Adapting to a warmer ocean--seasonal shift of baleen whale movements over three decades

Abstract

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 1 day 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.

Fig 1. The Study area.

The Gulf of St. Lawrence and eastern Canadian waters with 500m bathymetric line representing the shelf edge. The research area is marked as JCP (Jacques Cartier Passage). The other boxes show the extent of the areas for which SST data were used.

Fig 2. Mean arrival and departure date of fin (Bp) and humpback whales (Mn).

The difference between trends in arrival and departure represent average measured residency time.

Fig 3. Mean annual arrival date of fin whales (Bp) and the first week the Gulf of St. Lawrence was ice-free.