| Project/Area Number |
04041036
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| Research Category |
Grant-in-Aid for international Scientific Research
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| Allocation Type | Single-year Grants |
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| Section | Field Research |
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| Research Institution | The University of Tokyo |
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Principal Investigator |
SHIMIZU Makoto Professor, University of Tokyo, 農学部, 教授 (00011883)
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| Co-Investigator(Kenkyū-buntansha) |
SOSAーNISHIZA ニシザキ オスカー エンセナダ調査研究高等教育機関, 研究員
SERGIO Marti ニカラグア中央漁業研究所, 所長
TANAKA Sho Associate Professor, Tokai University, 海洋学部, 助教授 (90138636)
OTAKE Tsuguo Assistant Professor, Univ. Tokyo, 海洋研究所, 助手 (20160525)
ISHIHARA Hajime Senior researcher, Suidosha Co. Ltd., 主任研究員
OGAWA Kazuo Associate Professor, Univ. Tokyo, 農学部, 助教授 (20092174)
WATABE Shugo Associate Professor, Univ. Tokyo, 農学部, 助教授 (40111489)
TANIUCHI Toru Associate Professor, Univ. Tokyo, 農学部, 助教授 (00012021)
SOSA-NISHIZAKI Oscar Researcher, CICESE, Mexico
MARTINEZ C. Sergio Director, CIP, INPESCA, Nicaragua
OSCAR Sosa N エンセナダ調査研究高等教育機関, 研究員
佐野 光彦 東京大学, 農学部, 助手 (50178810)
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| Project Period (FY) |
1992 – 1993
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| Project Status |
Completed (Fiscal Year 1993)
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| Budget Amount *help |
¥13,800,000 (Direct Cost: ¥13,800,000)
Fiscal Year 1993: ¥10,000,000 (Direct Cost: ¥10,000,000)
Fiscal Year 1992: ¥3,800,000 (Direct Cost: ¥3,800,000)
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| Keywords | bull shark / sawfish / Usumacinta River / Lake Nicaragua / San Juan River / Colorado River / abundance / osmoregulation / 淡水 / サメ / エイ / ウシマシンタ川 / イサバル湖 / サンファン川 |
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| Research Abstract |
Field surveys of freshwater elasmobranchs have been conducted in Mexico and Central America during the period from 1992 to 1993 academic years. As a result, two species of elasmobranchs, the bull shark (Carcharhinus leucas) and sawfish (Pristis perotteti), have been captured in several places. In the Usumacinta River Basin, Mexico, five bull sharks were collected in May, 1993. Four small specimens estimated to be immediately after birth were taken in the main stream of the river while a mature male specimen attaining 210 cm in total length was captured by gill nets in a lagoon connecting with the main stream. This is the first record of its occurrence in the Usumacinta River. The male specimen was the largest among those ever reported from the freshwater. Osmorality of the sharks was approximately two-thirds of that of the marine form. Urea content of the sharks showed one-third concentration of the marine form. The large male was estimated to be five years old.
Only one sawfish was captured in Lake Nicaragua and its effluents, although extensive surveys were conducted in several places of the lake. No bull sharks were caught in the lake and upper reach of San Juan River. One bull shark was taken in Indios River along which the new village of San Juan de Norte was constructed after the destruction by hurricane. Another bull shark was captured in Samay inlet located near Barra del Colorado, Costa Rica. The result of the survey, combined with fishermen's witness, suggests that the population of elasmobranchs in Lake Nicaragua and upper reach of San Juan River might be greatly reduced in recent years. Osmorality and ionic concentrations have been under examination. Histological study has been done on the function of the rectal gland of both marine and freshwater forms.
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