The researchers combined the information with environmental data about factors that affect the whales’ foraging grounds, including the Pacific Decadal Oscillation, a fluctuation in sea temperatures that waxes and wanes about every 20 to 30 years in the Pacific Ocean. Current- and oscillation-driven upwelling is thought to push nutrient-rich phytoplankton into the whales’ foraging grounds, a process likely to be affected by climate change.

Though the whales’ lengths varied, the analysis found that overall they undergo a rapid growth phase between age 10 and 15, with growth slowing afterward. They typically grow to between 38 and 41 feet long at maturity.

But whales born after 2000 were significantly shorter than those born during the 20th century, the researchers found, with a whale born after 2000 likely to reach a maximum length about 5 feet, 4 inches shorter than that of a whale born before 2000. The decline has been greater in females, which are now comparable in length to males.

The length differences do not appear to be inherited, suggesting an environmental cause, the researchers said. However, the exact mechanism driving the shift is unclear.

“In general, size is critical for animals,” Enrico Pirotta, lead author of the study and a researcher at the University of St Andrews, said in a news release. “It affects their behavior, their physiology, their life history, and it has cascading effects for the animals and for the community they’re a part of.” The whales’ reproductive success and ability to adjust to environmental changes could be affected, the researchers noted.

Overall, they concluded, the study is more evidence that large marine predators are becoming smaller over time. Future research should test the potential effects of different climate conditions and see declining average size as a possible warning sign for other problems such as population declines and issues within the whales’ food chain, they added.