧ 18), the mixing ratio of hatchery-stocked ayu was estimated 31%. Consequently, the contribution ratio of hatchery-stocked ayu to the spawning population was around 30% at the Ota River