Data from: How to capture fish in a school? Effect of successive predator attacks on seabird feeding success

1. Prey aggregations, such as fish schools, attract numerous predators. This typically leads to the formation of multi-specific groups of predators. These aggregations can be seen both as a place of increased competition and as a place of possible facilitation between predators. Consequently, the functional role of such predator-prey aggregation is uncertain, and its effect on individual feeding success is virtually unknown. 2. Using underwater film footages of different predators feeding on fish schools during the sardine run in South Africa, we directly measured the in-situ feeding success of individual Cape gannets Morus capensis in different foraging situations. 3. We determined the types of Cape gannet attacks (direct plunge dive or plunge dive followed by underwater pursuit) and we measured the occurrences and timing of attacks from the different species (mostly Cape gannets and long-beaked common dolphins Delphinus capensis). We also estimated the size of the targeted fish schools. These observations were complemented with a simulation model to evaluate the cumulative effect of successive predator attacks on the prey aggregation structure. 4. The probability to capture a fish in one feeding attempt by Cape gannets averaged 0.28. It was lower when gannets engaged in underwater prey pursuit after the plunge compared to direct plunge (0.13 vs. 0.36). We found no effect of the number of prey on gannets’ feeding success. However, the timing and frequency of attacks influenced strongly and positively the feeding success of individuals. The probability to capture a fish was the lowest (0.16) when no attack occurred in the few seconds (1-15 s) prior to a dive, and the highest (~0.4, i.e. more than twice) when one or two attacks occurred during this time window. The simulation model showed that a prey aggregation disorganized just after an attack, and that the maximum of disturbance was obtained a few seconds after the initiation of the successive attacks. 5. Our study suggests that, in multi-species predator assemblages, the cumulative effect (through disorganisation of school cohesiveness) of the multiple species attacking a prey aggregation may increase the feeding success of each individual. Therefore, facilitation between predators is likely to overcome competition in these multi-specific assemblages.

1. 猎物集群（prey aggregations），例如鱼群（fish schools），会吸引大量捕食者。此类场景通常会催生多物种捕食者群落的形成。这类集群既是捕食者间竞争加剧的场所，也可能存在捕食者之间的互利易化作用。因此，这类捕食者-猎物集群的功能角色尚未明确，其对个体捕食成功率的影响几乎未被探明。
2. 借助南非沙丁鱼大洄游期间不同捕食者捕食鱼群的水下影像资料，我们直接测定了不同觅食场景下，个体南非鲣鸟（Cape gannets Morus capensis）的原位捕食成功率。
3. 我们记录了南非鲣鸟的攻击类型（直接俯冲入水，或俯冲入水后伴随水下追踪），统计了不同物种（主要为南非鲣鸟与长吻真海豚（long-beaked common dolphins Delphinus capensis））的攻击发生频次与攻击时机，同时估算了目标鱼群的规模。此外，我们结合仿真模型评估了连续捕食者攻击对猎物集群结构的累积效应。
4. 南非鲣鸟单次捕食尝试的成功捕获概率平均值为0.28。相较于直接俯冲入水（0.36），俯冲入水后伴随水下追踪的捕食成功率更低（0.13）。我们未观测到猎物数量对南非鲣鸟捕食成功率存在显著影响，但攻击时机与攻击频率对个体捕食成功率具有显著的正向影响。在俯冲前1至15秒内无任何攻击发生时，鱼类捕获概率最低（0.16）；而在该时间窗口内出现1至2次攻击时，捕获概率最高（约0.4，即两倍以上）。仿真模型结果显示，猎物集群会在攻击发生后立即解体，且连续攻击启动数秒后会达到最大扰动程度。
5. 本研究表明，在多物种捕食者群落中，多物种攻击猎物集群所产生的累积效应（通过瓦解鱼群凝聚力）或可提升每个个体的捕食成功率。因此，在这类多物种捕食者群落中，捕食者间的互利易化作用很可能超越种间竞争。