Risso's dolphins perform spinning dives to target deep-dwelling prey

Foraging decisions of deep-diving cetaceans can provide fundamental insight into food web dynamics of the deep pelagic ocean. Cetacean optimal foraging entails a tight balance between oxygen-conserving dive strategies and access to deep-dwelling prey of sufficient energetic reward. Risso's dolphins (Grampus griseus) displayed a thus far unknown dive strategy, which we termed the spin dive. Dives started with intense stroking and right-sided lateral rotation. This remarkable behaviour resulted in rapid descent. By tracking the fine-scale foraging behaviour of 7 tagged individuals, matched with prey-layer recordings, we tested the hypothesis that spin dives are foraging dives targeting deep-dwelling prey. Hunting depth traced the diel movement of the deep scattering layer, a dense aggregation of prey, that resides deep during the day and near-surface at night. Individuals shifted their foraging strategy from deep spin dives to shallow non-spin dives around dusk. Spin dives were significantly faster, steeper and deeper than non-spin dives, effectively minimizing transit time to bountiful mesopelagic prey, and were focussed on periods when the migratory prey might be easier to catch. Hence, whereas Risso's dolphins were mostly shallow, nocturnal foragers, their spin dives enabled extended and rewarding diurnal foraging on deep-dwelling prey.深潜鲸类（deep-diving cetaceans）的觅食决策，可为深海远洋（deep pelagic ocean）食物网的动态特征提供基础性认知。鲸类的最优觅食策略需要在保氧潜水策略与获取能量报酬充足的深海栖息猎物之间达成精细平衡。灰海豚（Risso's dolphins，学名Grampus griseus）展现出一种此前未被报道的潜水行为模式，我们将其命名为旋转潜水（spin dive）。此类潜水以剧烈划动与右侧侧向旋转为起始动作，可实现快速下潜。我们通过追踪7只佩戴追踪标记个体的精细尺度觅食行为，并同步结合猎物水层的观测记录，验证了"旋转潜水为针对深海栖息猎物的觅食潜水"这一假说。个体的狩猎深度匹配深海散射层（deep scattering layer）的昼夜迁移规律——该层为密集的猎物聚群，白天栖息于深水层，夜间则移动至近表层水域。个体的觅食策略会在黄昏前后从深潜旋转潜水转变为浅潜非旋转潜水。旋转潜水的速度、下潜倾角与深度均显著高于非旋转潜水，可有效缩短抵达丰饶中层带猎物栖息环境的通行时间，且此类潜水集中在迁徙猎物更易被捕食的时段开展。因此，尽管灰海豚原本多为夜间浅水区觅食者，但其旋转潜水行为使其能够在日间对深海栖息猎物开展持续且收益充足的觅食活动。