The European and Japanese eel are catadromous fish that migrate back and force between ocean and river. The eel larvae hatched in open ocean are transported by ocean currents from spawning area to the continental shelf during larval phase and recruit to estuaries and coastal waters as glass eel. Glass eel recruitment of both eel species has decreased dramatically during recent decades. It is believed that influence of oceanic environmental fluctuations on larval transport is one of the most important causes of their decline. However, it is not known what is the main cause of their recruitment decline and relation between recruitment decline and oceanic environmental fluctuations.

In this study, we set revealing effects of oceanic environmental fluctuations on larvae during early life stage and subsequent recruitment for the Japanese eel in the Pacific and the European eel in the Atlantic as goals. First, larval duration of both species were estimated using Lagrangian simulations. Larval duration for the European eel especially is controversial because the estimation of it can vary between 6 months and more than 2 years depending on the method of analysis. Additionally, for the European eel, whether low ambient temperature of larvae can explain the underestimation of larval duration using otolith microstructure analysis were tested. This information is a prerequisite to a better understanding of the effects of the oceanic environment on those eel larvae and subsequent recruitment. Second, it was investigated that whether oceanographic conditions encountered by the Japanese eel larvae during their early life stages may explain the patterns of variability and the decline in glass eel recruitment. Third, we analysed the effects of the oceanic environmental fluctuations in the North Atlantic on the European eel recruitment. Finally, a general conclusion is made from the findings of these studies.

Estimation of larval duration

Mean duration of larval stages in certain species has been estimated by cohort analysis, otolith microstructure analysis and Lagrangian simulations. Some previous studies indicated that the age of otoliths appears to be underestimated in anguillid species because of the decrease in the number of increments caused by low ambient temperature. Using Lagrangian simulations, we estimated the mean larval duration and the number of otolith increments during the larval stage dependent on ambient temperature of the European eel (Anguilla anguilla) and the Japanese eel (A. japonica). Lagrangian simulations were performed using an Ocean General Circulation Model for the Earth Simulator (OFES) developed by the Frontier Research System for Global Change in the Japan Agency for Marine-Earth Science and Technology. The mean larval durations were estimated as approximately 2 years in the European eel and 7 months in the Japanese eel. Larvae are transported from the spawning to recruitment areas where temperature are within a range of 13.0℃-22.5℃ in the European eel and 20.0℃-27.0℃ in the Japanese eel. The number of increments dependent on ambient temperature was estimated to be 313 in the European eel and 208 in the Japanese eel. These estimated values corresponded with otolith microstructure analyses in both species. We suggested that, larval duration only based on otolith microstructure analysis can be underestimated, and that durations of the larval stage confirmed by otolith increment depend on ambient temperature are approximately 2 years in the European eel and 7 months in the Japanese eel.

The effects of oceanic environmental fluctuations in North Pacific on the Japanese eel (Anguilla japonica) recruitment

The spawning area of the Japanese eel is situated in the North Equatorial Current (NEC) west of the Mariana Islands. After hatching, larvae are transported westwards by the NEC which bifurcates off the east coast of the Philippines into the north flowing Kuroshio and the south flowing Mindanao Currents. Only larvae that enter the Kuroshio can migrate successfully to the normal yellow eel growth habitats. Thus, the bifurcation could be an important oceanic structure related to larval transport and ultimately, recruitment of the Japanese eel. Its precise structure and its effects on larval transport, however, have not yet been clarified. The purposes of the study in this chapter, therefore, were to investigate interannual variability of the NEC-bifurcation and to estimate the proportion of larvae transported from the spawning area into the Kuroshio by a Lagrangian numerical simulation of particle transport. Moreover, by investigation relationship between interannual variations of the NEC-bifurcation latitude and glass eel recruitment index, important factors that might control recruitment success of the Japanese eel can be clarified, such as the El Nino-Southern Oscillation (ENSO). To clarify latitudinal fluctuation of the NEC bifurcation, the NEC-bifurcation latitude was defined as the position where the averaged meridional velocity in the 2° longitude band along the eastern coast of the Philippines is equal to zero, the Kuroshio as northward flow and the Mindanao Current as southward flow. The movement of the NEC-bifurcation latitude was significantly correlated with the Southern Oscillation Index (SOI) (r = -0.40, p < 0.01). Simulation showed that the number of particles transported from the NEC to the Kuroshio tended to be lowest during El Nino years, and differences between La Nina and regular years were small. Testing for synchrony showed a significant negative correlation between recruitment index in Tanegashima island and time series of the NEC bifurcation latitude (r = 0.61, p < 0.05). These results suggested that the bifurcation could be related to larval transport and northward change of the NEC bifurcation latitude during El Nino in negative SOI negatively affect the Japanese eel recruitment. Because NEC bifurcation latitude is on a northward trend over the 50 years (τ = -0.40, p <0.01), downward trend in glass eel catch appears to be correlated with the NEC bifurcation latitude.

