Hawaiʻi’s Last False Killer Whales Are Starving in a Warming Sea

From a few hundred feet up, a drone can read a whale’s body the way a doctor reads a chart. Width at the head, width across the midsection, the long taper toward the flukes: feed those numbers into the right equations and you get volume, and from volume, an estimate of mass. One false killer whale off the main Hawaiian Islands gave up a number that stopped the researchers in their tracks. Over roughly ten weeks, it had shed about 28 percent of its body mass, something on the order of 500 pounds, vanished from a single animal in two and a half months.

That whale belongs to one of the rarest populations in American waters. Fewer than 140 insular false killer whales remain around the islands, and the number falls by an average of 3.5 percent every year.

For a long time, nobody could say with any precision how the survivors were actually faring. False killer whales are not, despite the name, killer whales at all; they are large oceanic dolphins, fast and far-ranging, and they do not hold still for a vet. So a team led by Jens Currie of the Pacific Whale Foundation and the Marine Mammal Research Program at the University of Hawaiʻi at Mānoa went up instead of in. Between 2019 and 2025 they used high-resolution aerial photogrammetry to size up 68 individuals, roughly half the entire population, across 142 separate measurements. The result, published in Endangered Species Research, is the first real ledger of how much these animals weigh and how that weight rises and falls.

The trouble is that it falls rather a lot. And the worst year on record was not subtle.

The Year the Ocean Ran Hot

The population’s body condition index, a rough measure of how plump or how lean an animal is for its length, hit its lowest point in 2020. That same year saw the largest single-year drop in the population in recent memory, and record sea surface temperatures rolled across the region during a severe marine heatwave. Three things lining up in the same calendar year does not prove cause, but it is hard to look at and call it coincidence.

“Our findings suggest that many individuals are living on a thin metabolic margin,” says Currie, who is lead author of the study and a PhD candidate at the Marine Mammal Research Program. The phrase does a lot of work. A thin margin means there is no buffer, no comfortable reserve to draw on when the fish move or the water warms or a calf needs feeding.

The mechanism the team suspects is, at bottom, an accounting problem. False killer whales in Hawaiʻi hunt the same large, oily, high-value fish that people prize: mahi-mahi, ono, aku, and ʻahi, the yellowfin tuna that anchors half the menus on the islands. When a whale and a longline are after the same tuna, the whale does not always win. The paper traces a developmental signature in the bodies it measured, with relative width across the midsection declining as animals age, a pattern the authors read as fat laid down early in life and then steadily burned through by the energetic demands of adulthood. An adult false killer whale, in other words, is an expensive animal to run.

Not All Whales Are Struggling Equally

What the aerial data also showed is that hardship is not evenly shared. The population sorts itself into social clusters, and when the researchers ran a standardized major axis regression on the length-to-volume relationship, the clusters came out genuinely different from one another. One group in particular, known for ranging broadly across the archipelago, showed wide swings in condition. Cover more ground to find your dinner and you spend more energy doing it; the whales that travel furthest may be paying the steepest metabolic toll.

None of this would mean much if the measurements were soft. To check, the team flew their method against a known quantity, validating the drone-derived volumes on false killer whales held in human care at the Okinawa Churashima Foundation in Japan, whose bodies could be 3D-scanned directly. The aerial estimates landed within 3 percent of the scans. “This level of precision allows us to pinpoint exactly when and where these whales are struggling, which is key for directing conservation efforts,” says Lars Bejder, who directs the Marine Mammal Research Program and co-authored the study.

Precision is the point, because the alternative is flying blind while a population thins out. A 3.5 percent annual decline sounds gentle until you do the arithmetic on a group already below 140. There is not a great deal of room for error, and managers cannot protect what they cannot measure.

The loss, if it comes, will not be only an ecological one. False killer whales are woven into Hawaiian cultural memory, and the people who carry that memory are themselves disappearing. “Hawaiian culture has been losing many kūpuna, elders who carry the libraries of knowledge in cultural practices,” says Kaʻapuni, cultural advisor at the Pacific Whale Foundation. “We cannot afford to lose any more pieces of Hawaiʻi.”

This study is a baseline, not a verdict. It cannot yet say for certain that hunger is what is killing these whales, only that hunger is real, measurable, and worst in the years the ocean turns hostile. The next question is whether the fish are simply running out, and whether sharing them more carefully with the fleets that chase the same tuna might buy the whales the margin they no longer have. “These findings highlight the need to better understand the energetic requirements of these whales and how external stressors may be affecting them,” says Bejder. For now the drones keep flying, logging each animal’s width against the sea, watching to see which of the survivors is quietly getting lighter.

DOI / Source: https://doi.org/10.3354/esr01505

Frequently Asked Questions

Are false killer whales actually a type of killer whale?

No, despite the name. They are among the largest members of the dolphin family, fast-moving and capable of ranging across huge stretches of open ocean. The “killer whale” label comes from a passing resemblance in skull shape, not a close relationship. The Hawaiian group is a distinct, island-resident population that does not mix with the others.

How do you weigh a wild whale without touching it?

From the air. Researchers fly a drone overhead and photograph each animal, then measure its width at multiple points along the body and use those dimensions to calculate volume, which converts to an estimated mass. In this study the method was checked against whales whose bodies could be 3D-scanned directly, and the aerial estimates came within 3 percent. That accuracy is what lets scientists track individuals losing condition over time.

Why does a warming ocean make whales lose weight?

Marine heatwaves reshuffle where prey lives and how abundant it is, and they can push the fish whales depend on deeper, farther, or out of reach entirely. The population’s leanest year on record, 2020, coincided with record sea surface temperatures and the largest single-year drop in its numbers. When food gets harder to find, an animal already living on a thin energy reserve has nothing to fall back on.

Could protecting fish stocks actually save these whales?

That is the hope, though it is not yet proven. The whales hunt the same high-value species, mahi-mahi, ono, aku, and ʻahi, that commercial and recreational fisheries target, raising the possibility that competition for prey is squeezing them. Establishing this body-condition baseline is the step that lets managers test whether prey limitation is the real driver, and whether managing those stocks more carefully would help.

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