Nature
PUBLISHED
How do you monitor the health of a wild marine mammal – or how about 139? For researchers in Hawai‘i working with an endangered false killer whale (Pseudorca crassidens) population, the answer has been drone photography. But the photographs have revealed surprising insights into the body condition of the false killer whales year over year, and unfortunately it's not looking good.
The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.In Hawai‘i lives a resident population of 139 false killer whales, split into four clusters. These whales are adapted to the region's coastal areas and depend on them for survival. Because they are such a small population, the loss of any of the individuals can have knock-on consequences for the rest of the group.
The whales are thought to need to eat around 3-6 percent of their own body weight and experience high demands on their bodies to maintain their temperature and the energetic costs of locomotion. This is why changes in the availability of prey can have a devastating effect on both the body condition of the whales and, in extreme cases, the population.
“This study is a critical step in understanding whether prey limitation is driving the extinction risk for these whales,” explained Jens Currie, Chief Scientist at the Pacific Whale Foundation, Marine Mammal Research Program (MMRP) PhD candidate, and lead author of the study, in a statement.
“Our findings suggest that many individuals are living on a thin metabolic margin. We are now examining how competition with fisheries for high-energy prey like ‘ahi (yellowfin tuna) and mahimahi may be forcing these whales into a state of chronic nutritional stress.”
From 2019 to 2025, 68 individual false killer whales were photographed via drones and assessed across different metrics to track their health and body condition in their different clusters. In some cases the results were extreme; one whale lost 28 percent of its body mass over a two-and-a-half month period, probably amounting to 226 kilograms (500 pounds). This individual belongs to the cluster that has the broadest range, further underscoring the high energetic demands these whales face.
The researchers also found that during an extreme marine heatwave in 2020, the population suffered the biggest drop in overall body condition. To make sure their results were accurate they compared their body metrics with known 3D scans of captive populations of false killer whales and found they had an accuracy within 3 percent.
“This level of precision allows us to pinpoint exactly when and where these whales are struggling, which is key for directing conservation efforts,” notes Lars Bejder, MMRP Director, Hawai‘i Institute of Marine Biology Professor, and co-author of the study.
The team found that the calves in the study had the largest width, reflecting high levels of maternal involvement and high-fat-content milk.
By monitoring the whales over a period of seven years, the team could see shifts in the body conditions of the individuals by age range. Overall, the population is declining at 3.5 percent per year, but this study allows the team to establish a baseline level of condition, helping to monitor the population's health and make decisions surrounding fish stocks, and the survival of the species in the long term.
The study is published in Endangered Species Research.
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