鶴田 哲也

It is important to select the most appropriate method for fish sampling in streams and rivers; the sampling efficiency as well as any negative effect of the method of fish sampling on fishes such as stress, injury, and mortality must be considered. This study aimed to investigate stress hormone responses in ayu Plecoglossus altivelis in reaction to direct current (DC) electrofishing, alternating current (AC) electrofishing, and cast netting in artificial streams. The mortality, injury rate, and catch efficiency of these catching methods were also compared. It was found that immediately after capture, fish caught using a cast net exhibited significantly higher cortisol levels than did control fish that were caught rapidly by a hand net. Cortisol levels did not differ between electroshocked fish and the controls. Time required to catch 12 fish was shorter when DC was used (20 s) than when AC was used (45 s). The time required to catch fish greatly increased when a cast net was used (840 s). Cortisol levels in DC electroshocked and control treatment groups were at resting levels 24 and 48 h after capture. However, higher cortisol levels were found in fish captured using a cast net at both these time points. Cortisol levels in AC electroshocked fish returned to lower levels at 24 h, but increased again at 48 h. Furthermore, 48 h following capture, the fish caught by AC electrofishing exhibited higher mortality (7.5 %) than those captured by other methods (0 %). Spinal injury was also detected in one of the fish in this group. Considering these findings, DC electrofishing is the most effective and least damaging method for collecting P. altivelis in streams with respect to stress, physical damage, and efficiency.

To clarify food resource use and niche breadths of the Japanese fluvial sculpin Cottus pollux (large-egg type), listed by over half the prefectures as an endangered species, we examined the diet and food selectivity of 168 specimens at 7 locations in 3 tributaries that drain into the upper regions of the Chikuma River. A stomach pump was used to assess the stomach contents. In total, 2,833 aquatic organisms from more than 22 families were found in the stomachs. An index of relative importance (IRI) indicated that chironomid, baetid, and ephemerellid larvae were the major prey for the sculpin. Despite considerable variation in the composition of aquatic organisms among locations, sculpin exhibited strong food selectivity for baetid, glossosomatid, chironomid, and tipulid larvae at multiple locations. Each individual sculpin preyed upon 17.2 prey items on average, and the number of prey items found in the stomach positively correlated with the number of prey families, regardless of sculpin body size. Of 22 prey families found form all stomachs we examined, each individual sculpin preyed upon only 3.54 families on average. These findings suggest that sculpin may have relatively narrow niche breadths regardless of a considerable number of prey individuals in their stomachs. GLM results indicated that a model that incorporated water depth and sculpin body size was the most parsimonious model to explain the prey diversity index in the stomachs of the sculpins. The diversity index of aquatic organisms positively correlated with the mean water depth at each location. These data suggest that rivers with deeper water depths might allow colonization of a variety of aquatic organisms, which might be expected to enhance availability of prey potentially available for the sculpin.

Social and foraging modes in fish often vary between individuals with different body sizes and between populations under different ecological conditions. We studied social and foraging behaviour of algae-grazing fish Plecoglossus altivelis ryukyuensis (Ryukyu-ayu) inhabiting Japanese subtropical island streams. Ryukyu-ayu exhibited four behavioural modes: territorial, schooling, solitary and floating. Their body sizes differed among these modes. Territorial fish predominantly foraged on benthic algae, whereas schooling and solitary individuals frequently consumed drifting materials as well. Schooling fish were smaller than territorial fish, but, unlike some other algae-grazing fish species, did not use schooling to gain access to food within territories. Territorial fish attacked smaller conspecifics but exhibited lateral display towards larger fish and schooling fish while occasionally attacking grazing gobies. Larger fish maintained larger feeding territories and occupied territories for longer periods than smaller fish did. This suggests that, in Ryukyu-ayu, intra- and interspecific food competitions and relative body size can influence diverse behavioural modes and duration of territory occupation. We also found that Ryukyu-ayu foraged more often and had larger feeding territories than ayu P.a.altivelis inhabiting temperate streams. We conclude that foraging strategies of Ryukyu-ayu may have adapted to subtropical island streams, where algal productivity is much lower than that in temperate regions.

Rice-fish culture, which means the simultaneous culture of rice and fish, is one of the best options to increase food production from limited land and is practiced in many countries in the world. Although many researchers and farmers believe that the rice yield is increased by fish farming in paddy fields, this hypothesis has never been fully tested. Here, we report ecological processes leading to higher rice yields in the rice-fish culture using crucian carp (Carassius complex), which have adapted to the paddy field ecosystem in Japan. We compare the rice-fish and rice-only plots in the experimental paddy field for biota, water quality, and rice yield. Coverage of duckweed and densities of zooplankton and benthic invertebrates in the rice-fish plots were lower than those in the rice-only plots, indicating that fish utilized them as food. NO(3)-N concentration in the rice-fish plots was higher than that in the rice-only plots, indicating that the increase in NO(3)-N concentration results from excretion of unutilized food nutrients by the fish. Consequently, rice yield in the rice-fish plots was 20% higher than that in the rice-only plots. The fertilizing effect of the fish excrement probably increased rice yield.

