Adapting to a Warmer Ocean—Seasonal Shift of Baleen Whale Movements over Three Decades

Global warming poses particular challenges to migratory species, which face changes to the multiple environments occupied during migration. For many species, the timing of migration between summer and winter grounds and also within-season movements are crucial to maximise exploitation of temporarily abundant prey resources in feeding areas, themselves adapting to the warming planet. We investigated the temporal variation in the occurrence of fin (Balaenoptera physalus) and humpback whales (Megaptera novaeangliae) in a North Atlantic summer feeding ground, the Gulf of St. Lawrence (Canada), from 1984 to 2010 using a long-term study of individually identifiable animals. These two sympatric species both shifted their date of arrival at a previously undocumented rate of more than 1day per year earlier over the study period thus maintaining the approximate 2-week difference in arrival of the two species and enabling the maintenance of temporal niche separation. However, the departure date of both species also shifted earlier but at different rates resulting in increasing temporal overlap over the study period indicating that this separation may be starting to erode. Our analysis revealed that the trend in arrival was strongly related to earlier ice break-up and rising sea surface temperature, likely triggering earlier primary production. The observed changes in phenology in response to ocean warming are a remarkable example of phenotypic plasticity and may partly explain how baleen whales were able to survive a number of changes in climate over the last several million years. However, it is questionable whether the observed rate of change in timing can be maintained. Substantial modification to the distribution or annual life cycle of these species might be required to keep up with the ongoing warming of the oceans.

全球变暖对迁徙物种构成了特殊挑战，这些物种在迁徙过程中所占据的多种环境发生了变化。对于许多物种而言，在夏季和冬季栖息地之间以及季节内迁徙的时间点至关重要，这是为了充分利用临时丰富的猎物资源，而这些资源本身正在适应变暖的地球。我们通过长期研究个体可识别的动物，对1984年至2010年间在北大西洋夏季觅食地——圣劳伦斯湾（加拿大）——出现的鳍鲸（Balaenoptera physalus）和座头鲸（Megaptera novaeangliae）的时间变化进行了调查。这两种同域物种在研究期间以先前未记录的每年超过1天的速度提前了到达日期，从而保持了两种物种到达时间的约两周差异，并使时间生态位分离得以维持。然而，两种物种的离港日期也提前了，但速度不同，导致在研究期间时间重叠不断增加，这表明这种分离可能开始受到侵蚀。我们的分析显示，到达趋势与更早的冰融和上升的海表温度密切相关，这很可能是触发初级生产提前的原因。对海洋变暖反应的物候变化是表型可塑性的显著例证，可能部分解释了须鲸如何在过去数百万年的气候变化中幸存下来。然而，观察到的这种时间变化的速度是否能够维持是值得怀疑的。为了跟上海洋持续变暖的步伐，这些物种的分布或年度生命周期可能需要进行实质性的调整。