Variation in the isotopic composition of striped weakfish Cynoscion guatucupa of the Southwest Atlantic Ocean in response to dietary shifts

Abstract The aim of this study was to analyze the isotopic composition in muscle of striped weakfish Cynoscion guatucupa from Southwest Atlantic Ocean in order to evaluate a possible variation in δ13C and δ15N in response to dietary shifts that occur as animals grow. We also explored for isotopic evidence of differences between sample locations. The results showed an agreement between isotope analysis and previous conventional studies. Differences in the isotope composition between sampling location were not observed. A positive relation exists between isotope values and total body length of the animals. The Cluster analysis defined three groups of size classes, validated by the MDS. Differences in the relative consumption of prey species in each size class were also observed performing isotope mixing models (SIAR). Variation in δ15N among size classes would be associated with the consumption of a different type of prey as animals grow. Small striped weakfish feed on small crustaceans and progressively increase their consumption of fish (anchovy, Engraulis anchoita), increasing by this way their isotope values. On the other hand, differences in δ13C values seemed to be related to age-class specific spatial distribution patterns. Therefore, large and small striped weakfish remain specialized but feeding on different prey at different trophic levels. These results contribute to the study of the diet of striped weakfish, improve the isotopic ecology models and highlight on the importance of accounting for variation in the isotopic composition in response to dietary shifts with the size of one of the most important fishery resources in the region.摘要 本研究旨在分析西南大西洋海域条纹弱鱼（*Cynoscion guatucupa*）肌肉组织的同位素组成，以探究其δ¹³C与δ¹⁵N比值随个体生长发生食性转变时的潜在变化。本研究同时探寻了不同采样点位间同位素特征差异的相关证据。结果显示，同位素分析结果与既往传统研究结论一致，未观测到不同采样点位间同位素组成存在显著差异。同位素比值与动物总体长呈显著正相关关系。聚类分析划分出3个体长组，该分组结果经多维标度分析（MDS）验证。通过同位素混合模型（SIAR）分析，还发现不同体长组间猎物物种的相对摄食比例存在差异。不同体长组间δ¹⁵N比值的差异，可能与个体生长过程中摄食不同类型猎物相关：小型条纹弱鱼以小型甲壳类为食，随后逐渐增加对鱼类（阿根廷鳀*Engraulis anchoita*）的摄食比例，进而使其同位素比值升高。另一方面，δ¹³C比值的差异似乎与体长组特异性的空间分布模式相关。因此，不同大小的条纹弱鱼虽均为特食性物种，但在不同营养级摄食不同猎物。本研究结果可为条纹弱鱼的食性研究提供参考，完善同位素生态学模型，并强调了针对该区域最重要的渔业资源之一，考虑同位素组成随体长变化（由食性转变驱动）的重要性。