異なる懸濁物質濃度の濁水を短時間、もしくは長時間アユに暴露し、本種におけるストレス応答の実態を調査した。その結果は、低レベルの濁水であっても長時間曝されることによって、アユにとってストレッサーとして作用することが示唆された。 安房田智司、武島弘彦、鶴田哲也、矢田 崇、井口恵一朗 共同研究につき本人の担当部分の抽出不可能

Changes in mRNA levels of prolactin (PRL) during seaward migration and after experimental transfer from fresh water (RN) to seawater (SW) were examined in larvae of the amphidromous fish, ayu Plecoglossus altivelis. In the field study, ayu larvae caught in the surf zone showed lower levels of PRL mRNA than those in the river, while growth hormone (GH) levels showed no significant change. Decrease in PRL gene transcription was also observed 24 h after direct transfer from RN to SW, whereas there was no significant influence of water temperature. On the other hand, there was no significant change in GH mRNA levels in relation to SW transfer or environmental temperature. In a raceway with a vertical salinity gradient. PRL mRNA levels of ayu larvae showed a significant reduction during spontaneous migration from FIN to SW, which mimicked the results from the field observation and the transfer experiment, and then a gradual increase during the course of development. Whole body water and sodium contents of larvae in a salinity gradient were stable during migration to SW. Results in this study indicated the importance of regulation of PRL gene expression in the downstream migration and acclimation to SW during the early development of ayu. (C) 2010 Elsevier Inc. All rights reserved.

アユ仔魚に対する昼間と夜間における捕食圧を評価する目的で、オイカワ、ウグイおよびスミウキゴリによって消費されたアユ仔魚の個体数を明・暗実験条件間で比較した。その結果、オイカワとウグイでは明条件より暗条件において、より多くのアユ仔魚を捕食する傾向が認められた。したがって、昼行性であるこれら2種の捕食者が高密度で生息する河川では、アユ仔魚は夜間より昼間のほうが強い捕食圧にさらされると推察される。 鶴田哲也、小池亮人、武島弘彦、 棗田孝晴、井口恵一朗 共同研究につき本人の担当部分の抽出不可能

In Japan, the numbers of common cormorants (Phalacrocorax carbo hanedae) feeding in inland waters have shown a rapid increase since the 1980s, which has resulted in concerns of predation impacts on inland fisheries. We had an opportunity to examine stomach contents of cormorants in an inland area during winter, 2007-2008, Of the 23 cormorants examined, 19 had fish in their stomachs. The mean wet weight of identifiable prey fish in stomachs (129.3 g) amounted to 25.7 % of the daily food requirement of the cormorants. Of the four fish species found in the stomachs of the cormorants, pale chub (Zacco platypus) and Japanese dace (Tribolodon hakonensis), school-forming species, numerically dominated. Sixteen of 19 cormorants had either or both of these two species in their stomachs; only four cormorants preyed upon the barbel steed (Hemibarbus labeo), which implies a benthic foraging mode. A calculated index of relative importance indicated that pale chub and Japanese dace were important prey for the cormorants. Coefficient of variation of prey fish size was large for crucian carp (19.0), pale chub (18.8), and Japanese dace (15.9). We conclude that fish forming dense size-structured schools are vulnerable to predation by the cormorants, and cormorants may have started to use inland pools of the Chikuma River during winter as feeding environments.

絶滅のおそれのある汽水・淡水魚類226種と非該当種132種を対象とし、生息場所、体サイズ、食性などの20変数に着目して、国内広域分布種から絶滅危惧種へと移行した魚種の生態的特性の抽出を試みた。その結果、人間活動における各魚種への干渉や馴染みの程度を示す、人間との距離が、普通種から絶滅危惧種への移行を予測するうえで有効であることが示された。

絶滅が危惧されているリュウキュウアユを対象に、保全プログラムの構築を目指して、産卵条件に関する現地調査を行った。産卵床の集まるエリア内の環境変数にロジスティック回帰分析を適用すると、産着卵の有無を説明する要因として底質硬度が抽出された。また、産卵床付近に多量に出現したオキナワヒゲナガカワトビケラは、同所的なアユの産卵を阻害する可能性が示唆された。

