It has been known that small fish often swim in front of large fish, but the reason for this remains unclear. We hypothesized that small fish could swim more efficiently in front of large fish than when swimming singly. Computational fluid dynamical (CFD) analysis was conducted to verify this hypothesis. We calculated and compared the drag that affects small fish (bigeye trevally Caranx sexfasciatus) when swimming alone and that when tandem swimming in front of large fish (whale shark Rhincodon typus); the analysis was performed by changing the distance between the two fish. The results show that the drag coefficient of Caranx sexfasciatus when tandem swimming is smaller than the drag coefficient when swimming singly, and as the distance decreases, so does the drag coefficient. As the mechanism of this phenomenon, the stagnation point located in front of the large fish (Rhincodon typus) reduces the pressure difference between the small fish's (Caranx sexfasciatus) front and rear. The strength of the pressure drag depends on the pressure difference before and after the object. Therefore, we consider it to be the reason for the reduced drag on Caranx sexfasciatus. Moreover, the relationship between distance and drag reduction effect was the exact opposite of a previous study. This is most likely due to the fact that the Reynolds number, body-length ratio, and dimensions of analysis are different from previous studies. This paper suggests that Caranx sexfasciatus swims in front of Rhincodon typus to reduce their travel cost and insights for a more detailed understanding of tandem swimming.

众所周知，小鱼经常在大鱼面前游来游去，但其原因尚不清楚。我们假设小鱼在大鱼面前比单独游泳时更有效。进行了计算流体动力学 (CFD) 分析以验证这一假设。我们计算并比较了单独游泳时影响小鱼（bigeye trevally Caranx sexfasciatus）和在大鱼（鲸鲨Rhincodon typus）面前串联游泳时的阻力；通过改变两条鱼之间的距离来进行分析。结果表明, Caranx sexfasciatus的阻力系数双人游泳时的阻力系数小于单人游泳时的阻力系数，随着距离的减小阻力系数也随之减小。作为这种现象的机制，位于大鱼（Rhincodon typus）前面的停滞点降低了小鱼（Caranx sexfasciatus）前后之间的压力差。压力阻力的强弱取决于物体前后的压力差。因此，我们认为这是减少对Caranx sexfasciatus 的阻力的原因. 此外，距离和减阻效果之间的关系与之前的研究完全相反。这很可能是因为雷诺数、体长比和分析维度与以前的研究不同。本文建议Caranx sexfasciatus在Rhincodon typus前面游泳，以减少它们的旅行成本和洞察力，以便更详细地了解双人游泳。