ABSTRACT: Yellowfin tuna Thunnus albacares is a highly exploited species in the Indian Ocean. Yet, its stock structure is still not well understood, hindering assessment of the stock at a suitable spatial scale for management. Here, young-of-the-year (<4 mo) yellowfin tuna otoliths were collected in 2018 and 2019, from 4 major nursery areas in the Indian Ocean: Madagascar, Seychelles-Somalia, Maldives and Sumatra. First, direct age estimates were made in a subset of otoliths by visually counting microincrements to identify the portion of the otolith corresponding to the larval stage. We then developed 2-dimensional maps of trace element concentrations to examine spatial distribution of elements across otolith transverse sections. Different distribution patterns were observed among the elements analysed; Li, Sr and Ba were enriched in the portion of the otolith representing early life, whereas Mn and Mg concentrations were heterogeneous across growth bands. Last, we analysed inter-annual and regional variation in otolith chemical composition using both trace elements (Li, Mg, Sr, Ba and Mn) and stable isotopes (δ¹³C and δ¹⁸O). Significant regional variation in otolith chemical signatures was detected among nurseries, except between Madagascar and Seychelles-Somalia. Otolith δ¹³C and δ¹⁸O were important drivers of differentiation between western (Madagascar and Seychelles-Somalia), Maldives and Sumatra nurseries, whereas the elemental signatures were cohort specific. Overall nursery assignment accuracies were 69-71%. The present study demonstrates that baseline chemical signatures in the otoliths of yellowfin tuna are regionally distinct and can be used as a natural tag to investigate the nursery origin of older individuals in the Indian Ocean.

中文翻译：

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使用耳石化学鉴别印度洋育苗区之间的黄鳍金枪鱼 Thunnus albacares

摘要：黄鳍金枪鱼Thunnus albacares是印度洋高度开发的物种。然而，其种群结构仍不清楚，阻碍了在合适的空间尺度上对种群进行评估以进行管理。在这里，2018 年和 2019 年从印度洋的 4 个主要育苗区收集了当年（<4 个月）黄鳍金枪鱼耳石：马达加斯加、塞舌尔-索马里、马尔代夫和苏门答腊。首先，通过目视计算微增量来确定与幼虫阶段相对应的耳石部分，对耳石子集进行直接年龄估计。然后，我们开发了微量元素浓度的二维图，以检查耳石横截面中元素的空间分布。在所分析的元素中观察到不同的分布模式；Li、Sr 和 Ba 在代表早期生命的耳石部分中富集，而 Mn 和 Mg 浓度在整个生长带中是不均匀的。最后，我们使用微量元素（Li、Mg、Sr、Ba 和 Mn）和稳定同位素（δ¹³ C和δ ¹⁸ O)。除了马达加斯加和塞舌尔-索马里之外，在托儿所中检测到耳石化学特征的显着区域差异。耳石 δ ¹³ C 和 δ ¹⁸ O 是西部（马达加斯加和塞舌尔-索马里）、马尔代夫和苏门答腊苗圃之间分化的重要驱动因素，而元素特征是特定于群体的。总体托儿所分配准确率为 69-71%。本研究表明，黄鳍金枪鱼耳石中的基线化学特征在区域上是不同的，可用作研究印度洋老年人苗圃起源的自然标记。