Unagi is popular fish in Japan. People enjoy Unagi in Kabayaki style on Doyo-no Ushinohi. We have traditional culture related to this fish. However, Unagi's ecology - where they breed and how they come to Japan - has been in mystery until recently. Progress of technology reveals their life cycle little by little. Our lab use various approaches for the Unagi researches. We analyze pelagic ocean structure and its fluctuation to elucidate transportation mechanism of eggs and larvae. On the other hand, stable isotpope analysis is one of the main approaches for their early life ecology. Eel catch data and ocean structure around Japan are considered for eel resource fluctuation mechanisms.
1. Spawning Ground of Japanese Eel
Japanese eel has their spawning ground in North equatorial currents, west of Mariana islands near Guam (Figure 1). Japanese eel only distributes in Japan, China, Taiwan, Korea and North of Luzon island, Philippine. No genetic differences between these distribution areas, and leptocephali (eel larvae; Picture 1) of few weeks old have been found only the estimated spawning ground, thus the North equatorial area seems the only one spawning ground for them. In 2005 Research Cruise of Hakuho-maru, pre-leptocephali of few days old were obtained (Picture 2).
How the spawning eels find the exact point in pelagic ocean? It is impossible for one eel to meet another coinsidentaly regarding the migration scale. There should be a certain mechanism. Previous Cruises of Hakuho-maru revealed that the salinity dividing the surface layer of North equatorial current from North to South could be used as a landmark. This suggestion came from the catches of the leptocephali were concentrated in southern of the salinity front, and the ocean current structure is suited for west-ward transportation of the larva.
The salinity front is formed by high-salinity water brougt by strong evaporation effect offshore of Hawaii and low-salinity water brought by rainfall of tropical climate. When El Niño happens, shift of cumulonimbus to east pushes the salinity front south direction. If the salinity front is important landmark for Japanese eel spawning, the arrival of eel larva to Japan should fluctuate coresponding to El Niño, and in fact, the trend was observed in eel larvae cathes (Figure 2-1, 2-2). In addition, the distribution of leptocephali moved to south following the salinity front shifted to south in El Nino year. On the other hand, the carbon stable isotope ration of POM (particulate organic matter), which is food source of leptocephali, differs significantly in North and South of the salinity front. Thus, the water condition not only salinity may influence the spawning migration.
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| Figure 1. Spawning Ground of Japanese Eel, their Distribution and West Subtropical Gyre |
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| Picture 1. Leptocephalus (larvae of Japanese eel) | Picture 2. Pre-leptocephalus |
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| Figure 2-1. No El Niño | Figure 2-2. El Niño |
2. Marine Environment affects Larval Transportation
Our lab research forcusing physical and environmental aspects of marine condition influencing leptocephalus transportation. Leptocephali are transported by North equatorial current, via East of Philippine the Kuroshio. If they take wrong direction current, Mindanao current, the migration would be unsuccessful. Thus, it is important to switch the current correctly for survival (Figure 3).
However, the branching point of the Kuroshio and the Mindanao Current from North Equatorial Current changes in a moment. Recent studies revealed that the shifts of the branching point influences the survival of leptocephali strongly. Besides, analysis of stable isotope ratios leading us to detailed feeding condition of leptocepahli during the migration to nursery areas.
After the long journey, leptocephali metamorphose into Shirasu form. With high swimming ability of Shirasu, they get off the Koroshi and take a departure for new life style in rivers and estuaries. From spawning ground to coastal area of Japan, they migrate about 3,000 nautical miles in 4-5 months. Far pelagic ocean condition actually have a strong impact to our summer meal Kabayaki, which is mysterious and advocates our interests.
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| Figure 3. Japanese Eel Larvae Transportation Path |
3. Resource Evaluation of Young and Adult Eels
Since eels reached to Japan coast, what happens until they become Kabayaki? Nowadays, eels that we eat heavily depends on the farming. It uses naturally breeded Shirasu catches as seed source. However, the source of Shirasu declined significantly in recent years (Figure 4). What is happening to eel resource? As it was mentiioned, Japanese eel migrates to Mariana ridge for spawning. Their complicated ecology makes the recource assessment and management difficult. There are so many issues to solve for proper management of eel resource, such as population size of spawning adults and recruits, time required for sexual maturation and its success rate and so on. The problem is, on the other hand, we do not have concrete and accurate data of eel catches and the data needs unification for close analysis.
We aim to grasp the current situation of eel resource by gathering and analyzing eel catch data. Besides, we also focus on the local fluctuation of physical environment and eel catch.
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| Figure 4. Year Fluctuation of Shirasu Catch in Japan |
References
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- Kimura, S. and Tsukamoto, K. (2006) Possible spawning of the Japanese eel (Anguilla japonica) near a salinity front. Globec International Newsletter, 12(2), 35-36.
- 木村伸吾．(2005) 仔魚の輸送と加入量変動．海洋生命系のダイナミクス-海の生物資源，渡邊良朗編，東海大出版，pp436，241-258．
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