Data from: Empirically testing the influence of light regime on diel activity patterns in a marine predator reveals complex interacting factors shaping behaviour

Diel cycles in marine predator diving behaviour centre around the light-mediated diel vertical migration (DVM) of prey, and are considered critical for optimizing foraging and limiting competition across global seascapes. Yet our understanding of predator diel behaviour is based primarily on examining relative depth usage between constant day/night cycles with no formal investigation of how varying light regimes interact with abiotic factors to shape diel activity. The extreme seasonal light regimes (midnight sun, polar night, day/night cycle) in the Arctic provide a unique natural experimental setting to empirically investigate the occurrence and intensity of diel behaviour in marine predators relative to changing light levels while concomitantly assessing interacting abiotic factors. Depth time series data from satellite-linked tags deployed on six belugas for up to 12 months were used to quantify diel behaviour by calculating dissimilarity in time-at-depth between periods of low and high solar altitude on each day. Generalized additive mixed effects models were used to examine the influence of hours of daylight across extreme light cycles, coupled with bathymetry and sea ice concentration; focal diel patterns were further examined relative to the thermal structure of the water column. As predicted, belugas exhibited cathemerality during the midnight sun, and initiated diel behaviour with the onset of the fall day/night cycle, with a marked increase in its intensity with the progression to equal day/night length. Occurrence of diel patterns, however, was complex; ceasing in regions with seafloor depths < 700 m, and occurring with greatest intensity when the water column was thermally homogeneous within the upper 150 m. Through empirical investigation, this study demonstrates that the onset of day/night light cycles and presumably associated prey DVM can modulate predator diel dive behaviour under certain circumstances, but highlights how the complex interaction of abiotic factors with light regime shape dynamic spatiotemporal patterns. These findings, building on a body of recent work, emphasize that the traditional view of the ubiquitous occurrence of diel behaviour tied to DVM at the base of the food web oversimplifies vertical predator-prey interactions, identifying the need for more structured investigation.

海洋捕食者潜水行为的昼夜节律以猎物的光介导昼夜垂直迁移（DVM）为核心，被认为是优化全球海洋生境中觅食效率、限制种间竞争的关键环节。然而，当前学界对捕食者昼夜行为的认知，主要基于恒定昼夜周期下相对深度利用情况的分析，尚未正式探究变化的光环境如何与非生物因子相互作用，进而塑造捕食者的昼夜活动模式。北极地区存在极端的季节性光环境——极昼、极夜与昼夜交替周期——为实证研究海洋捕食者的昼夜行为发生规律与强度随光照水平的变化情况提供了独特的天然实验场景，同时可同步评估相关非生物因子的交互影响。本研究使用部署于6头白鲸身上、最长记录时长达12个月的卫星追踪标签（satellite-linked tags）获取的深度时间序列数据，通过计算每日太阳高度高低时段内的时间-深度分布差异，量化了白鲸的昼夜行为特征。研究采用广义加性混合效应模型（Generalized Additive Mixed Effects Models），检验了极端光周期下的日照时长、水深地形与海冰浓度的联合影响；同时结合水柱的热结构特征，进一步聚焦分析了目标昼夜行为模式的相关特征。正如研究预期，白鲸在极昼期间表现出泛日性（cathemerality）活动模式，并在秋季昼夜交替周期启动时开始出现昼夜行为，且随着昼夜时长趋于均等，该行为的强度显著提升。不过，昼夜行为模式的出现情况较为复杂：在海底深度小于700米的区域，该模式会消失；而当上层150米水柱呈现热均匀状态时，昼夜行为模式的强度达到最高。通过本次实证研究，本研究证实，昼夜光周期的出现，以及推测中伴随的猎物DVM现象，可在特定条件下调控捕食者的昼夜潜水行为；同时也凸显了非生物因子与光环境的复杂交互如何塑造动态的时空分布格局。本研究成果依托近期一系列相关研究，表明传统观点——即与食物网基础层的DVM相关的昼夜行为普遍存在——过度简化了捕食者与猎物的垂直交互关系，凸显了开展更系统性结构化研究的必要性。