Interannual variation of the diet shifts and their effects on the fatness and growth of age‐0 Pacific bluefin tuna (<i>Thunnus orientalis</i>) off the southwestern Pacific coast of Japan

<jats:title>Abstract</jats:title><jats:p>We investigated the effect of feeding habits on the fatness (length–weight relationship (<jats:styled-content style="fixed-case">LW</jats:styled-content>)) and otolith growth of age‐0 Pacific bluefin tuna (<jats:styled-content style="fixed-case">PBF</jats:styled-content>) (fork length (<jats:styled-content style="fixed-case">FL</jats:styled-content>) of 15.0–29.9 cm) collected off the southwestern Pacific coast of Japan from July to August during 2010–2015. The feeding rate on Clupeiformes (<jats:styled-content style="fixed-case">FR</jats:styled-content>c) was substantially influenced by the path of the Kuroshio Current during early summer. We suggest that the following patterns occurred in the feeding habits of <jats:styled-content style="fixed-case">PBF</jats:styled-content>. In 2012 and 2014, the short distance from the coast to the Kuroshio provided high overlap between <jats:styled-content style="fixed-case">PBF</jats:styled-content> and Clupeiformes and increased the opportunity for feeding, and this pattern likely caused the observed diet shift from Crustacea to Clupeiformes by smaller <jats:styled-content style="fixed-case">PBF</jats:styled-content> in the studied years than those in other years. In 2011 and 2015, the encounter probability for Clupeiformes might have been low due to the long distance to the Kuroshio Current, which led <jats:styled-content style="fixed-case">PBF</jats:styled-content> to actively consume non‐Clupeiformes Teleostei. In contrast, in August 2010 and 2015, <jats:styled-content style="fixed-case">PBF</jats:styled-content> did not encounter Clupeiformes or other Teleostei and may have been forced to feed on a large number of Crustacea; this effect was likely due to high water temperatures. Despite the interannual fluctuation in the timing of the diet shift and the feeding intensity (<jats:styled-content style="fixed-case">FI</jats:styled-content>), these factors did not contribute to growth in length; rather, they contributed to fatness. Considering size‐dependent mortality in the larval stage, these results suggest that only fast‐growing individuals survive, even if the feeding conditions drastically change.</jats:p>