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）必须应对分布不规则且时空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移动集中于短暂的海面锋面——不同水团间的梯度带——这是由于锋面区域本地生产力提升且饵料鱼密度更高。垂直锋面（如温跃层（thermoclines）、氧跃层（oxyclines））可在时空上保持稳定，因水体密度或可利用氧气的剧烈变化，会聚集低营养级生物与昼夜垂直迁移生物（diel vertically migrating organisms）。因此，垂直锋面对于潜水型远洋捕食者而言，是一处稳定且潜在能量富集的生境特征，但其在提升觅食机会方面的作用仍鲜有探索。本研究借助一套全新的高分辨率生物记录数据（biologging data）——包括原位获取的氧饱和度数据与视频资料——阐明了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氧最小值带（oxygen minimum zone）形成的垂直锋面。猎物搜索行为与潜水形态相关，蓝枪鱼（Makaira nigricans）和旗鱼（Istiophorus platypterus）的搜索行为分别在温跃层与低氧边界附近显著增强。进一步，我们发现了一种此前未见报道的远洋捕食者行为：捕食者会反复潜至温跃层与低氧边界下方（进而处于猎物下方）。我们推测该行为用于从下方伏击聚集在锋面边界的猎物。本研究阐述了低氧环境形成的生境锋面如何影响远洋生态系统，在全球变化与氧最小值带不断扩张的背景下，理解这一过程的重要性与日俱增。我们预计该研究发现可推广至存在强垂直锋面的诸多远洋捕食者类群，未来需开展更多高分辨率标记研究以验证这一推测。