Data for: Patrolling the border: billfish exploit the hypoxic boundary created by the world's largest oxygen minimum zone

Pelagic predators must contend with low prey densities that are irregularly distributed and dynamic in space and time. Based on satellite imagery and telemetry data, many pelagic predators will concentrate horizontal movements on ephemeral surface fronts—gradients between water masses—because of enhanced local productivity and increased forage fish densities. Vertical fronts (e.g., thermoclines, oxyclines) can be spatially and temporally persistent and aggregate lower trophic level and diel vertically migrating organisms due to sharp changes in water density or available oxygen. Thus, vertical fronts represent a stable and potentially energy-rich habitat feature for diving pelagic predators but remain little explored in their capacity to enhance foraging opportunities. Here, we use a novel suite of high-resolution biologging data, including in situ derived oxygen saturation and video, to document how two top predators in the pelagic ecosystem exploit the vertical fronts created by the oxygen minimum zone of the eastern tropical Pacific. Prey search behavior was dependent on dive shape, and significantly increased near the thermocline and hypoxic boundary for blue marlin (Makaira nigricans) and sailfish (Istiophorus platypterus), respectively. Further, we identify a behavior not yet reported for pelagic predators, whereby the predator repeatedly dives below the thermocline and hypoxic boundary (and by extension, below the prey). We hypothesize this behavior is used to ambush prey concentrated at the boundaries from below. We describe how habitat fronts created by low-oxygen environments can influence pelagic ecosystems, which will become increasingly important to understand in the context of global change and expanding oxygen minimum zones. We anticipate that our findings are shared among many pelagic predators where strong vertical fronts occur, and additional high-resolution tagging is warranted to confirm this.

远洋捕食者（pelagic predators）必须应对空间与时间上分布不均且动态变化的低猎物密度环境。基于卫星影像与遥测数据，诸多远洋捕食者会将水平运动集中于临时性表层锋面——水团间的梯度区域——这是因为该区域本地生产力提升、饵料鱼密度更高。垂直锋面（vertical fronts，如温跃层thermoclines、氧跃层oxyclines）可在空间与时间上持续存在，并因水体密度或可用氧气的剧烈变化，聚集低营养级生物与昼夜垂直迁移生物。因此，垂直锋面对于潜水型远洋捕食者而言，是一类稳定且潜在能量富集的生境特征，但其对捕食机会的提升作用却鲜有研究。本研究采用一套全新的高分辨率生物记录数据（biologging data），包括原位测得的氧饱和度与视频数据，以此阐明远洋生态系统中的两类顶级捕食者如何利用东热带太平洋（eastern tropical Pacific）低氧区（oxygen minimum zone）形成的垂直锋面。研究发现，猎物搜寻行为与潜水形态相关，其中蓝枪鱼（Makaira nigricans）的搜寻行为在温跃层附近显著增强，而旗鱼（Istiophorus platypterus）则在低氧边界附近显著提升。此外，本研究还发现了一种此前未在远洋捕食者中被报道过的行为：捕食者会反复潜入温跃层与低氧边界下方（进而抵达猎物下方）。我们推测该行为是为了从下方伏击聚集在锋面边界处的猎物。本研究阐明了低氧环境形成的生境锋面如何影响远洋生态系统，而在全球变化与低氧区持续扩张的背景下，理解这一过程的重要性将日益凸显。我们预计，在存在强垂直锋面的诸多远洋捕食者类群中，本研究的发现具有普适性，未来需开展更多高分辨率标记研究以验证这一推测。