Migratory whales track the phytoplankton spring bloom
It is commonly believed that the world’s largest animals, baleen whales, are fasting during their spring migration towards the summer feeding grounds in the Arctic Ocean. However, a new study by Fleur Visser and Jef Huisman of the University of Amsterdam and international colleagues overthrows this classic paradigm. They observed that baleen whales tune their spring migration to the timing of the phytoplankton spring bloom, and utilize mid-latitude areas as feeding stops en route.
Marine phytoplankton flourishes once temperature, nutrient and light conditions are suitable for growth. Hence, each spring produces a wave of enhanced phytoplankton production across the North Atlantic Ocean, known as the North Atlantic spring bloom. The bloom propagates from subtropical latitudes in late winter to Arctic waters in early summer. Migratory baleen whales in the North Atlantic Ocean follow a similar trajectory in spring, travelling from their subtropical breeding grounds to the summer feeding grounds in the Arctic, a journey spanning several thousands of kilometres. Thus far, baleen whales were thought to be fasting during spring migration, surviving on the fat layer built up during the preceding summer period.
Fleur Visser and colleagues studied baleen whales at the islands of the Azores for several years, and related their whale observations to satellite remote sensing of phytoplankton abundance. They observed peak abundances of fin whales, blue whales, sei whales and humpback whales at 13-16 weeks after the onset of the phytoplankton spring bloom. Moreover, these baleen whales were not fasting, but feasting on swarms of their favourite prey, northern krill (Meganyctiphanes norvegica). Northern krill are known to hatch as small larvae at the onset of the spring bloom and require about 13-16 weeks to mature to a suitable size for foraging baleen whales. Baleen whales did not visit the Azores in autumn, during their southward migration, consistent with the low phytoplankton concentrations during summer and autumn when nutrients have been exhausted.
The results, published in Marine Ecology Progress Series, suggest a new conceptual model of whale migration. As the phytoplankton spring bloom propagates across the North Atlantic Ocean, it induces krill growth, and thereby creates a consecutive series of feeding areas en route that ultimately lead migratory whales to their summer feeding grounds in the Arctic.
Several recent studies indicate that global warming will lead to a stronger and more permanent thermal stratification of the subtropical oceans. Enhanced stratification will reduce the nutrient influx from deep ocean waters into the surface layer, which tends to suppress the phytoplankton spring bloom at subtropical latitudes. According to Visser and colleagues, further monitoring of the migratory behaviour of baleen whales in relation to the phytoplankton spring bloom will therefore be highly desirable.