The Osumi Strait and its neighboring waters are major spawning grounds for commercially important small pelagic and coastal fish in the Pacific coastal area of Japan. Mesopelagic fish larvae are also abundant here, co-occurring with the larvae of commercial species. However, it is unknown whether these fish groups compete for prey. We examined the diet of four commercial fish groups ( Engraulis japonicus , Trachurus japonicus , Triglidae spp., and Sebastiscus spp.) and two mesopelagic species ( Sigmops gracilis , Myctophum asperum ) during their larval stages to determine whether they had the same prey source. Morphological analysis and DNA metabarcoding of gut contents showed that the main prey species of all six fish groups were calanoid copepods. However, results from metabarcoding differed greatly from morphological analysis, showing that appendicularians were abundant in the guts of S. gracilis , M. asperum , and Sebastiscus spp. This may be because the methods differ in how they determine prey composition, with morphological analysis relying on counts of identifiable prey parts and metabarcoding being only a semi-quantitative method. In addition, appendicularians might be underestimated by metabarcoding analysis due to variation in genome size, copy number of target genes, and primer mismatches. Stable isotope analysis supported the importance of appendicularians as prey for S. gracilis , M. asperum , and Sebastiscus spp. Our results indicate that the two mesopelagic species and Sebastiscus spp. may compete for prey, but E. japonicus , T. japonicus, and Triglidae spp. occupy niches different from those of the two mesopelagic species. Our results imply that the trophic pathway via appendicularians may support the feeding of dominant mesopelagic species and enable coexistence with commercial species in the study area.

中文翻译：

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通过形态学、DNA 宏条形码和稳定同位素分析评估的大隅海峡中层和商业上重要鱼类共同出现的幼虫阶段的饮食生态位分离

大隅海峡及其邻近水域是日本太平洋沿岸地区具有重要商业价值的小型远洋和沿海鱼类的主要产卵场。中层鱼类幼体在这里也很丰富，与商业物种的幼体同时发生。然而，这些鱼群是否会争夺猎物尚不得而知。我们检查了四种商业鱼类（Engraulis japonicus、Trachurus japonicus、Triglidae spp. 和 Sebastiscus spp.）和两种中层鱼类（Sigmops gracilis、Myctophum asperum）在幼虫阶段的饮食，以确定它们是否有相同的猎物来源。肠道内容物的形态学分析和 DNA 元条形码显示，所有六个鱼群的主要猎物物种都是桡足类。然而，元条形码的结果与形态学分析有很大不同，表明附肢在 S. gracilis 、 M. asperum 和 Sebastiscus spp 的内脏中含量丰富。这可能是因为这些方法在确定猎物成分的方式上有所不同，形态分析依赖于可识别猎物部位的计数，而元条形码只是一种半定量方法。此外，由于基因组大小、靶基因拷贝数和引物错配的变化，元条形码分析可能会低估阑尾。稳定同位素分析支持附肢作为 S. gracilis、M. asperum 和 Sebastiscus spp 猎物的重要性。我们的结果表明，这两种中层海洋物种和 Sebastiscus spp。可能会争夺猎物，但 E. japonicus 、 T. japonicus 和 Triglidae spp。占据与两种中层海洋物种不同的生态位。