The study, published January 15 in Global Change Biology, is based on eight years of data on humpback whale pregnancies (2013 to 2020) in waters along the Western Antarctic Peninsula, where krill fishing is concentrated.

Krill availability in the year before a humpback pregnancy is crucial because females need to increase their energy stores to support the upcoming pregnancy. In 2017, after a year in which krill were abundant, 86% of the humpback females sampled were pregnant. But in 2020, following a year in which krill were less plentiful, only 29% of humpback females were pregnant.

Lead author Logan Pallin, a postdoctoral researcher in the Ocean Sciences Department at UC Santa Cruz, said 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, who earned his Ph.D. in ecology and evolutionary biology at UCSC while working on this study. "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."