Fish otoliths' chronometric properties make them useful for age and growth rate estimation in fisheries management. For the Eastern Baltic Sea cod stock (Gadus morhua), unclear seasonal growth zones in otoliths have resulted in unreliable age and growth information. Here, a new age estimation method based on seasonal patterns in trace elemental otolith incorporation was tested for the first time and compared with the traditional method of visually counting growth zones, using otoliths from the Baltic and North seas. Various trace elemental ratios, linked to fish metabolic activity (higher in summer) or external environment (migration to colder, deeper habitats with higher salinity in winter), were tested for age estimation based on assessing their seasonal variations in concentration. Mg:Ca and P:Ca, both proxies for growth and metabolic activity, showed greatest seasonality and therefore have the best potential to be used as chemical clocks. Otolith image readability was significantly lower in the Baltic than in the North Sea. The chemical (novel) method had an overall greater precision and percent agreement among readers (11.2%, 74.0%) than the visual (traditional) method (23.1%, 51.0%). Visual readers generally selected more highly contrasting zones as annuli whereas the chemical readers identified brighter regions within the first two annuli, and darker zones thereafter. Visual estimates produced significantly higher, more variable ages than did the chemical ones. Based on the analyses in our study, we suggest that otolith micro-chemistry is a promising alternative ageing method for fish populations difficult to age, such as the Eastern Baltic cod. This article is protected by copyright. All rights reserved.

中文翻译：

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寻找真实的时间：探索耳石化学作为年龄测定工具

鱼耳石的计时特性使其可用于渔业管理中的年龄和增长率估计。对于东波罗的海鳕鱼种群 (Gadus morhua)，耳石季节性生长区不明确导致年龄和生长信息不可靠。在这里，首次测试了一种基于微量元素耳石掺入季节性模式的新年龄估计方法，并与使用来自波罗的海和北海的耳石直观计算生长区的传统方法进行了比较。与鱼类代谢活动（夏季较高）或外部环境（冬季迁移到较冷、较深、盐度较高的栖息地）相关的各种微量元素比率，基于评估其浓度的季节性变化进行了年龄估计测试。镁：钙和磷：钙，生长和代谢活动的两个指标都表现出最大的季节性，因此最有可能用作化学钟。波罗的海的耳石图像可读性明显低于北海。与视觉（传统）方法（23.1%、51.0%）相比，化学（新型）方法在读者之间（11.2%、74.0%）具有更高的总体精度和一致性百分比。视觉阅读器通常选择对比度更高的区域作为环，而化学阅读器在前两个环中识别出较亮的区域，然后识别出较暗的区域。与化学估计相比，视觉估计产生了显着更高、更多变的年龄。根据我们研究中的分析，我们认为耳石微化学是一种有前途的替代老化方法，可用于难以老化的鱼类种群，例如东波罗的海鳕鱼。本文受版权保护。版权所有。