慣行水田における底生動物の群集構造に影響を及ぼす要因を解明することを目的として、千曲川流域の水田で調査を実施した。その結果、肉食性水生昆虫の個体数が、優占種である貧毛類やユスリカ幼虫の個体数に影響している可能性が示唆された。また、異なる種類の除草剤が使用されていた水田間でも底生動物相に違いが認められた。この原因として、除草剤の種類による生態毒性の強さの違いがもたらす直接的・間接的影響が考えられた。

稲田養魚に対する一般意識を知るために、炊飯米の味覚官能試験を実施した。実験田にフナ放養区と対照区を設けて米を栽培し、両試験米の食味を比較した。その結果、米の出自を伏せた試験では、フナ米と対照米の間の食味評価に差異が検出されなかったのに対し、出自を開示した試験では、フナ米の方が高い評価を得ることが明らかとなった。食の安全と環境の保全に敏感な消費者は、稲田養魚に対して良好なイメージを抱いていることが推測された。

ワウによる放流アユの食害防除に資することを目的として、富士川水系の2河川でカワウの摂餌場所の特性を調査した。目視観察の結果、カワウは河畔林等に見通しが遮られた人目に付き難い場所で採食すること、および水深20cm程度の浅瀬であっても、毎秒160cmを超える急流であっても摂餌が可能であることが明らかとなった。また、採食場所の魚類相調査の結果、カワウは魚影の濃い場所、特にアユの多い場所を選んで摂餌していることが明らかとなった。

3種のトミヨ属魚類（汽水型、淡水型およびエゾトミヨ）が共存する北海道東部の河川において、河川内営巣分布パタンと営巣微細環境を調査した。河川内営巣分布調査の結果、汽水型は主に塩分濃度と水温の高い下流域に、淡水型およびエゾトミヨは塩分濃度と水温の低い上流域に営巣する傾向が認められた。この結果は、汽水型と他の2種との間には物理環境要素に起因する場所的隔離が生殖的隔離機構として機能していることを示唆する。

The freshwater and Omono types of ninespine stickleback, Pungitius pungitius (L., 1758), coexist in several ponds and a stream in the Omono River system, Japan. We tested the hypothesis that coexistence of the two types is accomplished by resource partitioning through interspecific competition. First, the stomach contents of the two types were compared between sympatric and allopatric populations: the stomach contents of the sympatric freshwater type consisted of almost all Copepoda, whereas those of the sympatric Omono type consisted of not only Copepoda, but also many large benthic invertebrates; the stomach contents of both allopatric types were similar, consisting of Copepoda and large benthic invertebrates. Second, behaviour related to resource use in a sympatric pond was observed in the nonbreeding season: the freshwater type showed little aggressive behaviour, but the Omono type had a high frequency of aggressive behaviour. These results indicate that the sympatric freshwater type does not hold a feeding territory and its food resource is almost all Copepoda, whereas the Omono type has a feeding territory and its food resources are various. This suggests that asymmetric interference competition causes a diet shift of the sympatric freshwater type, allowing the two types to coexist by their resource partitioning.

雄物川水系におけるトミヨ属淡水型と雄物型の遺伝的集団構造の詳細を、アロザイム多型解析によって調査した。その結果、雄物型には地域性を反映した遺伝的集団構造が認められたのに対し、淡水型にはそのような構造が認められなかった。これは、雄物型のほうが歴史的に古い時代に本水系に侵入し、過去に起こった歴史的イベントによって雄物型内の地域集団間で長期間にわたって遺伝子流動が制限されていたことを示唆する。

トミヨ属淡水型と汽水型の生態的特性の相違が、両型の個体群構造にどの様に影響しているのかを明らかにするため、mtDNAの遺伝子型組成に基づく個体群間の遺伝子流動の推定を行った。その結果、2型は異なる遺伝的特徴を維持しているとはいえ、各型の遺伝子型間の塩基配列の違いは小さいことが明らかとなった。これは、2型の間でmtDNAの遺伝子移入が生じたことを示唆する。

A total of 123 adult specimens of the nine-spined stickleback Pungitius pungitius were identified as either the freshwater or Omono types, based on diagnostic allozyme loci, and their allozyme patterns compared with restriction fragment length polymorphism patterns of polymerase chain reaction-amplified mtDNA segments (mtDNA PCR-RFLP pattern). All specimens of the two types were clearly identified by the mtDNA PCR-RFLP patterns, except for a single putative hybrid progeny. The mtDNA PCR-RFLP patterns of 133 clutches from 21 nests were also compared with those of their parental males, and 126 out of 127 clutches were found to be consistent with the parental male pattern, the remaining six clutches being guarded by an F 1 hybrid male. The results demonstrated that type assortative mating had occurred between the two types in nature, indicating the existence of a premating isolating mechanism between them. © 2002 The Fisheries Society of the British Isles. Published by Elsevier Science Ltd. All rights reserved.