The effects of oceanic environmental fluctuations in North Atlantic on the European eel (Anguilla anguilla) recruitment

The decline in the European glass eel recruitment is alarming. This species has been listed by the Convention on International Trade in Endangered Species of Wild Fauna and Flora regarding its current level of abundance. Thermal fronts in the Sargasso Sea act as cues that help adult eels locate the spawning area. In addition to, temperature at surface can be a useful indicator of nutrient availability in this area. Hence, temperature fluctuation in this area affected the European eel recruitment though larval survival caused by starvation and by unfavorable currents that prolong the duration of oceanic migration. In the present analysis, horizontal and vertical thermal structure and its temporal fluctuations in all of the Sargasso Sea were investigated based on CTD data over 45 years. Secondly, the relationship between the recruitment of the European glass eel and temperature descriptors related to transport and food availability. In order to investigate relationship between recruitment and oceanic environment, the four time series of glass eel recruitment were used from ICES, Ems in Germany, Den Oever in the Netherlands, Loire in France and Nalon in Spain. Temperature data in the Sargasso Sea (10°N-40°N, 40°W-80°W) were obtained from World Ocean Database 2001 (WOD01) web site in National Oceanographic Data Center. Above 100m depth, three water mass in the Sargasso Sea, that is area of the thermal front and its north-south, were horizontally defined by thermal structure. Temperature of three areas has increase since 1960, with interannual fluctuations. A negative relationship was found between fluctuations in sea temperature (2-4years lagged) and in recruitment indices over the period 1961-2004. Especially, significant negative relationships were confirmed area of the thermal front and its southern area where the European eel spawn. The results of the regime shift detection show that positive regime shift occurred from 1976 to 1980 in the Sargasso Sea temperature. The sequential regime shift detection showed a negative shift in all glass eel recruitment indices in 1980-1982. These results suggested that linkages between recruitment decline with fluctuation of the European eel and change of Sargasso Sea temperature though early larval development. Significant negative correlations were found using a 2 or 3-yr delay between recruitment and temperature time-series. In our analysis, this time lag accounts for transatlantic migration, metamorphosis, and estuarine arrival as glass eel. This is consistent with estimates of larval duration in this study.

On the whole, this study revealed that recruitment of the European and Japanese eel are affected by oceanic environmental fluctuations. The findings provided through these studies can be utilized for the resource management and recovery of eel populations.

On the whole, this study revealed that recruitment of the European and Japanese eel are affected by oceanic environmental fluctuations. The findings provided through these studies can be utilized for the resource management and recovery of eel populations.

OFES(OGCM for the Earth Simulator)によって算出された月平均3次元流速を使用し、粒子追跡シミュレーションから仔魚期間を推定した。粒子投入から生息域沿岸に輸送されるまでの期間を仔魚期間と定義して、日周鉛直移動や自然死亡を考慮したモデルであり、推定されたヨーロッパウナギ、アメリカウナギ、ニホンウナギの仔魚期間のピークはそれぞれ約700,330,180日であった。ヨーロッパウナギとアメリカウナギの推定値は、耳石輪紋解析から求められた190-360日と160-300日を大きく上回るものである。そこで、水温に依存した耳石輪紋数をモデルに組み入れて検討を行い、すべての種で耳石輪紋解析による仔魚期間の推定とよく一致する結果を得た。この結果は、経験水温が低い場合には耳石成長率低下に伴う耳石輪紋不検出の影響を受け、耳石輪紋解析では仔魚期間の推定が過小評価となる可能性が高いことを初めて示唆した意義深い研究成果である。

北赤道海流域に存在する塩分フロント域で孵化したニホンウナギ仔魚は、北赤道海流によって西方へ輸送され、その後フィリピン東岸で分岐した北向きの黒潮に乗り換え、東アジア沿岸に輸送される。本論文では、この分岐緯度をニホンウナギ仔魚の輸送に関わる環境要因として捉え、フィリピン東岸で仔魚の分布する水深50-150mの南北流速の平均値が無流となる緯度を分岐緯度として定量化した。エルニーニョの指標となる南方振動指数と分岐緯度の間には負の相関が認められ、エルニーニョ年に北上傾向を示すことを明らかにした。産卵場の指標となる塩分フロントも南方振動指数と関連しており、塩分フロント位置を産卵場とした粒子追跡シミュレーションでは、南方振動に関連した塩分フロントと分岐緯度の変動により、エルニーニョ年には黒潮に取り込まれる粒子数が減少することを明らかにした。一方、ラニーニャ年は流速が遅く、小中規模渦の多い北赤道海流北部で産卵が行われるため、エルニーニョ年や通常年に比べて輸送期間が遅くなる傾向にあり、自然死亡を考慮した輸送成功粒子数は通常年に比べて少なくなることを示した。この研究成果により、ニホンウナギのシラス来遊量を定量的に予測できる可能性があることが初めて示された。

大西洋のヨーロッパウナギとアメリカウナギは、シラスの来遊量と肥満度の間に正の相関関係があり、初期仔魚期における餌料環境変動に伴う生残率の変動が来遊量を決定する大きな要因であることが示唆されている。そこで、初期仔魚の分布域であるサルガッソ海の水温を餌料環境の指標と見なし、シラス来遊量とサルガッソ海の水温との関係を検討した。その結果、ヨーロッパウナギで2-3年、アメリカウナギで1年のタイムラグで水温と来遊量との間に有意な負の相関があることが分かった。また、水温には1979年にレジームシフトがあり、このレジームシフト後となる1981-82年、1980年にヨーロッパウナギとアメリカウナギのシラス来遊量が低水準に移行した。つまり、産卵場での水温上昇が仔魚の餌量環境を悪化させ、仔魚の生残が低下させることで低来遊量をもたらしたと推測できる成果であり、メカニズムの解明に大きな指針を与えた。

上述した研究成果は、気候変動に伴う海洋環境変動がウナギ属魚類の加入量を変動させていることのメカニズムを明らかにしたものであり、地球温暖化やレジームシフトなどの気候変動に伴う海洋環境変動に対するニホンウナギ、ヨーロッパウナギの応答の予測に貢献する極めて意義深い研究成果であると判断できる。

なお、本論文第2章は、北川貴士、宮崎幸恵、笹井義一、佐々木英治、木村伸吾との共同研究であるが、論文提出者が主体となって分析および検証を行ったもので、論文提出者の寄与が十分であると判断する。

したがって、博士(環境学)の学位を授与できると認める。