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. 猎物聚群（prey aggregation，如鱼群）会吸引大量捕食者，进而通常形成多物种捕食者类群。这类聚群既是捕食者间竞争加剧的场所，也可能是捕食者间产生协同促进作用的场景。正因如此，此类捕食-猎物聚群的生态功能仍不明确，其对单个捕食者摄食成功率的影响几乎未被探明。
2. 本研究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，直接测定了不同觅食场景下单个南非鲣鸟（Morus capensis）的原位摄食成功率。
3. 我们明确了南非鲣鸟的攻击类型（直接俯冲突入，或俯冲突入后接水下追踪），统计了以南非鲣鸟和长喙真海豚（Delphinus capensis）为主的不同物种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，本研究辅以模拟模型，以评估连续捕食者攻击对猎物聚群结构的累积效应。
4. 南非鲣鸟单次摄食尝试成功捕获鱼类的概率平均为0.28。相较于直接俯冲突入（0.36），当其俯冲后开展水下猎物追踪时，捕获概率更低（0.13）。研究发现，猎物数量对南非鲣鸟的摄食成功率无显著影响，但攻击的时机与频率会对个体摄食成功率产生显著正向影响：在俯冲前1~15秒内无任何攻击发生时，鱼类捕获概率最低（0.16）；而当该时间窗口内出现1~2次攻击时，捕获概率最高（约0.4，即超过两倍）。模拟模型结果显示，猎物聚群会在攻击发生后即刻解体，且连续攻击发起数秒后会达到最大扰动程度。
5. 本研究表明，在多物种捕食者集合群中，多个物种攻击猎物聚群所产生的累积效应（通过破坏鱼群凝聚力）或可提升每个个体的摄食成功率。因此，在这类多物种捕食者类群中，捕食者间的协同促进作用大概率会抵消竞争效应。