The decline in grey and humpback whales in the Pacific and Atlantic Oceans has been traced to food shortages caused by rising ocean temperatures.
Between December and February every year, thousands of grey whales (Eschrichtius robustus) migrate from the cold waters of the northern Pacific Ocean to the warmer ones of Baja California, in Mexico, to give birth to their cubs. Before embarking on this strenuous journey, mothers need to accumulate large amounts of energy, mainly stored as fat. However, due to ocean warming populations of amphipods, small crustaceans that are the base of their diet, are declining dramatically. As a result, whales don’t have enough food and many of them fail to finish their voyage or die before returning home.
This analysis was presented by Fredrik Christiansen from the University of Aarhus in Denmark, and Lars Bejder from the University of Hawaii in Manoa. The two researchers began monitoring the state of cetaceans in 2017 by using drones. In 2018 they found their condition was deteriorating, coinciding with a decline in their reproduction rate.
That same year, what scientist call an unusual mortality event began; now in its third consecutive year, it has led to the death of 378 specimens. “It appears that a large number of grey whales are leaving their feeding grounds already in a poor nutritional state and by the time they’ve completed the breeding season in Mexico they’ve depleted their energy reserves and starve to death,” Christiansen explains. Further research will be carried out to prove this thesis with certainty.
A similar situation has been recorded in the Atlantic Ocean, where the decline is affecting humpback whales (Megaptera novaeangliae). In the Gulf of Saint Lawrence on the Canadian coast, in the last 15 years births have declined significantly. The culprit in this case too is global warming, which is reducing the number of herring available for females to feed on. According to Joanna Kershaw, who leads the team that surveyed the area, without enough food mothers are struggling to breastfeed their cubs.
Protecting whales is fundamental both to preserve the well-being of the ecosystems they exist in and because they represent a precious weapon against climate change. Their dung, in fact, fertilises phytoplancton, which plays a crucial role in regulating the climate as it absorbs carbon dioxide and, at the end of its lifecycle, sinks into the abyss, dragging carbon along with it.
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