Saturday, February 11, 2023

Have we already seen peak humpback whale populations? Maybe.

 Long-time Hawai’i residents have seen the remarkable increase in humpback whale numbers since the 1960s, but that increase could be over.

Whale numbers dropped catastrophically in the middle of the last decade, and there are suggestions that climate changes mean we may have already seen peak whale numbers. 

Although whales are impacted by lots of things, including entanglement with marine debris, swallowing marine debris, overfishing of prey resources and other issues, the key threat may be reduced food availability associated with a warming climate.

A little background.

The numbers of humpback whales got so low that the International Whaling Commission banned humpback hunting in 1966, although a couple of countries continued hunting them for several more years. They were placed on the Endangered Species List in 1970. 

In 1992, the Hawaiian Islands Humpback Whale National Marine Sanctuary was established to add protection to the Hawaiian Islands breeding grounds for the central North Pacific population of humpbacks. 

Today, there are close to 100,000 humpbacks alive globally, living in 14 identified breeding groups that do not often interbreed. At the peak population, before whaling, there may have been 125,000 to 150,000. See this source.

Around Hawai’i, at their lowest, in about 1965, there were only a few hundred animals. Hawai’i residents back then would spot an occasional spout, but nothing like the great shows of spouting and slapping and leaping that we see commonly today. Today the Hawai’i humpback whales number about 12,000 in the Islands.

The animals summer in their feeding grounds off Alaska, where they feed on the huge schools of small fish and krill, a shrimp-like creature. The whales that come to the Hawaiian Islands winter in shallow coastal waters, where they give birth, mate, and feed their young, but do not eat much. They rely on the fat stores from all that krill they have eaten during the summers up north.

But after decades of increase from their low numbers in the 1960s, the whale numbers stumbled 10 years ago, particularly from 2015 to 2016.

“Numbers then declined, including a precipitous 60% drop between 2015 and 2016,” wrote Adam Frankel, Christine Gabriele, Susanne Yin and Susan Rickards, of the Hawai’i Marine Mammal Consortium in Kamuela.

It seemed to have been linked to warming waters in the feeding grounds. An extended warm period described as “the largest marine heatwave event ever recorded in the Northeast Pacific Ocean” started in late 2013. It caused serious disruption to the ocean ecosystem. These authors didn’t know exactly how that impacted whales, but they suggested that the temperature changes impacted the food web, reducing humpback feeding success.

They are not alone. The authors of this 2022 paper said that North Pacific Heat Wave was associated with fewer surviving female whales, fewer calves, and a lower survival among those calves that were born.

“Calf survival dropped tenfold,” the authors said, and older animals also were impacted. They said “documented changes to the forage fish and zooplankton prey base” were the likely suspect.

Now, a new paper seems to confirm that hypothesis—although in the Antarctic rather than the North Pacific. It was published last month in the journal Global Change Biology, under the title “A surplus no more? Variation in krill availability impacts reproductive rates of Antarctic Baleen Whales.” 

The authors found that “krill availability is in fact limiting and affecting reproductive rates” and that humpbacks “may be at a threshold for population growth.” They said similar issues are occurring with several species of whales, as krill numbers decline in their traditional grounds, and some of them move to different parts of the sea.

That heat wave of a decade ago ended. But researchers say such heat waves have been increasing and are going to be happen more often. And that’s not good for the humpback population.

“Climatic extremes are becoming increasingly common against a background trend of global warming. In the oceans, marine heatwaves—discrete periods of anomalously warm water—have intensified and become more frequent over the past century, impacting the integrity of marine ecosystems globally,” wrote the authors of the 2023 Annual Review of Marine Science.  

These marine heat waves, the authors write, “are emerging as pervasive stressors to marine ecosystems globally.”

© Jan TenBruggencate 2023

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