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Humpback whale reproduction is linked to krill abundance

Each year, humpback whales migrate from warmer, temperate waters to the icy Antarctic to gorge themselves on krill. These small, shrimp-like crustaceans are rich in nutrients and in the southern summer they become so abundant that the whales’ journey is well rewarded. The frigid waters become so crowded with krill that they were once considered an unlimited source of food, especially in the light of the removal of more than 2 million baleen whales from the Southern Ocean during the 20th century.

However, as with most ecosystems, the components of the Antarctic food webs are interconnected and none of them is inexhaustible. There is no surplus of krill, especially as whale populations have shown recovery and as global warming begins to change the Antarctic ice sheets.

In new collaborative research led by scientists from the University of California, Santa Cruz, the rate of pregnancy in a population of humpback whales was determined over an eight year period (2013 to 2020). Biopsy samples taken from the whales found off the West Antarctic Peninsula were used for genetic analysis, and to determine the levels of hormones, particularly progesterone. The results, published in the journal Global Change Biology, indicate that the proportion of females that were pregnant varied considerably each year over the research period. 

When the researchers linked the pregnancy rates to the abundance of krill in the waters of the West Antarctic Peninsula, they found that more females were pregnant in years following seasons of krill abundance. In 2017, after a year in which krill populations flourished, 86 percent of the female humpbacks were pregnant. However, in 2020, following a year in which krill populations were less successful, the pregnancy rate plummeted to only 29 percent. 

Humpback whales only feed in the Southern Ocean for a few months of the year, and the females fast for most of the rest of the time, even while they are pregnant and nursing calves. This means that the nutrient and energy boost obtained from feeding on thousands of tons of krill during the brief summer visits to the Antarctic, is critical for reproductive success. 

According to lead author Logan Pallin, the study demonstrates for the first time the link between population growth and krill availability in Antarctic whales.

“This is significant because, until now, it was thought that krill were essentially an unlimited food source for whales in the Antarctic,” said Pallin. “Continued warming and increased fishing along the Western Antarctic Peninsula, which continue to reduce krill stocks, will likely impact this humpback whale population and other krill predators in the region.”

Professor Ari Friedlaender noted that the Western Antarctic Peninsula is experiencing some of the fastest climatic warming of any region on the planet. Winter air temperatures have risen significantly since the 1950s, and the annual sea ice extent is, on average, 80 days shorter than four decades ago. These conditions are likely to affect the survival of krill populations negatively.

“Krill supplies vary depending on the amount of sea ice because juvenile krill feed on algae growing on sea ice and also rely on the ice for shelter,” Friedlaender said. “In years with less sea ice in the winter, fewer juvenile krill survive to the following year. The impacts of climate change, and likely the krill fishery, are contributing to a decrease in humpback whale reproductive rates in years with less krill available for whales.”

In recent decades, people have found uses for krill as a food source and medicinal treatments, and there is now a commercial fishery operating in the waters off the West Antarctic Peninsula. The total global harvest of krill from all fisheries is about 150–200,000 tons a year, though only a small percentage of this is for human consumption. The majority is utilized as fish food in aquaculture, bait, and livestock and pet food. Krill oil and enzymes are also used in various medicinal treatments.

Study co-author Chris Johnson is the global lead of the World Wide Fund for Nature’s Protecting Whales & Dolphins Initiative. He said this research shows that highly precautionary management measures are needed to protect all Antarctic marine life that depends on krill for its survival, including blue, fin, humpback, minke, and southern right whales, as well as other krill predators such as penguins, seabirds, seals, and fish.  

Furthermore, understanding how climate-mediated variation in prey availability influences humpback whale population dynamics is critical for focused management and conservation actions, particularly in light of changing temperatures in the Antarctic. 

“This information is critical as we can now be proactive about managing how, when, and how much krill is taken from the Antarctic Peninsula,” added Pallin. “In years of poor krill recruitment, we should not compound this by removing krill from critical foraging areas for baleen whales.”

“Krill are not an inexhaustible resource, and there is a growing overlap between industrial krill fishing and whales feeding at the same time,” Johnson said. “Humpback whales feed in the Antarctic for a handful of months a year to fuel their annual energetic needs for migration that spans thousands of kilometers. We need to tread carefully and protect this unique part of the world, which will benefit whales across their entire range.”

By Alison Bosman, Staff Writer

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