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Taking a positive approach to the gray whale die-off

The stranding of more than 200 gray whales on the Pacific coast last year has been classified as an unusual mortality event by the NOAA Fisheries. Apart from frequent signs of emancipation, the overall cause of the die-off could not yet be determined, causing scientists and the general public to worry about the whale population and the migratory process of this year.


Bruce Mate, a marine mammal researcher at Oregon State University who has conducted research on the animals for more than 50 years, has a different idea on the topic. He and his colleagues had started tracking the gray whales in 2005 to follow their journey from the breeding grounds in Mexico to their feeding grounds in the North via satellites.


Based on his observations, he has a different approach to the recent gray whale die-off. Several years of research have shown that the animals did not just migrate to the Alaskan Bering Sea as usual but moved beyond into the high arctic. Similar phenomena occurred for western populations that usually fed around the east coast of Russia, before moving down along the Asian coast. Instead, they kept travelling north, eventually ending up in the Bering Sea and even in Mexico like the eastern North Pacific Whales. One animal even travelled a round-trip of 12,500 miles (20,112 km) without stopping – the longest migration ever recorded for a mammal – to feed substantially anywhere.


Mate suggests food depletion as the reason for this unexpected behavior. The animals move past their usual feeding grounds to explore more environments to find food. Getting enough nutritional intake during the feeding season is essential, as the mothers make their way to Mexico to bear young and travel all the way back without eating. An inability to forage enough and build sufficient energy storages would explain the increased mortality, decreased calving rate and the emancipation signs.


The gray whale population is thought to have been reduced to 1,000 to 2,000 whales in 1900 due to commercial whaling. Thanks to protections against over-exploitation and later adoption under the Endangered Species Act, the population was able to rebound. During their last big mortality event in 1999, annual counts estimated a population of 20,000 animals – likely close to the original population size. Currently, the population is estimated at about 27,000 animals. Mate assumes that this number might simply be too high for the ecosystem to support. The animals cannot feed sufficiently to go through the whole migration while bearing the calves. Therefore, many either die or go without calving altogether.


However, he states that instead of a “woe for the great whales, we should think about this adjustment as the result of the species’ successful recovery and having overshot their carrying capacity as they find a population that is suitable over an extended period of time”. After the natural regulations, the grey whale population will likely level out at a number substantially lower than the current population – with around 20,000 animals being the estimated carrying capacity the Bering Seafood could sustain. He therefore likes to “see it as a celebration of this animal returning to its proper place in the marine ecosystem”.

Marine mammal expert Bruce Mate sees last year’s gray whale die-off as sign that the species has rebounded.

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