Inter-individual variability of stable-isotope ratios in marine organisms

使用氮和碳稳定同位素比率分析营养水平比通过肠道内容物检查进行传统的食物习性分析更有利，因为前者反映了单个动物的长期食物历史，而后者仅提供捕获前的食物组成。采用氮碳稳定同位素比值法和常规肠道含量分析相结合的方法，研究了相模湾及其附近海域中上层鱼类、中上层鱼类和中上层鱼类和对虾的食性个体差异。在日本凤尾鱼中，仔鱼和成鱼的个体稳定同位素比率聚为两类，氮的变异约为1.7%。在中、深海对虾中，非迁徙的深海对虾Bentheogennema borealis表现出显著低于两种垂直迁徙的Delta值和较高的Delta值。在后者中，Sergiaprehensilis在体型较大的个体中表现出较高的Delta值，这与肠道内容物分析(强烈肉食性)的结果相一致。另一种迁飞种四尾无尾轮虫的增量N值与卷尾轮虫相似，但大小差异不大，表明四尾扁尾轮虫从相对年轻的阶段摄取营养水平较高的食物。垂直迁徙的Maurolicus muelleri表现出约1.5倍于环礁属3种非迁徙中层鱼类的Delta^&lt；15&gt；N的个体间变异，表明前者利用了表层和中层的食物资源。综上所述，本研究结果表明，delta值提供了关于个体动物食物特征和食物历史的有用信息，这些特征与个体的其他生态特征，如生长、垂直迁移和分布深度密切相关。

Analysis of trophic levels by using nitrogen and carbon stable-isotope ratios is advantageous to conventional food-habit analysis by gut-content examination, in that the former reflects a long-term food history of an individual animal, while the latter gives a food composition only immediately before capture. The present studies examined inter-individual variability of food-habits of pelagic fishes, meso-and bathypelagic fishes and shrimps in Sagami Bay and adjacent waters, by applying nitrogen and carbon stable-isotope ratio method along with conventional gut-content analysis.In Japanese anchovy, the individual stable-isotope ratios of both larvae and adults were clustered into two groups, with a variation of ca. 1.7% in nitrogen. In meso-and bathypelagic shrimps, Bentheogennema borealis, a non-migrant, deep-sea species, showed significantly lower delta^<13>C and higher delta^<15>N values than the two vertical migrants. Of the latter, Sergiaprehensilis showed higher delta^<15>N values in larger individuals, coinciding to the result of gut-content analysis (strong carnivory). Acanthephyra quadrispinosa, another migrant, showed delta^<15>N values similar to S.prehensilis but difference between sizes was less marked, suggesting that A.quadrispinosa take foods of higher trophic levels from relatively young stages. Maurolicus muelleri, a vertical migrant, showed inter-individual variations of delta^<15>N of ca. 1.5 times those of the three non-migrating mesopelagic fishes of the genus Cyclothone, suggesting that the former utilizes food resources from both the epi-and mesopelagic layers.In summary, the present results demonstrated that the delta values provide useful information on food characteristics and food history of individual animals and that these characteristics are closely related to other ecological characteristics of individuals, such as growth, vertical migration, and distributional depths.