Data supplement from Injury-mediated decrease in locomotor performance increases predation risk in schooling fish

The costs and benefits of group living often depend on the spatial position of individuals within groups and the ability of individuals to occupy preferred positions. For example, models of predation events for moving prey groups predict higher mortality risk for individuals at the periphery and front of groups. We investigated these predictions in sardine (<i>Sardinella aurita</i>) schools under attack from group hunting sailfish (<i>Istiophorus platypterus</i>) in the open ocean. Sailfish approached sardine schools about equally often from the front and rear, but prior to attack there was a chasing period in which sardines attempted to swim away from the predator. Consequently, all sailfish attacks were directed at the rear and peripheral positions of the school, resulting in higher predation risk for individuals at these positions. During attacks, sailfish slash at sardines with their bill causing prey injury including scale removal and tissue damage. Sardines injured in previous attacks were more often found in the rear half of the school than in the front half. Moreover, injured fish had lower tail-beat frequencies and lagged behind uninjured fish. Injuries inflicted by sailfish bills may, therefore, hinder prey swimming speed and drive spatial sorting in prey schools through passive self-assortment. We found only partial support for the theoretical predictions from current predator–prey models, highlighting the importance of incorporating more realistic predator–prey dynamics into these models.This article is part of the themed issue ‘Physiological determinants of social behaviour in animals’.

群体生活的成本与收益往往取决于个体在群体内的空间位置，以及个体占据偏好位置的能力。例如，针对移动猎物群体的捕食事件模型预测，处于群体外围与前端的个体面临更高的被捕食致死风险。我们针对开阔大洋中受群猎旗鱼（Istiophorus platypterus）捕食的沙丁鱼（Sardinella aurita）鱼群，对上述预测展开了实证研究。旗鱼从沙丁鱼鱼群的前方与后方发起接近的频率大致相当，但在发起攻击前存在一段追逐阶段，此时沙丁鱼会试图逃离捕食者。因此，所有旗鱼的攻击都瞄准了鱼群的后方与外围位置，使得这些位置的个体承受了更高的捕食风险。攻击过程中，旗鱼会用吻部撕咬沙丁鱼，造成猎物受伤，具体表现为鳞片脱落与组织损伤。相较于鱼群前端，此前遭遇攻击受伤的沙丁鱼更多集中分布在鱼群的后半段。此外，受伤个体的尾拍频率更低，且整体落后于未受伤的同伴。由此可见，旗鱼吻部造成的损伤可能会阻碍猎物的游泳速度，并通过被动自我分选机制推动猎物鱼群内的空间排序。我们仅为当前捕食者-猎物模型的理论预测提供了部分实证支持，这凸显了在这类模型中纳入更贴合现实的捕食者-猎物动态的重要性。本文属于主题特刊《动物社会行为的生理决定因素》的组成部分。